This circuit is useful in liquids level or proximity detection. It operates detecting the distance from the target by reflection of an infrared beam. It can safely detect the level of a liquid in a tank without any contact with the liquid itself.
Power supply must be regulated (hence the use of IC3) for precise reference voltage. The circuit can be fed by a commercial wall plug-in adapter, having a DC output voltage in the range 12-24V.
Current drawing: LED off 40mA; LED and Relay on 70mA @ 12V DC supply.
R10, C6, Q1, D6, D7, RL1 and J1 can be omitted if relay operation is not required.
The infra-red Photo Diode D2, should be of the type incorporating an optical sunlight filter: these components appear in black plastic cases. Some of them resemble TO92 transistors: in this case, please note that the sensitive surface is the curved, not the flat one.
Avoid sun or artificial light hitting directly D1 & D2.
Usually D1-D2 optimum distance lies in the range 1.5-3 cm.
If you are needing a similar circuit driving 3 LEDs in sequence, also suitable as a parking aid
The device’s range can be set from a couple of cm. to about 50 cm. by means of a trimmer
Range can vary, depending on infra-red transmitting and receiving LEDs used and is mostly affected by the color of the reflecting surface. Black surfaces lower greatly the device’s sensitivity
This comparator switches on and off the LED and the optional relay via Q1, comparing its input voltage to the reference voltage at its non-inverting input set by the Trimmer R7.
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