The GP2Y0D02 is an infrared proximity sensor with a detection field that extends 80cm. This type of sensor can be used to build collision avoidance systems for robots. We’ll demonstrate this sensor using a single resistor and a multimeter.
Sharp GP2Y0D02 fixed 80cm IR proximity detector (Digikey #425-2064-ND, $14.38). Datasheet (PDF).
The GP2Y0D02 requires a 5volt power supply (not shown). A 0.1uF bypass capacitor between power and ground (C1) is a good idea, but we didn’t use it in our demonstration. The open collector output (pin 1) pulls to ground when no object is detected, a 12K pull-up resistor (R1) holds the signal high when an object is detected.
In the demonstration we connected the output of the sensor to a multimeter. When nothing is in front of the sensor, the detector holds the output low (0.40volts). When we put a PCB in front of the sensor, the output changes to high-impedance and the pull-up resistor (R1) holds the signal high (5volts).
Why open collector?
An open collector output doesn’t toggle between high and ground, it toggles between ground and unconnected. The unconnected state, also called high impedance, exerts nothing on the output and allows the signal line to float. This is an undefined state for most microcontrollers that returns rapidly varying values, so we use a resistor (R1) to hold the signal high. The open collector output overcomes the small amount of current flowing through resistor to register the low state. Without this resistor, the output will never reach a proper high state.
Open collector outputs are useful when several sensors need to share the same microcontroller pin. Multiple sensors outputting high to the same microcontroller pin is generally considered a bad practice that can damage parts of a circuit. Multiple open collector outputs, however, can only switch to ground; a single resistor holds the signal high. In the case of multiple GP2Y0D02s, the signal will be high only when all connected sensors detect an object and switch to high impedance state.
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