4 Photo-Detector With Delayed Release And Bipolar Output

- The Outputs Of The 556 Timers Are HIGH When The Phototransistors Are Dark -

- The LEDs Can Be ON or OFF When The Phototransistors Are Dark -

  The circuit on this page is for a visible and infrared light detector circuitboard that has 4 independant detectors. The inputs are sensed by LM339 voltage comparators and the outputs are from LM556 timers.

  The 4 outputs have selectable delayed release times and can source or sink up to 200 milliamps. The output's release delay is set by changing the values for the resistors and capacitors at the TRIGGER and THRESHOLD terminals of the LM556 timers.

  This circuit is ideal for driving signals without flicker or relays without chatter.

A printed circuitboard and parts are available for this circuit.


  If you are interested in printed circuit boards please send an email to the following address: rpaisley4@cogeco.ca Subject: Bipolar Output Photodetector

Basic Time Delay Output Detector Circuit

  The following diagram shows the basic circuit on the detector circuitboard using a LM555 timer instead of an LM556 timer.

Selecting A Value For The Input Resistor (R1)

  The value of resistor R1 depends on the type of sensor and the desired sensitivity. See below for more details.

  For phototransistors a value of 470K ohms will work for most room light situations. Selecting a higher value resistor such as 1 Megohm will give better sensitivity in low light. This High Impedance Test Voltmeter circuit can also be used for testing phototransistors installations.

  For CdS photocells it is usually best to install the cell and then measure its resistance under the normal lighting conditions. A resistor with a value that is 3 to 5 times the measured resistance of the cell is then selected for R1.

Selecting A Value For The Time Delay Resistor (RT) and Capacitor (CT)

  To calculate the delay time in seconds for specific values: Multiply RT X CT X 1.1. There is a calculator in section 2 of the - 555 Timer information page at this site.

  As the values of the resistors and capacitors increase, the time calculation becomes less accurate.

  The values for the timing resistors are selected from the standard values for 1/4 watt resistors. Typical values for the delay capacitors are: 1.0uF, 2.2uF, 3.3uF, 4.7uF, 10uF, 16uF, 22uF, 33uF and 47uF.

Selecting A Value For LED Current Limiting Resistors

  This circuitboard does not have built in current limiting resistors at the output of the timers. An external resistor will be needed if LEDs are connected to the circuits.

  A 1K ohm resistor will allow about 10 milliamps to flow through a typical LED if the supply voltage is 12 volts. The value of the resistors at the outputs of the comparators can changed depending on the desired current through the LEDs.

  For information on calculating the value of current limiting resistors see the Current Limiting Resistor Calculator page at this site.

Typical Phototransistor Installation

Driving LEDs With The Photo-Detector Circuit

  The next schematic shows some signals options for the photo-detector circuit's output.

  To drive common anode or common cathode connected LEDs this adapter circuit can be used.

4 - Photo-Detector PCB Circuit

  The following diagram shows the circuit that is on the printed circuit board. The board contains 4 photo-detector circuits.

Circuit Notes

  The next diagram can be printed in the center of a sheet of paper then and used to record wiring connections when installing the circuit, sensors and LEDs. This diagram will be easier to use than the actual schematic for the circuitboard. Below that is an example of a connection diagram.

Full Circuit Schematic

  The next schematic shows the circuitboard schematic with input phototransistors and several output options.

Printed Circuit Boards

For The

4 Photo-Detector Circuit

  The following is an assembled example of the circuit board which is 1.9 inches by 3.9 inches and is drilled to fit DigiKey part ED1624 - 3 position position terminal blocks.

  The INPUTs of the detectors are on the left and the OUTPUTs are on the right side of the picture. As in the full circuit schematic, the INPUTs and OUTPUTs of the detectors are directly across from each other on the circuit board.

Circuitboards and Parts Prices

Option - 1

1 - Bare 4 Photo-Detector circuitboard is 12.00 dollars US, plus postage. (Each additional, bare circuitboard is 11.50 dollars.)


Option - 2

1 - Assembled - 4 Photo-Detector circuitboards, with 4 phototransistors is 26.00 dollars US each, plus postage.


Option - 3

1 - Assembled 4 Photo-Detector circuit boards, without phototransistors is 22.50 dollars US each, plus postage.


Option - 4

1 - Kit - 4 Photo-Detector circuitboard including all parts and with 4 phototransistors is 25.00 dollars US each, plus postage.


