IR Sensor using LM 311

September 22, 2009:

My IR sensor using LM311 and IR LED – Photodiode pair is done. Please see the pics below:

IR sensor circuit
The LED on the right side is the IR LED. The one on the Left is a Photodiode

IR Light

The circuit seems to work like this:

If you take a white reflective surface close to the Transmitter – Receiver pair, the light from the IR LED is reflected on the Photodiode and hence a high signal is received at the output.

If you take a black surface close to the sensor, they seem to show no response. Black surfaces usually absorb IR radiations and hence there is no effect on the output.

However, there seems to be a problem with the output voltage. The high signal seems to be a mere 1.5 V which doesn’t seem to “Switch off” the Red LED at the output instead it seems to glows dimmer. I am trying to find out the reason.

IR LED emits IR Radiations in narrow angles. So, the IR LED/Photodiode need to be placed as close as possible.

Both the IR LED and the photo diode look similar to the naked eye. To check for an IR LED, connect the LED to a battery with a 100 ohm resistor. Use a camera to observe the IR radiations from the LED. If the LED glows in shades of violet and red, Bingo! you have chosen an IR LED!


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