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Laboratory of Neuropsychology

Photodetector Circuit for Computer Monitors

General Description

We have developed a photodetector circuit to indicate the precise moment an image appears on the display screen of NIMH Cortex or the computer screen of any other behavioral control program. The photodetector circuit works with a sensitive silicon photodetector diode. The diode is mounted on one corner or edge of the display screen so it can detect light from the display. When an image is projected/displayed on the screen, that image includes a dot at the photodetector's location. The dot must be large enough and bright enough for the photodetector diode to sense. The photodetector circuit amplifies the light, sets a threshold, and then provides a digital output to indicate the detection.

The silicon photodetector itself is a pensive photodiode with a 6mm diameter window.

silicon detector

silicon detector

The circuit board for the detector is approximately 34 x 54 mm (25 x 44 mm for surface mount version). It uses two integrated circuits (TLC2262 operational amplifier and 74HC14 CMOS Schmitt trigger) and provides a CMOS logic output. The board operate on 5 volts and requires less then 1/3 mA when idling. The output can drive a long cable, which allows the circuit to reside on the video monitor, near the detector itself.

Detector circuit board populated with 1/8 w resistors and sockets for the integrated circuits

Detector circuit board populated with 1/8 w resistors and sockets for the integrated circuits

The detector is typically mounted into a piece of black plastic and affixed near the edge or one corner of the video screen with black masking tap e. In this example we used plastic "shim stock".

Photodetector press-fit into 3.35 mm plastic shim stock

Photodetector press-fit into 3.35 mm plastic shim stock

The photodetector circuit board has a threshold control and a built-in LED to help with precise placement of the silicon detector. With a jumper on JP1, the LED will turn on when light is detected. When the silicon photodetector properly mounted, properly positioned, and the threshold control properly set, the LED will remain off unless the video image (usually a round spot) is lit up immediately under the silicon photodetector.


Fabricating small printed circuit boards

A number of projects on this web site use small custom printed circuit boards. We don't sell the boards, but we provide a cheap and easy way for you to have them sent to you. The ExpressPCB "MiniBoard" service will send you three circuit boards for about $60. We supply the board design using the free ExpressPCB software, and this software allows you to order the boards on-line from the company. Here is how to use the designs we provide.

  1. Download the free ExpressPCB CAD software from
  2. Install the software, which gives you two applications, ExpressSCH and ExpressPCB. You only need ExpressPCB to get the boards; ExpressSCH is needed only to modify the circuit design.
  3. Download the printed circuit board design file from this web site. The printed circuit board design file will have the .pcb extension.
  4. Start ExpressPCB and use it to open the .pcb file.
  5. Under the Layout menu, select Order Boards Via the Internet.
  6. Step through the process, filling in your name, contact information and shipping address.
  7. Select MiniBoard Service.
  8. Enter your credit card information.
  9. When you complete the process, the boards will be manufactured and shipped to you. Boards take about one week to arrive on the East Coast of the US.

While we have been very happy with the designs and with the ExpressPCB service, we make no warrantee on the suitability, quality, or any other aspect of the process or product. If you follow these directions, use the service, or use our files, you do so at your own risk.