A few weeks ago, our son came to visit. On this occasion, he mentioned that the parking assistance at the back of his car probably no longer worked. Research on the Internet revealed that the on-board computer of his car initially checks the sensors every time it is put into reverse. As soon as a sensor is defective, there is a long warning tone and the entire parking assistance is deactivated.

Bat Detector

I thought for a while about how to check the sensors. Then I remembered the prototype of a bat detector that I had built for fun some time ago. Figure 1 shows its block diagram. The detector is logically constructed: a microphone that is also sensitive to ultrasonic signals is followed by an amplifier with an automatic level control. After the signal bandwidth has been limited by the series connection of a high- and low-pass filter, a further amplification with a second level control is carried out.

Bat detector block diagram
Figure 1: The block diagram of the bat detector. The last module on the far right is not needed here.

A suitable microphone of the type SPU0410HR5H is available from the usual distributors or at a reasonable price on eBay. The second block from the left in Figure 1 (preamp/AGC/limiter) is a MAX9814 on a small circuit board, which is also available ready-made for a few euros on eBay. The original microphone on this module has been removed. The high-pass and low-pass filters are built on a prototype board. The signal was routed from directly after the low-pass filter cable to the input of my oscilloscope using a coaxial cable. The output amplifier of the bat detector with an AGC and a limiter based on an SSM2167 was not used here.

Parking sensor prototype
Figure 2: The prototype of my repurposed bat detector built into a plastic casing.

Parking Sensor Tester Prototype

Figure 2 shows my prototype in its plastic casing. Figure 3 shows how I connected the new microphone to the module board of the MAX9814. The real measurement in the car is shown in Figure 4. Figure 5 shows the oscillogram of the measured signal of a functioning sensor.

The SPU0410HR5H microphone connected to the MAX9814 module from Adafruit.
Figure 3: The SPU0410HR5H microphone connected to
the MAX9814 module from Adafruit.
Testing the parking sensors
Figure 4: Testing the parking sensors on my son’s car.
This one worked fine, as you can see on the oscilloscope.
Oscilloscope screenshot. This is what the signal from a functioning parking sensor looks like.
Figure 5: Screenshot of my oscilloscope. This is what the signal from
a functioning parking sensor looks like.

I redrew the circuit diagram (Figure 6) without the second amplifier of Figure 1 (SSM2167). IC2B serves here as a dual-amplifying buffer for the output.

Schematic of the adapted version of bat detector
Figure 6: Schematic of the adapted version of my bat detector without the superfluous output stage of Figure 1.

Since the circuit is simple, and I had quickly built it on a prototyping board, there was no time to design a circuit board. However, there is nothing to stop you from doing so and, if necessary, turning it into a compact device — possibly with a small oscilloscope built into the case. Ideally, the power should then be supplied via the car battery.
 

Parking sensor components

You can test the circuit with a function generator and a 40-kHz ultrasonic transmitter or a suitable tweeter. An alternative signal source is, for example, the inexpensive Grove ultrasonic distance sensor from Seeed Studio, which is available from Elektor. In the case of our son’s car, it was very easy to find the fault using this test circuit: One sensor was no longer emitting an ultrasonic signal.

Questions or Comments?

Do you have questions or comments about the parking sensor tester or this article? Email the author at alfred_rosenkraenzer@gmx.de or contact Elektor at editor@elektor.com.

Editor's Note: This article (240323-01), translated by Jörg Starkmuth, appears in the Elektor Circuit Special 2025.

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