Electronics Project Circuts: IC555 Project

4 Way Traffic Lights

Sunil Jadhavar IC555 Project, LED Projects 7:54 AM

4 Way Traffic Lights

This circuit produces traffic lights for a "4-way" intersection. The seemingly complex wiring to illuminate the light...

Traffic Lights

Sunil Jadhavar IC555 Project, LED Projects 7:50 AM

<span style="font-family: Arial; font-size: x-small;"><span style="color: navy; font-family: Arial;"><span style="color: black; font-family: Arial; font-size: x-small;">Here's a clever circuit using two 555's to produce a set of traffic 
   lights for a model layout. <br />
   The animation shows the lighting sequence and this follows the Australian-standard. 
   The red LED has an equal on-off period and when it is off, the first 
   555 delivers power to the second 555. This illuminates the Green LED 
   and then the second 555 changes state to turn off the Green LED and 
   turn on the Orange LED for a short period of time before the first 
   555 changes state to turn off the second 555 and turn on the red 
   LED. A supply voltage of 9v to 12v is needed because the second 555 
   receives a supply of about 2v less than rail. This circuit also 
   shows how to connect LEDs high and low to a 555 and also turn off 
   the 555 by controlling the supply to pin 8.  Connecting the 
   LEDs high and low to pin 3 will not work and since pin 7 is in phase 
   with pin 3, it can be used to advantage in this design.  </span>
   <table border="0" id="table215" style="width: 483px;">
    <tbody>
<tr>
     <td width="63"><img border="0" height="178" src="http://www.talkingelectronics.com/projects/30%20LED%20Projects/images/TrafficLight.gif" width="38" /></td>
     <td><img border="0" height="240" src="http://www.talkingelectronics.com/projects/30%20LED%20Projects/images/TrafficLights-1.gif" width="385" /></td></tr>
</tbody></table>
</span></span>

Traffic Lights

Here's a clever circuit using two 555's to produce a set of traffic lights for a model layout. The animation shows the light...

Flashing Railroad Lights

Sunil Jadhavar IC555 Project, LED Projects 7:27 AM

Flashing Railroad Lights

This circuit flashes two red LEDs for a model railway crossing.

DC motor control circuit using NE555

Sunil Jadhavar IC555 Project 4:30 AM

<h3>
<b>Description.</b></h3>
<div style="text-align: justify;">
A simple DC motor controller circuit using NE555 is shown here. Many <a href="http://www.circuitstoday.com/pwm-motor-speed-controller" target="_blank">DC motor speed control</a>
 circuits have been published here but this is the first one using NE555
 timer IC. In addition to controlling the motors speed its direction of 
rotation can be also changed using this circuit.</div>
<div style="text-align: justify;">
A PWM circuit based on timer NE555 is 
the heart of this circuit. NE555 is wired as an astable multivibrator 
whose duty cycle can be adjusted by varying the POT R1. The output of 
IC1 is coupled to the base of transistor Q1 which drives the motor 
according to the PWM signal available at its base. Higher the duty cycle
 the average voltage across motor will be high which results in higher 
motor speed and vice versa. Change of DC motor direction is attained 
using the DPDT switch S1 which on application just toggles the polarity 
applied to the motor.</div>
<b> </b><br />
<h3>
<b>Circuit diagram of DC motor controller.</b></h3>
<div class="mceTemp">
<b> </b>
<br />
<div class="wp-caption alignleft" id="attachment_4948" style="width: 862px;">
<b><a href="http://www.circuitstoday.com/wp-content/uploads/2011/02/dc-motor-controller.png"><img alt="Dc motor control circuit diagram" class="size-full wp-image-4948 " height="363" src="http://www.circuitstoday.com/wp-content/uploads/2011/02/dc-motor-controller.png" title="dc motor controller" width="640" /></a></b><br />
<div class="wp-caption-text">
DC motor controller using NE555</div>
</div>
<b>Notes.</b></div>
<ul>
<li>The circuit can be assembled on a Vero board or PCB.</li>
<li>Use 12V DC for powering the IC.</li>
<li>Vm is the power supply for motor and its value depends on the motors
 voltage rating. Any way maximum Vceo for BD139 is 80V and so Vm should 
not be exceeded 80 volts.</li>
<li>Maximum collector current BD139 can handle is 1.5A and so do not use a motor that consumes more than 1.5 amperes of current.</li>
<li>A heat sink is necessary for BD139.</li>
</ul>
<b>Few other  DC motor speed control circuit you may be interested in:</b>

DC motor control circuit using NE555

Description. A simple DC motor controller circuit using NE555 is shown here. Many DC motor speed control circuits have been published h...

