8 Photo-Detector PCB - Non Inverting

- The LEDs are OFF when the phototransistors are covered -

( The comparator's outputs are HIGH when its inputs are HIGH )

  The circuit on this page is for a visible and infrared light detector circuitboard that has 8 detectors. LM339 voltage comparators are the active element.

  These detectors can be used as part of other light detector circuits shown on other pages at this site such as these Light Activated Detector Circuits at this site.

Basic Non Inverting Detector Circuit

  The following diagram shows the basic circuit on the Non Inverting circuitboard.

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. If the light is dim, selecting a higher value resistor such as 1Megohm will give better sensitivity. 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 Output Resistors (R4)

  The value of resistor R4 is chosen to give a desired current flow though the LEDs See below for more details.

  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 please see the Current Limiting Resistor Calculator page at this site.

Non Inverting 8 - Photo-Detector PCB Schematic

  The following diagram shows the circuit that is on the printed circuit board. There are 8 independent photo-detectors with open collector outputs that can sink up to 15 milliamps each.

Circuit Notes

  The diagram above 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.

Typical Detector Installation Circuit

  The following schematic is for a typical installation circuit using phototransistors and LEDs. Multiconductor cable can be used to keep the wiring runs neat as sensors and indicators are likely to used in localized groups.

Typical Phototransistor Installation

Full Circuit Schematic

Typical 8 Photo-Detector Circuit

8 Detector Circuit Parts List

  The following is a parts list for a typical parts list. The values of the resistors can be changed to suit the needs of the user.

2 - LM339 Quad Comparator 14-DIP - IC 1, IC 2 - 296-1393-5
8 - 470K Ohm 1/4W Resistor - R1, 2, 3, 4, 11, 12, 13, 14 - 470KQBK
4 - 10K OHM 1/4W Resistor - R9, 10, 19, 20 - 10KQBK
8 - 1K Ohm 1/4W Resistor - R5, 6, 7, 8, 15, 16, 17, 18 - 1.0KQBK
1 - 2.2uF 50V Miniature Aluminum Electrolytic - C2 - P10313-ND
2 - 2 Position Terminal Block - 5mm - - - ED1601-ND
5 - 3 Position Terminal Block - 5mm - - - ED1602-ND

  The DigiKey part number for suitable phototransistors is 160-1988-ND.

  The output Light Emitting diodes would be selected depending on the needs of the user.

Other Circuit Examples

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.

  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.

Muliptle Input Transistors

  Multiple phototrasistors can be connected to each input.

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


LM339 Data sheet - National Semiconductor (.pdf)

LM393 Data sheet - National Semiconductor (.pdf)

Pinout Diagram For Various Devices.

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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.

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02 April, 2018