Standard Delay Times Available

  When ordering, please specify the delay times needed. (Different times can be used on each of the timers if needed.)

  Calculated time delays for various timing capacitor (C2, C3, C4, C5) values are:

1.0uF - 1.1 seconds   /   2.2uF - 2.4 seconds   /   3.3uF - 3.6 seconds

4.7uF - 5.2 seconds   /   10uF - 11.0 seconds   /   16uF - 17.6 seconds

22uF - 24.2 seconds

  The phototransistors have Red and Black, 5 inch long leads attached.

  All phototransistors will have been tested before shipment.

  The circuitboard is also available without terminal blocks.


  If you are interested in printed circuit boards please send an email to the following address: rpaisley4@cogeco.ca Subject: Bipolar Output Photodetector

  Your message will be answered as soon as possible.

PCB Parts List

  The following is a parts list for a typical parts list for use with the 4 - PhotoDetector circuit board. The values of resistors and capacitors can be changed to suit the needs of the user.

1 - LM339 Quad Comparator 14-DIP - IC 1 - 296-1393-5
2 - LM556 Dual timer - IC2, 3 - LM556CNFS-ND
4 - 470K Ohm 1/4W Resistor - R1, 2, 3, 4 - 470KQBK
2 - 10K OHM 1/4W Resistor - R5, 6 - 10KQBK
4 - 1M OHM 1/4W Resistor - R7, 8, 9, 10 - 1.0MQBK
5 - 2.2uF 50V Miniature Aluminum Electrolytic - C1, 2, 3, 4, 5 - P5175-ND
4 - 3 Position Terminal Block - 5mm - - - ED1624-ND

  DigiKey part numbers for suitable phototransistors are 160-1988-ND.

  T42-1 Push-In Terminals can be substituted for the terminal blocks to reduce costs if desired. The DigiKey part number for the T42-1 Terminals is V1069-ND.

4 Photo-Detector Printed Circuit Board Parts Layout

Other Circuits

Diamond Protection Circuit

  Two of the circuits on the circuitboard can be cross-connected so that when one timer is active the other timer is disabled. This is done by short circuiting its phototransistors of the inactive timer with a NPN transistor connected to the output of the active timer.

  A relay is used to disconnect the power to the opposing track when a train is in the protected section.

Muliptle Input Transistors

  Multiple phototrasistors can be connected to each input.

First Across The Line Race Timer Circuit

Using Infrared LEDs As Artificial Light

  Infrared - light emitting diodes can be used to light the phototransistors in areas such as tunnels and hidden yards. The diagram shows 8 LEDs but only 4 would be needed for this circuit.

  To reduce the current require, the LEDs can be wired in series so that four LEDs use the same amount of current as one LED. In the example shown, approximately 8 milliamps.

Is The IR LED Working?

  Infrared light is not visible to the naked eye, however, a digital camera can be used to view the IR light if it does not have an IR blocking filter on the lens.

  The image may not be very bright but close-up or in a darkened area the light should be visible on the camera's display screen.

Circuit Board Assembly

  When installing the components on the circuit board start with parts with the lowest height and work up to the tallest parts. For example starting with the diodes then resistors, IC's, transistors, capacitors and terminal blocks.

Soldering Leads To Phototransistors And LEDs

  The following shows a method of attaching wires to the leads of phototransistors, LEDs and photocells. The wire is 26 gage.

Attaching Wires To Phototransistors, etc.

  The phototransistor is held by a piece of wood with a 3 or 5 millimeter hole drilled in it, depending on the case size of the device.

  The leads of the phototransistor were trimmed to 3/8th's of an inch and 9/16th's of an inch of insulation was stripped from the wire.

  Needle nose pliers are used to hold the wire while it was wrapped around the lead. Any excess wire can be trimmed away after soldering and the end tucked into the phototransistors lead. A small heat sink could also be used during soldering. (The out-of-focus line entering from the left in the fourth image is the solder.)

  The heat-shrink tubing was reduced by touching it to the side of the soldering iron's tip.

Recommended Tools


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Please Read Before Using These Circuit Ideas

  The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.

  If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

  Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.


  If you are interested in printed circuit boards please send an email to the following address: rpaisley4@cogeco.ca Subject: Bipolar Output Photodetector

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19 May, 2017