555 Duty Cycle Control

Sunil Jadhavar IC555 Project 6:22 PM

Here is a simple oscillator circuit that varies the duty cycle over a
 wide range without affecting the frequency.  It is a variation of the 
simple 555 astable oscillator.  Initially, I told a reader that there 
was no standard 555 circuit that could do this, but then the grey matter
 started working.   The use of an air-variable capacitor for frequency 
control is a mind-blower—nothing short of a time warp!<br />
<span id="more-6950"></span><br />
<ins style="border: none; display: inline-table; height: 280px; margin: 0; padding: 0; position: relative; visibility: visible; width: 336px;"><ins id="aswift_0_anchor" style="border: none; display: block; height: 280px; margin: 0; padding: 0; position: relative; visibility: visible; width: 336px;"></ins></ins><br />
<b>555 Duty Cycle Control Schematic</b>  <br />
<a href="http://electroschematics.com/wp-content/uploads/2012/10/555-Duty-Cycle-Control-Schematic.jpg" target="_blank"><img alt="555 Duty Cycle Control Schematic" class="alignnone size-large wp-image-6951" height="380" src="http://electroschematics.com/wp-content/uploads/2012/10/555-Duty-Cycle-Control-Schematic-550x380.jpg" title="" width="550" /></a><br />
<b>Overview</b><br />
When potentiometer R1 is centered, operation is obvious and the duty 
cycle is 50%.  However, as R1 is rotated in either direction the charge 
time and discharge times vary accordingly.  The two sides of R1 have 
independent steering diodes (D1 & D2).  C1 & C2 make up the 
timing capacitor.  Pins 2 & 6 of the 555 are the upper and lower 
thresholds of the input comparators.  The charge /discharge voltage is 
taken from pin 3 because it has rail-to-rail voltage swing, and the open
 collector output (pin 7) cannot do this.  The rectangular waveform 
output is taken from pin 7 instead.  R3 is the pull-up resistor.<br />
If constant frequency is desired, C1 could be padded for the correct 
frequency.  However, most experimenters also want variable frequency.  
Since R1 cannot be varied in total resistance, it cannot vary the 
frequency.  R2 could vary the frequency, but would also affect the duty 
cycle ratio limits as well.  The only practical means of obtaining 
variable frequency is to vary C1.<br />
<b>Mathematical proof</b><br />
Q = I * T – where Q is the charge, I is charge current and T is charge time<br />
Q = C * V – where Q is charge, C is capacitance, and V is voltage across the capacitor<br />
∴ I * T = C * V        <br />
E = I * R –  where E is voltage I is current and R is resistance<br />
∴ I * T = C * I * R – because E is simply another expression for V<br />
T = C * R – dividing by I—we have now proved that charge time is directly proportional to R<br />
So we have a charge resistance and a discharge resistance, the sum of
 which is constant and equal to the R1 potentiometer total resistance (1
 to 3).  Therefore, the sum of the charge and discharge times is also 
constant.  Since F = 1/T, the frequency is also be constant.<br />
In other words, the two resistances are complementary and the two time periods are likewise complementary.<br />
<b>The Air Variable Capacitor</b><br />
The old-fashioned air variable capacitor is old and klunky.  While 
DigiKey offers no such product, these devices remain available on eBay 
as used or old stock.  Every serious experimenter should have one of 
these.  The one I would buy is a 3-gang 440pf.  Wired in parallel the 
total capacitance is 1320pf.  Physical size limitations prevent higher 
capacitances.<br />
Actually, it is fun to play with air variable capacitors.<br />
<b>High impedances</b><br />
To obtain a reasonably low frequency, R1 had to be selected to have 
as high a resistance as possible.  2M is the highest value pot I had on 
hand.  A 5M would also be a good selection.  To function under such high
 impedance conditions, I selected the TLC555 CMOS 555.  A further 
advantage of this device is that it has rail-to-rail output 
voltage—something that the bipolar 555 cannot do.<br />
<b>Limits of accuracy</b><br />
The accuracy tends to degrade when the slew rate at C1 exceeds about 
0.25V/uS.  When this happens, the propagation delay of the comparators 
becomes significant and the frequency drops somewhat.  This causes the 
peaks of the saw-tooth waveform to “skid” past the thresholds before the
 output switches polarity.  This forces the maximum frequency to be 
lower than about 50kHZ for 50% duty cycle, or lower than 2kHZ with 98% 
duty cycle.  To maintain 1 or 2% frequency accuracy the duty cycle range
 must drop as frequency increases.<br />
<b>Bench test set-up</b><br />
<br />
<div class="ngg-galleryoverview" id="ngg-gallery-2-6950">
<div class="ngg-gallery-thumbnail-box" id="ngg-image-7">
<div class="ngg-gallery-thumbnail">
<a class="shutterset_set_2" href="http://electroschematics.com/wp-content/gallery/555-duty-cycle-control/bench-setup.jpg" title=" ">
        <img alt="bench setup" src="http://electroschematics.com/wp-content/gallery/555-duty-cycle-control/thumbs/thumbs_bench-setup.jpg" title="bench setup" />
       </a>
  </div>
</div>
<div class="ngg-gallery-thumbnail-box" id="ngg-image-8">
<div class="ngg-gallery-thumbnail">
<a class="shutterset_set_2" href="http://electroschematics.com/wp-content/gallery/555-duty-cycle-control/protoboard-closeup.jpg" title=" ">
        <img alt="protoboard closeup" height="75" src="http://electroschematics.com/wp-content/gallery/555-duty-cycle-control/thumbs/thumbs_protoboard-closeup.jpg" title="protoboard closeup" width="100" />
       </a>
  </div>
</div>
<div class="ngg-gallery-thumbnail-box" id="ngg-image-9">
<div class="ngg-gallery-thumbnail">
<a class="shutterset_set_2" href="http://electroschematics.com/wp-content/gallery/555-duty-cycle-control/protoboard.jpg" title=" ">
        <img alt="protoboard" height="75" src="http://electroschematics.com/wp-content/gallery/555-duty-cycle-control/thumbs/thumbs_protoboard.jpg" title="protoboard" width="100" />
       </a>
  </div>
</div>
</div>
<b>Oscillographs</b><br />
<br />
<div class="ngg-galleryoverview" id="ngg-gallery-3-6950">
<div class="ngg-gallery-thumbnail-box" id="ngg-image-10">
<div class="ngg-gallery-thumbnail">
<a class="shutterset_set_3" href="http://electroschematics.com/wp-content/gallery/555-duty-cycle-control-oscillographs/5-duty-cycle.jpg" title=" ">
        <img alt="5% duty cycle" height="75" src="http://electroschematics.com/wp-content/gallery/555-duty-cycle-control-oscillographs/thumbs/thumbs_5-duty-cycle.jpg" title="5% duty cycle" width="100" />
       </a>
  </div>
</div>
<div class="ngg-gallery-thumbnail-box" id="ngg-image-11">
<div class="ngg-gallery-thumbnail">
<a class="shutterset_set_3" href="http://electroschematics.com/wp-content/gallery/555-duty-cycle-control-oscillographs/50-duty-cycle.jpg" title=" ">
        <img alt="50% duty cycle" height="75" src="http://electroschematics.com/wp-content/gallery/555-duty-cycle-control-oscillographs/thumbs/thumbs_50-duty-cycle.jpg" title="50% duty cycle" width="100" />
       </a>
  </div>
</div>
<div class="ngg-gallery-thumbnail-box" id="ngg-image-12">
<div class="ngg-gallery-thumbnail">
<a class="shutterset_set_3" href="http://electroschematics.com/wp-content/gallery/555-duty-cycle-control-oscillographs/95-duty-cycle.jpg" title=" ">
        <img alt="95% duty cycle" height="75" src="http://electroschematics.com/wp-content/gallery/555-duty-cycle-control-oscillographs/thumbs/thumbs_95-duty-cycle.jpg" title="95% duty cycle" width="100" />
       </a>
  </div>
</div>
</div>
<b>For the future</b><br />
Novel VCO circuit<br />
<b>Glossary of undocumented words</b>   (for our ESL friends)<br />
Klunky – adjective – variation of clunky – big, cumbersome, ugly, outmoded…<br />
<div class="info">
<a href="http://electroschematics.com/650/lm555-datasheet/" target="_blank">555 datasheet</a></div>

555 Duty Cycle Control

Here is a simple oscillator circuit that varies the duty cycle over a wide range without affecting the frequency. It is a variation of the...

