parking aid senses distance to rear of vehicle

Posted by Author makecircuits

Parking aid senses distance to rear of vehicle

Three LEDs signal bumper-barrier distance

Infra-red operation, indoor use

Circuit diagram:


R1_____________10K 1/4W Resistor

R2,R5,R6,R9_____1K 1/4W Resistors

R3_____________33R 1/4W Resistor

R4,R11__________1M 1/4W Resistors

R7______________4K7 1/4W Resistor

R8______________1K5 1/4W Resistor

R10,R12-R14_____1K 1/4W Resistors

C1,C4___________1΅F 63V Electrolytic or Polyester Capacitors

C2_____________47pF 63V Ceramic Capacitor

C3,C5_________100΅F 25V Electrolytic Capacitors

D1_____________Infra-red LED

D2_____________Infra-red Photo Diode (see Notes)

D3,D4________1N4148 75V 150mA Diodes

D5-7___________LEDs (Any color and size)

IC1_____________555 Timer IC

IC2___________LM324 Low Power Quad Op-amp

IC3____________7812 12V 1A Positive voltage regulator IC

Device purpose:

This circuit was designed as an aid in parking the car near the garage wall when backing up. LED D7 illuminates when bumper-wall distance is about 20 cm., D7+D6 illuminate at about 10 cm. and D7+D6+D5 at about 6 cm. In this manner you are alerted when approaching too close to the wall.

All distances mentioned before can vary, depending on infra-red transmitting and receiving LEDs used and are mostly affected by the color of the reflecting surface. Black surfaces lower greatly the device`s sensitivity.

Obviously, you can use this circuit in other applications like liquids level detection, proximity devices etc.

Circuit operation:

IC1 forms an oscillator driving the infra-red LED by means of 0.8mSec. pulses at 120Hz frequency and about 300mA peak current. D1 & D2 are placed facing the car on the same line, a couple of centimeters apart, on a short breadboard strip fastened to the wall. D2 picks-up the infra-red beam generated by D1 and reflected by the surface placed in front of it. The signal is amplified by IC2A and peak detected by D4 & C4. Diode D3, with R5 & R6, compensate for the forward diode drop of D4. A DC voltage proportional to the distance of the reflecting object and D1 & D2 feeds the inverting inputs of three voltage comparators. These comparators switch on and off the LEDs, referring to voltages at their non-inverting inputs set by the voltage divider resistor chain R7-R10.


  • Power supply must be regulated (hence the use of IC3) for precise reference voltages. The circuit can be fed by a commercial wall plug-in power supply, having a DC output voltage in the range 12-24V.

  • Current drawing: LEDs off 40mA; all LEDs on 60mA @ 12V DC supply.

  • 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.

  • If your car has black bumpers, you can line-up the infra-red diodes with the (mostly white) license or number plate.

  • It`s wiser to place all the circuitry near the infra-red LEDs in a small box. The 3 signaling LEDs can be placed far from the main box at an height making them well visible by the car driver.

  • The best setup is obtained bringing D2 nearer to D1 (without a reflecting object) until D5 illuminates; then moving it a bit until D5 is clearly off. Usually D1-D2 optimum distance lies in the range 1.5-3 cm.

  • If you are needing a simpler circuit of this kind driving a LED or a relay, click Infra-red Level Detector