A simple ultrasonic radar (sonar) without microprocontroller to parking your car in reverse gear or for general purposes




Inspired by a Alberto Ricci Bitti's circuit (  I realized and then modified some parts of his ultrasonic radar. It is useful for different purposes and original idea is simple as ingenious.

Fortunately today a lot of cars are provided of some complex radar-sensor menaged by a microcontroller and its software and  modern devices are more and more reliable. This circuit has not a microprocessor but it is completly digital-sequential and analog ciruitry based.

However, the circuit treated in these pages does not guarantee secure and calmly  "eyes closed" car-reverse maneuver (where the caution is never too much), but it is suitable only to intercept  well-defined barriers and obstacles.







When I made Ricci Bitti's parking sonar (, it did not work.  I never understood what error I committed. So, I decided to investigate why and then to re-design entire RX section and to replace all transistors with op-amp. Principle is universal radar based. 


1) transmitter send pulses cyclically on  the air;
2) an obstacle or barrier reflects transmitted signal;
3) receiver detects the signal reflected from obstacle (echo);
4) core-circuit obtains the time-delay between  transmitted bursts and received echos.


This system uses sound waves: ultrasonic waves at 40 KHz.

Sound's speed in air depends by temperature and you can calculate it approximately:



T = temperature of air in  Celsious degrees.

To get an idea, the speed of sound at 20 Celsious is about 344 meters/second or 1128 feets per second.


The ultrasonic sound waves are not audible by the human ear (the human ear stops receive at 20 KHz). The pair of ultrasonic-piezo capsules (Transmitter or TX and Receiver or RX) used in system operates at ultrasonic frequency of 40kHz.


TX section (see electric diagram) is made by  three  CMOS logic gates of  NOT smith-trigger 40106;  gate 3/6 of 40106, capacitor C11, trimmer TR3 and resistor R11 create a 40 KHz oscillator together. Two gates (2/6 and 1/6) act as a buffer. Ultrasonic TX capsule is placed between input and output of 1/6 gate and it'll have two 40KHz rectangular waves alternating  and 180 phased. 

The 40kHz oscillator generates burst when it is excited from pin Q0 of the CMOS counter 4017 (Ic04).


Clock section is realized by gate 4/6 of IC0,  R10, C10 and Tr2. Tr2 trims clock from 400Hz to 3KHz frequency range. This setting affects the distance's range of each LED. In pic 1 you can see “timing” of TX section. Instead, in photo 1 you can see 40 KHz bursts ready to be irradiated on air by TX piezo-capsule. In photo 2 there is single burst  zoomed-in by an analog scope. You can see that  Ic03 3/6, R11, Tr03, C11 array provides a damping auto-oscillations.

Then the bursts are irradiated by the TX piezo-capsule and they wander through to front space as ultrasonic wave. If this wave encounters an obstacle, it is reflected as any sound would be reflected, creating the echo. Likewise, the return ultrasonic echo is received by the RX piezo-capsule and transformed into an electrical signal (photo 3).  This photo is obtained with an oscilloscope probe inserted between RX capsule and circuit's ground. The two highest amplitude signal peaks are the main echo, while in central zone, between the two peaks, there are secondary reflections (the secondary reflections are not good and should be deleted). Then the RX echo is sent to the first amplifier stage and amplified in voltage about 70 times (x 70) and then sent to the second stage (x 10). Amplified echo at D2 anode (amplified 700 times in voltage or about 55dB) is sent to D2 that cuts negative half-waves (photo 4). Network R6-C8 is an envelope-detector  (photo 5).

The third stage (op-amp 3/4 of Ic02) is a voltage comparator and receives the envelope. You can see like the echo-signal is transformed at output of comparator in photo 6. The fourth opamp is an emitter-follower (gain=1). In photo 7 you can see an oscilloscope comparison of the eco taken at RX capsule and pin 14 of Ic02.

Signal outgoing from Ic02 photo 6 is used to drive monostable multivibrator. Multivibrator  is made by NOT gate 6/6, R8 -C9 net and D3 array.  Multivibrator function is much longer time duration of the  photo 6 's signal like in  picture 2. This timing extension  slows down the fast scanning of the Ic04 counter, disabling counting for its entire duration.

Results are:



More distant is the obstacle, later comes the echo, and then later the count is disabled, so you see one of the last LED (among green LEDs) has more light compared to others.

If the obstacle is very near, the echo comes immediately and immediatly counting is disabled, and one of the first LED (among  red LEDs) remains significantly lit than others. For example, if the obstacle is very near, the burst has been received by the RX section quikly and counting is stopped soon so, while Q1 is active, the counter is interdicted and you see the first red LED "Q1" on.

My friend Farid Morteza Agaie has improved the circuit by introducing nine electrolytic capacitors (100 uF) on the LEDs to reduce even more the flashing/blinking.

In photo 8 , I analyzed by scope  general operation of the circuit: a trace showed  bursts generated by TX section and matched with the echo signals taken at pin 14 of Ic02. You can see the distance-time between bursts and  echoes;  distance is the time taken from ultrasonic wave to travel from TX capsule until to return to RX capsule after its reflection, of course.



To calibrate this circuit 10 centimeters (4 inches) is a good distance for each LED, so the last LED (the one on Q9 output) indicates a distance of about 90 cm (about 35 inches). Over 90cm/35", the sensitivity of the circuit must be calibrated to not "see" any obstacle.






Realizing this circuit is not very difficult although it is not the simplest.

Farid has developed an excellent prototype designing a printed circuit board (photo 9 ). Bottom layout of PCB is here: Picture2/B ,.

 If mounted on a car, the power of this device can be taken from the reverse-gear lamp.

Ideal for calibration is an oscilloscope, but you can try  empirical approach with a voltage multimeter.


The first step is to place entire circuit on a chair or on a mobile-table at 60cm (23/24 inches) distance  from a wall and power-on the device by 12.5Vdc power sypply. With a multimeter applied between pin 10 of Ic02 and ground, adjust Tr03 for maximum voltage. This should ensure correct calibration of 40KHz burts generator.

Rotate Tr01 antclockwise and adjust Tr02 until  the sixth LED (output Q6) is turn-on. Now, increase the distance from wall (about 90cm/35") to turn-on the last LED. Place the device over 90cm/35" and trim Tr01 to desensitize receiver  that no LED is lit. Desensitization, over a distance-threshold is important otherwise if the circuit is too sensitive the first LED light-on  for distances above the threshold.

You can choose just parameters to your liking: by acting on Tr02 you can determine how much distance from obstacle must be associated to each LED. Clearly Tr01 must be readjusted and to seek a fair compromise agreement with the maximum sensitivity of the receiving section. If you want to increase the sensitivity you can try to increase R6 or R4 values.

Buzzer is DC type (direct-current) at 6 Volt.

If schottky diodes are unavailable, you can try to use generic 1N4148.


Thanks to all! :D

Ciro  &  A.Farid








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