555 (TLC555) Relay Driver Circuit

Sunil Jadhavar IC555 Project 6:21 PM

Many integrated circuits have undocumented features or abilities.  
This is one of them.  The TLC555 output (pin 3) can sink a 100mA load to
 1.28V.  The open drain transistor reset (pin 7) can sink 100mA to 1V.  
Tying both lines together is permissible because they are logically the 
same polarity and this potentially doubles the sink current ability to 
200mA.  This is ideal for driving my 133mA relay coil.<br />
The input also has the undocumented, but better known, feature of a Schmitt trigger that provides positive switching.<br />
<span id="more-7103"></span><br />
<ins style="border: none; display: inline-table; height: 280px; margin: 0; padding: 0; position: relative; visibility: visible; width: 336px;"><ins id="aswift_0_anchor" style="border: none; display: block; height: 280px; margin: 0; padding: 0; position: relative; visibility: visible; width: 336px;"></ins></ins><br />
<span style="font-family: "Trebuchet MS",sans-serif; font-size: large;">555 Relay Driver Schematic</span><br />
<a href="http://electroschematics.com/wp-content/uploads/2012/11/TLC555-Relay-Driver-Schematic.jpg" target="_blank"><img alt="TLC555 Relay Driver Schematic" class="alignnone size-large wp-image-7106" height="392" src="http://electroschematics.com/wp-content/uploads/2012/11/TLC555-Relay-Driver-Schematic-550x392.jpg" title="" width="550" /></a><br />
<b>Protoboard setup </b> <br />
<a href="http://electroschematics.com/wp-content/uploads/2012/11/Protoboard.jpg" target="_blank"><img alt="" class="alignnone size-large wp-image-7105" height="412" src="http://electroschematics.com/wp-content/uploads/2012/11/Protoboard-550x412.jpg" title="" width="550" /></a><br />
<b>Output current rating (pin 3): TLC555  vs.  NE555</b><br />
<table style="width: 500px;">
<tbody>
<tr>
<th>(Vcc = 15V)</th>
<th>TLC555</th>
<th>NE555</th>
</tr>
<tr>
<td>Source current</td>
<td>10mA</td>
<td>200mA</td>
</tr>
<tr>
<td>Source mode saturation voltage</td>
<td>Vcc-0.8V</td>
<td>Vcc-2.5V</td>
</tr>
<tr>
<td>Sink current</td>
<td>100mA</td>
<td>200mA</td>
</tr>
<tr>
<td>Sink mode saturation voltage</td>
<td>1.28V</td>
<td>2.5V</td>
</tr>
</tbody></table>
While it can be seen that the NE555 has the higher current rating, 
its saturation voltage is grossly inferior and this is a detriment in 
driving loads without excessive voltage drop.  Also it can be seen that 
the TLC555 is much like TTL in that its sourcing ability is far less 
than its sinking ability.  However for driving a relay, we are 
interested only in current sinking properties.<br />
<b>Relay out of my junk box</b><br />
<a href="http://electroschematics.com/wp-content/uploads/2012/11/12V-Contactor.jpg" target="_blank"><img alt="12V Contactor" class="alignnone size-large wp-image-7104" height="412" src="http://electroschematics.com/wp-content/uploads/2012/11/12V-Contactor-550x412.jpg" title="" width="550" /></a><br />
This is a 40A automotive relay (contactor) that I selected for this 
application.  Since its coil current exceeds the TLC555 output sink 
current rating, it is a good candidate.  Manufacturer or part number is 
unknown.<br />
<ul>
<li>Contact rating:  40A (from memory)</li>
<li>Coil resistance:  90Ω cold</li>
<li>Coil current:  133mA</li>
<li>Coil power:  1.6W</li>
<li>Coil voltage:  12V</li>
<li>Pickup voltage:  4.8V cold</li>
<li>Dropout voltage:  2.1V cold</li>
<li>Operate time:  4.5mS</li>
</ul>
What is the difference between a relay and a contactor?  There is no 
clear difference other than perhaps current rating and/or application—to
 me, relays are anything from signal devices to approx 20A. Anything 
rated at 40A or larger, or is used in power applications is a contactor.<br />
<b>Schmitt trigger input</b><br />
The trigger input (pin 2) and the threshold input (pin 6) pins are 
tied together—this is commonly done.  With a 12V supply, the upper 
threshold is 8V and the lower is 4V.  The two voltage levels, being far 
apart, make a great Schmitt trigger.  This may be driven directly by 
4000series CMOS logic that is also powered via 12V.<br />
To make it compatible with TTL logic levels, simply add the two 
resistors that are shown in the schematic.  This loads down the internal
 divider to a lower voltage.  The calculated levels are approx. 1.4 and 
2.8V respectively.<br />
<b>Testing the performance</b><br />
<ul>
<li>Output saturation voltage:
<ul>
<li>0.617V @ 133mA</li>
<li>1.043V @ 200mA</li>
</ul>
</li>
<li>TTL high level threshold:  3.0V</li>
<li>TTL low level threshold:  1.6V</li>
</ul>
<b>How about the bipolar 555 and other variants?</b><br />
The standard 555 saturates poorly (2.5V) at 200mA and its reset line 
(pin 7) is not specified above 15mA, so it is not recommended.  The 
intersil 7555 is a very low power device, so that is not recommended 
either.<br />
<b>For the future</b><br />
<ul>
<li>Assorted relay driver circuits</li>
<li>555 Voltage doubler relay driver circuit</li>
<li>Relay economy circuits</li>
</ul>
<div class="info">
<a href="http://electroschematics.com/650/lm555-datasheet/" target="_blank" title="tlc555 datasheet">555 datasheet</a></div>

555 (TLC555) Relay Driver Circuit

Many integrated circuits have undocumented features or abilities. This is one of them. The TLC555 output (pin 3) can sink a 100mA load to...

