This simple circuit can be used as an aid for sensing the distance between the rear bumper of the car and any obstacle behind the car. The distance can be understood from the combination of the LEDs (D5 to D7) glowing. At 25cm D7 will glow, at 20 cm D7&D6 will glow and at 5cm D7, D6 and D5 will glow. When the obstacle is beyond 25 cm none of the above LEDs will glow.
Two ICs are used in the circuit. The IC1 (NE555) is wired as an astable multivibrator for driving the IR Diode D1 to emit IR pulses. The operating frequency of the transmitter is set to be 120Hz.The IR pulses transmitted by D1 will be reflected by the obstacle and received by the D2 (IR photo diode).The received signal will be amplified by IC2a.The peak of the amplified signal will be detected by the diode D4 and capacitor C4.R5 and R6 compensates the forward voltage drop of D4.The output voltage of the peak detector will be proportional to the distance between car’s bumper and obstacle. The output of peak detector is given to the inputs of the other three comparators IC2b,IC2c and IC2d inside the IC2 (LM324).The comparators switch the status LEDs according to the input voltage their inverting inputs and reference voltages at their non inverting inputs. Resistances R7 to R10 are used to set the reference voltages for the comparators.
Circuit diagram with Parts list.
- Assemble the circuit on a good quality PCB or common board.
- The D1 & D2 must be mounted close (~2cm) to each other, looking in same direction.
- The D1 can be a general purpose IR LED.
- The D2 can be general purpose IR photo diode with sun filter.
- The transmitter as well as receiver can be powered from the car battery.
- For proper working of the circuit, some trial and error is needed with the position of D1 and D2 on the dash board.
- All capacitors must be rated 25V.
- The ICs must be mounted on holders.