Inverted 555 Timer Circuit

Sunil Jadhavar IC555 Project 6:20 PM

Whenever I use the 555 timer, it seems that the output polarity is 
invariably incorrect, and the way the 555 functions, it normally cannot 
generate a duty cycle of less than 50% — 90% yes, 10% NO!  This inverted
 555 circuit generates duty cycles of less than 50%.  Everything works 
the same except for the polarity.  Why not simply invert the output with
 a transistor?  Will not that do the same?  Legitimate question…  
Actually NO, because a transistor that inverts the output cannot provide
 the desirable totem pole source/sink output function of pin 3.  And 
there is another reason that we will get in the future—this is a 555VCO 
circuit that requires the inverted timer.<br />
<span id="more-7114"></span><br />
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<h2>
Inverted 555 Timer Schematic</h2>
<a href="http://electroschematics.com/wp-content/uploads/2012/11/Inverted-555-Timer-Schematic.jpg" target="_blank"><img alt="Inverted 555 Timer Schematic" class="alignnone size-large wp-image-7117" height="550" src="http://electroschematics.com/wp-content/uploads/2012/11/Inverted-555-Timer-Schematic-540x550.jpg" title="" width="540" /></a><br />
The schematic shows (3) circuits, because one circuit does not work well over the entire Vcc range.<br />
The first simply uses a “normal” 2N3904 garden variety transistor, 
and this works well when Vcc < 9V.  When Vcc > 9V, the base to 
emitter junction starts to zener and disrupts operation. Transistor base
 to emitter junction zeners a little above -6V.<br />
The second circuit adds D1 to the emitter of Q1 in order to increase 
VEBO.  With this mod, Vcc may be increased to the 18V limit.  However, 
D1 may be eliminated if we use the special 2SC2878 switching transistor 
that has a VBEO rating of -25V.  These devices are no longer in 
production, but are available on eBay—every serious experimenter should 
have a few in his component collection.  The types now in production use
 the tiny SO-23 SMD package.<br />
The third circuit uses a J112 N-Channel JFET for Q1.  The JFET is a 
curious device that works well for the application provided we allow for
 the VGS(OFF) parameter.  VGS must be less than Vcc /3.  (e.g. if Vcc = 
9V, VGS(OFF) must be less than 9V /3 or 3V for proper operation).  Since
 the VGS(OFF) parameter is sloppy (-1 to -5V), the device must be 
selected—to the left is a simple test circuit—actually, most devices 
will work OK.  The J112 should also be in every serious experimenter’s 
component collection.<br />
<b>Inverted 555 Oscillograph</b><br />
<a href="http://electroschematics.com/wp-content/uploads/2012/11/Oscillograph-Inverted-555.jpg" target="_blank"><img alt="Oscillograph Inverted 555" class="alignnone size-full wp-image-7118" height="408" src="http://electroschematics.com/wp-content/uploads/2012/11/Oscillograph-Inverted-555.jpg" title="" width="498" /></a><br />
<b>How it works</b><br />
You will note that C1 is tied to Vcc rather than common and that R1 
& R2 charge it in the negative direction.  By connecting C1 to Vcc, 
the relatively high discharge current does not run through the power 
source.  The reset transistor (Q1) discharges the capacitor in the 
positive direction.  R1 & R2 are determined in the same fashion as 
the normal 555 timer.  R3 is a pull-up resistor for the open collector 
output (pin 7) so it can drive the emitter follower (or source follower)
 reset transistor.<br />
R1 scales the output frequency—here it is about 1.25kHZ and it can be rescaled to whatever you need.<br />
R2 controls the reset pulse width for low duty cycle (short output 
pulse) output—the larger it gets, the more it affects output frequency.<br />
Pin 5 does not need a bypass capacitor unless you desire high stability—generally I omit this capacitor.<br />
<b>Protoboard setup</b><br />
<a href="http://electroschematics.com/wp-content/uploads/2012/11/Protoboard-Setup.jpg" target="_blank"><img alt="Inverted 555 Protoboard Setup" class="alignnone size-large wp-image-7119" height="412" src="http://electroschematics.com/wp-content/uploads/2012/11/Protoboard-Setup-550x412.jpg" title="" width="550" /></a><br />
<b>For the future</b><br />
555 VCO circuit<br />
<b>Preferred components for the serious experimenter</b><br />
2SC2878 – <a href="http://electroschematics.com/wp-content/uploads/2012/11/2sc2878-datasheet.pdf" target="_blank" title="2sc2878 datasheet">datasheet</a><br />
J112 N-Channel JFET transistor – <a href="http://electroschematics.com/wp-content/uploads/2012/11/J111-Datasheet.pdf" target="_blank" title="J111 Datasheet">datasheet</a><br />
NE555 timer IC – <a href="http://electroschematics.com/650/lm555-datasheet/" target="_blank" title="555 datasheet">datasheet</a><br />
2N4148 signal diode, 100V, 150mA (garden variety diode) (same as 1N916) (1N914 is rated at 75V)<br />
<b>Glossary of undocumented words and idioms</b> (for our ESL friends)<br />
garden variety –idiom—literally something that grows in your garden—commonly available component<br />
<div class="info">
<a href="http://electroschematics.com/650/lm555-datasheet/" target="_blank">555 datasheet</a></div>

Inverted 555 Timer Circuit

Whenever I use the 555 timer, it seems that the output polarity is invariably incorrect, and the way the 555 functions, it normally cannot ...

Wide Frequency Range 555 VCO

Sunil Jadhavar IC555 Project 6:19 PM

The 555 frequency can be varied via adjusting the voltage at pin 5.  
However, the range and linearity of frequency adjustment is very 
limited.  This is a way to greatly improve performance using the inverted 555 timer circuit that was previously posted.<br />
<span id="more-7170"></span><br />
<ins style="border: none; display: inline-table; height: 280px; margin: 0; padding: 0; position: relative; visibility: visible; width: 336px;"><ins id="aswift_0_anchor" style="border: none; display: block; height: 280px; margin: 0; padding: 0; position: relative; visibility: visible; width: 336px;"></ins></ins><br />
<span style="font-size: large;"><b>555 VCO Schematic</b></span><br />
<a href="http://electroschematics.com/wp-content/uploads/2012/11/Wide-Range-555-VCO-Circuit.jpg" target="_blank"><img alt="Wide Range 555 VCO Circuit Schematic" class="alignnone size-large wp-image-7178" height="270" src="http://electroschematics.com/wp-content/uploads/2012/11/Wide-Range-555-VCO-Circuit-550x270.jpg" title="" width="550" /></a><br />
<b>Current source</b><br />
Q2 is connected in the common base configuration.  In this mode of 
operation, the collector current is a function of emitter current 
regardless of the collector voltage.  In this way, it can perform a 
linear charge function upon C1.  The emitter must be driven from a 
negative supply voltage.  Frequency is scaled to 10V = 10kHZ via R5.  
Frequency range in this set-up is 180 to 10kHZ.<br />
<b>Bias transistor</b><br />
Q3 is wired as a self-biasing transistor by connecting the base to 
the collector and feeding it a bias current via R4.  Voltage drop is 
0.6V.  Originally, I had a 1N4148 diode and its voltage drop was 0.45V 
that added too much offset to the emitter of Q1.  Ideally, the voltage 
drop across Q3 should equal Vbe of Q2.  This is a compromise because 
they are matched only under the condition of Ie = (9V – 0.6V) /R4 = 
84uA.  The emitter current varies from about 3 to 303uA.  <br />
Lowest low end offset would occur with both transistors matched and 
R4 increased to 3M—in that case, the hFE @ 3uA must >100—this looks 
practical, but would require a different transistor selection—I did not 
experiment with this option.<br />
<b>Reset transistor</b><br />
Q1 is a 2N4401.  It varies from the 2N3904 in that its current gain 
(hFE) is optimized at a higher current.  The reset current must be high 
due to its low duty cycle.<br />
<b>Oscillographs</b><br />
<div class="ngg-galleryoverview" id="ngg-gallery-5-7170">
<div class="ngg-gallery-thumbnail-box" id="ngg-image-31">
<div class="ngg-gallery-thumbnail">
<a class="shutterset_set_5" href="http://electroschematics.com/wp-content/gallery/wide-frequency-range-555-vco-oscillographs/waveform-1.jpg" title=" ">
        <img alt="waveform-1" height="75" src="http://electroschematics.com/wp-content/gallery/wide-frequency-range-555-vco-oscillographs/thumbs/thumbs_waveform-1.jpg" title="waveform-1" width="100" />
       </a>
  </div>
</div>
<div class="ngg-gallery-thumbnail-box" id="ngg-image-32">
<div class="ngg-gallery-thumbnail">
<a class="shutterset_set_5" href="http://electroschematics.com/wp-content/gallery/wide-frequency-range-555-vco-oscillographs/waveform-2.jpg" title=" ">
        <img alt="waveform-2" height="75" src="http://electroschematics.com/wp-content/gallery/wide-frequency-range-555-vco-oscillographs/thumbs/thumbs_waveform-2.jpg" title="waveform-2" width="100" />
       </a>
  </div>
</div>
</div>
<b>Linearity</b><br />
<div class="ngg-galleryoverview" id="ngg-gallery-6-7170">
<div class="ngg-gallery-thumbnail-box" id="ngg-image-33">
<div class="ngg-gallery-thumbnail">
<a class="shutterset_set_6" href="http://electroschematics.com/wp-content/gallery/wide-frequency-range-555-vco-linearity/linear-graph.jpg" title=" ">
        <img alt="linear-graph" height="75" src="http://electroschematics.com/wp-content/gallery/wide-frequency-range-555-vco-linearity/thumbs/thumbs_linear-graph.jpg" title="linear-graph" width="100" />
       </a>
  </div>
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<div class="ngg-gallery-thumbnail-box" id="ngg-image-34">
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<a class="shutterset_set_6" href="http://electroschematics.com/wp-content/gallery/wide-frequency-range-555-vco-linearity/log-graph.jpg" title=" ">
        <img alt="log-graph" height="75" src="http://electroschematics.com/wp-content/gallery/wide-frequency-range-555-vco-linearity/thumbs/thumbs_log-graph.jpg" title="log-graph" width="100" />
       </a>
  </div>
</div>
</div>
Linearity is shown over two orders of magnitude.  While not perfect, 
it shows how the circuit performs.  The error on the low end is caused 
by input offset voltage that is not perfectly nulled via the voltage 
drop of Q3.  On the high end, non-linearity is caused by the finite 
reset time of the 555 that runs about 6% @ 10kHZ.<br />
<b>Spread sheet data </b> <br />
<div class="download-archive">
<a href="http://electroschematics.com/wp-content/uploads/2012/11/Wide-Range-555-VCO.rar" target="_blank" title="vco transfer function">link to wide range 555 VCO.xls</a></div>
<b>Protoboard setup</b><br />
<a href="http://electroschematics.com/wp-content/uploads/2012/11/Protoboard-setup.jpg" target="_blank"><img alt="" class="alignnone size-large wp-image-7175" height="412" src="http://electroschematics.com/wp-content/uploads/2012/11/Protoboard-setup-550x412.jpg" title="" width="550" /></a><br />
There is some extra stuff here to generate a -10V reference from a single 19V supply.<br />
<b>Disadvantages</b><br />
Frequency is a function of Vcc—requires stable supply<br />
High component count<br />
Requires negative reference voltage—no issue if already using split supply<br />
<b>Conclusion</b><br />
I do not generally recommend this circuit, but believe that it would 
make a great lab experiment for those who really want to learn more 
about VCO’s and the versatile 555.<br />
<b>For the future</b><br />
XR4151 VCO circuit<br />
Charge pump VCO circuit<br />
<b>Preferred components for the serious experimenter</b><br />
<div class="sectiune">
<a href="http://electroschematics.com/wp-content/uploads/2012/11/2N4401.jpg" target="_blank"><img alt="2N4401" class="alignleft size-thumbnail wp-image-7172" height="100" src="http://electroschematics.com/wp-content/uploads/2012/11/2N4401-100x100.jpg" title="" width="100" /></a><br />
2N4401, 600mA, 60V, hFE = 100 @ 150mA<br />
Digikey 2N4401-ND<br />
$0.20 each<br />
EBC pinout
<br />
<div class="download-pdf">
<a href="http://electroschematics.com/wp-content/uploads/2012/11/2N4401.pdf" target="_blank">2N4401 datasheet</a></div>
</div>
<div class="sectiune">
<a href="http://electroschematics.com/wp-content/uploads/2012/11/2N3904.jpg" target="_blank"><img alt="2N3904" class="alignleft size-thumbnail wp-image-7171" height="100" src="http://electroschematics.com/wp-content/uploads/2012/11/2N3904-100x100.jpg" title="" width="100" /></a><br />
2N3904, 200mA, 60V, hFE = 100 @ 10mA,<br />
Digikey 2N3904FS-ND<br />
$0.18 each<br />
EBC pinout
<br />
<div class="download-pdf">
<a href="http://electroschematics.com/wp-content/uploads/2012/11/MMBT3904.pdf" target="_blank">2N3904 datasheet</a></div>
</div>
<div class="info">
<a href="http://electroschematics.com/650/lm555-datasheet/" target="_blank" title="555 datasheet">555 datasheet</a></div>

Wide Frequency Range 555 VCO

The 555 frequency can be varied via adjusting the voltage at pin 5. However, the range and linearity of frequency adjustment is very limi...

Quirky 555 Timer Reset Function

Sunil Jadhavar IC555 Project 6:18 PM

How many have had legitimate problems with the 555 timer?  Let me 
guess…it involved the reset line, did it not?  We all know that pin 4 
must be set high before oscillation begins, but what is its threshold? 
input current? and what happens when it is operated slightly out of 
spec?  This may be the very first attempt in documentation of this 
obscure phenomenon.<br />
<span id="more-7195"></span><br />
<ins style="border: none; display: inline-table; height: 280px; margin: 0; padding: 0; position: relative; visibility: visible; width: 336px;"><ins id="aswift_0_anchor" style="border: none; display: block; height: 280px; margin: 0; padding: 0; position: relative; visibility: visible; width: 336px;"></ins></ins><br />
<span style="font-size: large;"><b>555 pin 4 reset behavior schematic</b></span><br />
<a href="http://electroschematics.com/wp-content/uploads/2012/12/Test-Circuit-1.jpg" target="_blank"><img alt="555 pin 4 reset behavior Test Circuit 1" class="alignnone size-large wp-image-7201" height="318" src="http://electroschematics.com/wp-content/uploads/2012/12/Test-Circuit-1-550x318.jpg" title="" width="550" /></a><br />
<b>Method</b><br />
To determine reset threshold, a transistor integrator generates a low
 impedance, negative-going ramp voltage signal that integrates from +5V 
to –0.65V.  To obtain the negative voltage, the power supply is split to
 provide –0.7V.  While it is slowly changing at the rate of -1V /S 
(trace 1), pins 2 & 6 are monitored for oscillation (trace 2).  A 
3rd trace monitors the output (pin 3).  All data is logged on the 
spreadsheet.<br />
A total of (8) devices were tested, including (2) CMOS TLC555N devices.<br />
When running, the 555 oscillates at approx. 100hZ.  Vcc = 5V.<br />
<b>Data</b><br />
<a href="http://electroschematics.com/wp-content/uploads/2012/12/Data.jpg" target="_blank"><img alt="" class="alignnone size-large wp-image-7198" height="184" src="http://electroschematics.com/wp-content/uploads/2012/12/Data-550x184.jpg" title="Data" width="550" /></a><br />
<b>Oscillograph</b><br />
<a href="http://electroschematics.com/wp-content/uploads/2012/12/555-Pin-4-Oscillograph.jpg" target="_blank"><img alt="" class="alignnone size-full wp-image-7196" height="408" src="http://electroschematics.com/wp-content/uploads/2012/12/555-Pin-4-Oscillograph.jpg" title="555 Pin 4 Oscillograph" width="498" /></a><br />
<b>Observations</b><br />
The threshold voltage is exceptionally low—lower than any true logic 
device.  Specifications indicate a threshold range of 0.4 to 1.0V in 
bipolar devices, and 0.4 to 1.5V in CMOS device.  Actual measurements 
indicate a range of 0.65 to 0.75V (bipolar) and 1.1V (CMOS).<br />
What is most interesting is what happens when the voltage drops a 
little below –0.12V.  At this voltage, pin 7 turns off and the voltage 
at pins 2 & 6 floats high while pin 3 demonstrates an abnormally 
high saturation voltage (0.8V)—normal is 0.1V.  While this is an 
out-of-spec condition, it remains important information to know because 
it occurs very close to zero volts and may be subject to noise.  <br />
In some cases, this anomaly can actually occur above 0V.  I recall 
from years ago some 555’s that exhibited this behavior at approx +20mV. 
 The solution at the time was the addition of a series resistor in the 
reset line.  Of course these devices were out of spec and therefore 
defective according to published specifications.  Since then, I think 
manufacturers have been testing for this specific condition.  I 
challenge readers to locate some of these strange 555’s and make 
comment—very simple to test for this condition by simply shorting pin 4 
to common and see if pin 7 allows capacitor voltage to float up.<br />
One of the CMOS devices actually started to oscillate again at 
-0.65V.  I have had issues with this chip before, so I think that I can 
write it off as defective—may have taken a static discharge…<br />
<b>Not recommended as a voltage threshold function input</b><br />
Due to the extremely low and variable threshold, the reset line must 
not be used as an input level threshold.  I know it is used this way in 
numerous electroschematics.com circuits, but I cannot recommend it—this 
may be an explanation why some circuits fail to operate.<br />
<b>Looks good on paper, but this is an out of spec condition</b><br />
<a href="http://electroschematics.com/wp-content/uploads/2012/12/Test-Circuit-3.jpg" target="_blank"><img alt="" class="alignnone size-large wp-image-7203" height="327" src="http://electroschematics.com/wp-content/uploads/2012/12/Test-Circuit-3-550x327.jpg" title="Test Circuit 3" width="550" /></a><br />
This is a typical interface to avoid—looks clean and simple and works
 with most logic interfaces, but does not work with the 555.  What a 
simulator would show?  A simulator cannot handle out of spec conditions.<br />
<b>Another way to preset /reset the 555</b><br />
<a href="http://electroschematics.com/wp-content/uploads/2012/12/Test-Circuit-2.jpg" target="_blank"><img alt="" class="alignnone size-large wp-image-7202" height="336" src="http://electroschematics.com/wp-content/uploads/2012/12/Test-Circuit-2-550x336.jpg" title="Test Circuit 2" width="550" /></a><br />
Open collector comparators etc. work well as an interface to the 555.<br />
<b>Input current</b><br />
Reset current maxed out at 0V.  Actual range was -13 to -33uA.  The 
minus sign indicates that current comes out of pin 4 (sourced) as 
opposed to going into the pin (sinked).  CMOS input current was 
understandably immeasurable.  Actual specification for the bipolar 
version is 400uA typical.  From this data you can determine what you 
need to drive it below the reset threshold voltage—Max series resistor 
value must not generally exceed 470Ω or so.<br />
<b>Photos</b><br />
<br />
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<div class="ngg-gallery-thumbnail-box" id="ngg-image-54">
<div class="ngg-gallery-thumbnail">
<a class="shutterset_set_8" href="http://electroschematics.com/wp-content/gallery/quirky-555-timer-reset-function/bench-setup.jpg" title=" ">
        <img alt="bench setup" height="75" src="http://electroschematics.com/wp-content/gallery/quirky-555-timer-reset-function/thumbs/thumbs_bench-setup.jpg" title="bench setup" width="100" />
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<div class="ngg-gallery-thumbnail-box" id="ngg-image-55">
<div class="ngg-gallery-thumbnail">
<a class="shutterset_set_8" href="http://electroschematics.com/wp-content/gallery/quirky-555-timer-reset-function/protoboard-setup.jpg" title=" ">
        <img alt="protoboard setup" height="75" src="http://electroschematics.com/wp-content/gallery/quirky-555-timer-reset-function/thumbs/thumbs_protoboard-setup.jpg" title="protoboard setup" width="100" />
       </a>
  </div>
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</div>
<b>For the future</b><br />
Quirky <a href="http://electroschematics.com/628/lm358-datasheet/" title="LM358">LM358</a> /LM324 /LM293 /LM339 common mode input range issues<br />
<b>Glossary of undocumented words and idioms</b> (for our ESL friends)<br />
write it off –idiom—consider defective—forget it—give up on that idea…<br />
<b>Recommended devices for the serious experimenter</b><br />
<div class="sectiune">
<a href="http://electroschematics.com/wp-content/uploads/2012/12/DIP-8.jpg" target="_blank"><img alt="" class="alignleft size-thumbnail wp-image-7199" height="100" src="http://electroschematics.com/wp-content/uploads/2012/12/DIP-8-100x100.jpg" title="DIP-8" width="100" /></a><br />
LM555CN<br />
DIP-8 Package<br />
DigiKey LM555CNFS-ND, $0.43 each
<br />
<div class="info">
<a href="http://electroschematics.com/650/lm555-datasheet/" target="_blank">LM555 datasheet</a></div>
</div>
<div class="sectiune">
<a href="http://electroschematics.com/wp-content/uploads/2012/12/DIP-8.jpg" target="_blank"><img alt="" class="alignleft size-thumbnail wp-image-7199" height="100" src="http://electroschematics.com/wp-content/uploads/2012/12/DIP-8-100x100.jpg" title="DIP-8" width="100" /></a><br />
TLC555CP<br />
DIP-8 Package, CMOS<br />
DigiKey 296-1857-5-ND, $0.74 each
<br />
<div class="info">
<a href="http://electroschematics.com/650/lm555-datasheet/" target="_blank">TLC555 datasheet</a></div>
</div>

Quirky 555 Timer Reset Function

How many have had legitimate problems with the 555 timer? Let me guess…it involved the reset line, did it not? We all know that pin 4 mu...

LDR Pole Light Switch

Sunil Jadhavar IC555 Project 6:16 PM

My vintage (62 year old) pole light has always been controlled by a 
timer—a source of continual frustration due the requirement of 
readjustment for the ever-changing seasons.  However, after knocking it 
over (I backed my car into it…), I decided it was time for an update.  
The circuit consists of a light dependent resistor (LDR), TLC555 
(applied as Schmitt trigger), and a TRIAC power switch.  The complete 
assembly neatly fits inside the steel pole light tube and the LDR peeks 
out through a hole in the side.  This was really a fun project, and 
useful to boot.<br />
<span id="more-7207"></span><br />
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<span style="font-size: large;"><b>Pole Light Switch Circuit Schematic</b></span><br />
<a href="http://electroschematics.com/wp-content/uploads/2012/12/LDR-Pole-Light-Switch.jpg" target="_blank"><img alt="LDR Pole Light Switch Schematic" class="alignnone size-large wp-image-7219" height="275" src="http://electroschematics.com/wp-content/uploads/2012/12/LDR-Pole-Light-Switch-550x275.jpg" title="" width="550" /></a><br />
<b>Power supply</b><br />
The power supply is the typical capacitor limited charge pump type 
that is zener regulated at 6.2V.  Due to the lack of isolation, I was 
careful to identify the return conductor so that the electronics 
(including the LDR) would not be floating on the hot lead.  For safety, 
most of the testing was done using an isolation transformer.  R1 must 
absorb a high peak power transient current when power is applied, so a 
carbon comp, ceramic comp, or wire wound resistor is recommended.  
Maximum DC current available is 16mA.  Actual load is about 6mA.  My 
application was 115V, 60hZ.  For 230VAC, the components are indicated on
 the schematic.  In this circuit, +6V is the power return lead, and 0V 
is the electronic circuit common—to visualize this, one must screw his 
head on backwards…<br />
<b>LDR</b><br />
The light dependent resistor (CdS photocell) that I used was in the 
TO-5 package that is well adapted for poking through a hole and is held 
in place with silicone rubber.  The Clairex CL703M19 LDR that I used is 
no longer available and I have been unable to locate the specs.  The 
DigiKey PDV-P8103-ND appears to be a reasonable choice, but may require 
bias current tweaking to set the threshold.<br />
<b>555 Schmitt trigger driver</b><br />
A TLC 555 was used as a voltage threshold detecting device with 
hysteresis.  Pin 7 drives the TRIAC gate directly via its open collector
 output.  This is a rather unconventional application.  The CMOS version
 is used to minimize power supply load—I tried a bipolar 555 and it 
worked, but the power supply ripple voltage doubled to about 0.5VP-P.<br />
<b>Constant current bias—Threshold adjustment</b><br />
Because the 555 has so much hysteresis, I feared that the ON & 
OFF thresholds would be too far apart.  To help reduce the hysteresis, 
the LDR is biased by a current source.  Q1 is wired as a current 
source—its collector current does not vary with collector voltage.  This
 technique essentially increases the “gain” of the LDR.  The current is 
set via adjusting the emitter resistor (R3)—it drops about 0.37V.  <br />
C3 makes the circuit insensitive to rapid changes in light intensity.  It takes about 60sec to turn on.<br />
<b>Logic TRIAC</b><br />
The logic TRIAC is an interesting device.  It can be triggered by 
either a positive or negative gate current regardless of voltage 
blocking polarity.  For maximum sensitivity, I used negative gate 
current.  The device I used had an actual Igt (gate current sensitivity)
 of 1.5mA that is well below the 5mA Max specification.  However, gate 
overdrive (5mA in my case) is recommended to assure that it fires at low
 winter temperatures.  Quencharc RC-1 is connected across the TRIAC to 
help control turn-off voltage transients.<br />
<b>Choice of lighting</b><br />
For the time being, I am sticking with the vintage incandescent 
lamp—it is a matter of aesthetics.  I will upgrade to LED technology 
only when its color balance matches incandescent.<br />
<b>Photo gallery</b><br />
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<div class="ngg-gallery-thumbnail-box" id="ngg-image-58">
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<a class="shutterset_set_9" href="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/getting-ready.jpg" title=" ">
        <img alt="Getting Ready with the pole light switch" height="75" src="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/thumbs/thumbs_getting-ready.jpg" title="Getting Ready with the pole light switch" width="100" />
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<div class="ngg-gallery-thumbnail-box" id="ngg-image-57">
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<a class="shutterset_set_9" href="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/finished-assembly.jpg" title=" ">
        <img alt="Finished pole light switch Assembly" height="75" src="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/thumbs/thumbs_finished-assembly.jpg" title="Finished pole light switch Assembly" width="100" />
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<div class="ngg-gallery-thumbnail-box" id="ngg-image-63">
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<a class="shutterset_set_9" href="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/test-setup.jpg" title=" ">
        <img alt="test setup" height="75" src="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/thumbs/thumbs_test-setup.jpg" title="test setup" width="100" />
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<div class="ngg-gallery-thumbnail-box" id="ngg-image-60">
<div class="ngg-gallery-thumbnail">
<a class="shutterset_set_9" href="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/installed.jpg" title=" ">
        <img alt="Installed pole light switch" height="75" src="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/thumbs/thumbs_installed.jpg" title="Installed pole light switch" width="100" />
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<div class="ngg-gallery-thumbnail-box" id="ngg-image-61">
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<a class="shutterset_set_9" href="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/ldr.jpg" title=" ">
        <img alt="LDR sensor" height="75" src="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/thumbs/thumbs_ldr.jpg" title="LDR sensor" width="100" />
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<div class="ngg-gallery-thumbnail-box" id="ngg-image-56">
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<a class="shutterset_set_9" href="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/day.jpg" title=" ">
        <img alt="pole light switch on day" height="75" src="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/thumbs/thumbs_day.jpg" title="pole light switch on day" width="100" />
       </a>
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<div class="ngg-gallery-thumbnail-box" id="ngg-image-62">
<div class="ngg-gallery-thumbnail">
<a class="shutterset_set_9" href="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/night.jpg" title=" ">
        <img alt="night pole light switch" height="75" src="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/thumbs/thumbs_night.jpg" title="night pole light switch" width="100" />
       </a>
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<div class="ngg-gallery-thumbnail-box" id="ngg-image-59">
<div class="ngg-gallery-thumbnail">
<a class="shutterset_set_9" href="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/gfi-convenience-outlet.jpg" title=" ">
        <img alt="GFI Convenience Outlet" height="75" src="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/thumbs/thumbs_gfi-convenience-outlet.jpg" title="GFI Convenience Outlet" width="100" />
       </a>
  </div>
</div>
<div class="ngg-gallery-thumbnail-box" id="ngg-image-64">
<div class="ngg-gallery-thumbnail">
<a class="shutterset_set_9" href="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/timer-not.jpg" title=" ">
        <img alt="timer not" height="75" src="http://electroschematics.com/wp-content/gallery/ldr-pole-light-switch/thumbs/thumbs_timer-not.jpg" title="timer not" width="100" />
       </a>
  </div>
</div>
</div>
<b>For the future</b><br />
TRIAC phase control<br />
<b>Glossary of undocumented words and idioms</b> (for our ESL friends)<br />
tweaking –small adjustment (electronics)<br />
to boot –idiom—literally, to kick around—In addition to—fringe benefit—“extra spending money to boot”<br />
time being –idiom—the present time, presently, for now…<br />
<b>Preferred components for the serious experimenter</b><br />
<div class="sectiune">
<a href="http://electroschematics.com/wp-content/uploads/2012/12/2N6073.jpg" target="_blank"><img alt="" class="alignleft size-thumbnail wp-image-7208" height="100" src="http://electroschematics.com/wp-content/uploads/2012/12/2N6073-100x100.jpg" title="2N6073" width="100" /></a><br />
2N6075B, Logic TRIAC, 4A, 600V, Igt = 3mA, TO-225AA<br />
DigiKey 2N6075BGOS-ND, $0.69 each
<br />
<div class="download-pdf">
<a href="http://electroschematics.com/wp-content/uploads/2012/12/2N6071-Datasheet.pdf" target="_blank">2N6071 datasheet</a></div>
</div>
<div class="sectiune">
<a href="http://electroschematics.com/wp-content/uploads/2012/12/Photocell.jpg" target="_blank"><img alt="" class="alignleft size-thumbnail wp-image-7216" height="100" src="http://electroschematics.com/wp-content/uploads/2012/12/Photocell-100x100.jpg" title="Photocell" width="100" /></a><br />
LDR (CdS Photocell), Advanced Photonics PDV-P8103<br />
DigiKey PDV-P8103-ND, $0.80 each
<br />
<div class="download-pdf">
<a href="http://electroschematics.com/wp-content/uploads/2012/12/PDV-P8103-ldr.pdf" target="_blank">PDV-P8103 datasheet</a></div>
</div>
<div class="info">
<a href="http://electroschematics.com/7207/ldr-pole-light-switch/electroschematics.com/650/lm555-datasheet/" rel="nofollow" target="_blank">555 datasheet</a></div>
<div class="info">
<a href="http://electroschematics.com/6355/ldr-light-dependent-resistor-photoresistor/" target="_blank" title="LDR = Light Dependent Resistor = Photoresistor">what is LDR</a></div>

LDR Pole Light Switch

My vintage (62 year old) pole light has always been controlled by a timer—a source of continual frustration due the requirement of readjus...

555 Door Ajar Flasher

Sunil Jadhavar IC555 Project 6:15 PM

A magnet is placed on the door and a magnetic reed switch on the door
 casing so when the door is closed the circuit is disabled.  When the 
door is open, the LED flashes VERY BRIGHTLY—when closed, the circuit is 
disabled in such a way that it draws virtually no current for maximum 
battery life.<br />
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<ins style="border: none; display: inline-table; height: 280px; margin: 0; padding: 0; position: relative; visibility: visible; width: 336px;"><ins id="aswift_0_anchor" style="border: none; display: block; height: 280px; margin: 0; padding: 0; position: relative; visibility: visible; width: 336px;"></ins></ins><br />
<span style="font-size: large;"><b>Door Ajar LED Light Schematic</b></span><br />
<a href="http://electroschematics.com/wp-content/uploads/2012/12/555-Door-Ajar-Flasher-Circuit.jpg" target="_blank"><img alt="555 Door Ajar Flasher Circuit Schematic" class="alignnone size-large wp-image-7235" height="330" src="http://electroschematics.com/wp-content/uploads/2012/12/555-Door-Ajar-Flasher-Circuit-550x330.jpg" title="" width="550" /></a><br />
<b>Bill of material </b><br />
<div class="download-archive">
<a href="http://electroschematics.com/wp-content/uploads/2012/12/555-Door-Ajar-Flasher-BOM.rar" target="_blank">555 door ajar BOM</a></div>
<b>How it works</b><br />
The flasher is the standard 555 oscillator circuit that drives an 
ultrabright white LED for daylight visibility.  Repetition rate is about
 2hZ and the duty cycle is about 10%.  Peak LED current is 60mA, but 
average is 6mA.  <br />
Since the door is supposed to remain closed most of the time, the 
circuit power is controlled by a 2N7000 N Channel MOSFET.  When the reed
 switch is closed (sensing the magnet), it shorts out the gate drive 
signal to the transistor and turns off the oscillator.  At this time the
 battery drain is only 9uA.  <br />
Note that there seems to be no convenient way to turn off the 555 in 
such a way that it does not draw much current.  Grounding pin 4 stops 
oscillation, but turns on the LED continuously.<br />
C2 provides voltage transient protection for the MOSFET gate.<br />
LED brightness is controlled by R5—YIKES!  do not look into the LED like I did…<br />
<b>The reed switch</b><br />
This circuit uses the commonly available NC (normally closed) reed 
switch.  A NO variety would be easier to use, but such is rare and 
expensive.  If the circuit is packaged inside a small plastic enclosure,
 the reed switch could be cemented into one edge of the box thus 
protecting the glass tube from damage.  The reed switch I used for the 
test is packaged in a plastic tube.  It is quite sensitive — can sense 
the magnet at 2 to 3cm.<br />
<b>Door Ajar Flasher Photos</b><br />
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        <img alt="Door Ajar Light Flasher Components" height="75" src="http://electroschematics.com/wp-content/gallery/555-door-ajar-flasher/thumbs/thumbs_getting-parts-togther.jpg" title="Door Ajar Light Flasher Components" width="100" />
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        <img alt="Door Ajar Light Flasher Testing" height="75" src="http://electroschematics.com/wp-content/gallery/555-door-ajar-flasher/thumbs/thumbs_flasher.jpg" title="Door Ajar Light Flasher Testing" width="100" />
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<b>For the future</b><br />
555 Strobe Circuit<br />
<div class="info">
<a href="http://electroschematics.com/650/lm555-datasheet/" target="_blank">555 datasheet</a></div>

555 Door Ajar Flasher

A magnet is placed on the door and a magnetic reed switch on the door casing so when the door is closed the circuit is disabled. When the ...

Electronics Project Circuts