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I want to use a parking distance sensor, specifically the common ones found on vehicle bumpers.

parking sensor

There are only two wires. How can I drive the sensor and read distance from it?
I specifically need to know what the input and output pulses should look like (voltage/duration).

Conceptually, I need to send a pulse of some magnitude and duration, and then measure the time before a signal is received back. That seems simple enough to be done with a 5V microcontroller and few other components, but I don't know for sure because I can't find much data on these sensors.

A common bidding site has the following 'data' for these sensors:

  • Allowable Maximum Input Voltage ( Vp-p): 140 (40KHz)
  • Pulse width: 0.5ms
  • interval: 20ms
  • Decay Time: ≤1.2ms

140V input pulse? Is that right or just a mistranslation? If it's right, I think it would be more cost effective to buy a driver for it.

This link suggests that the sensors in vehicles perform "both emission and reception", though that's not any sort of confirmation.

Note that I do not have of these sensors, and I do not have any supporting hardware for them (like a stock PCB).

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  • \$\begingroup\$ These typically us a ~12v rail, a transistor, and a transformer (or inductor, don't remember) to boost voltage - the driver circuit is not hard to trace and duplicate, and you can extract the transformer before bothering to source one. The main thing you are overlooking is that the receiver is designed to be distinct (and while visually similar the two transducers are not interchangeable without performance loss). If you want to avoid unintended coupling of transmitter ringing over to the detector creating a high minimum range, you will need to carefully isolate the two circuits. \$\endgroup\$ – Chris Stratton Mar 20 '17 at 17:53
  • \$\begingroup\$ You could also consider keeping the stock PCB, and tapping out transmit and receive signals to time with your own MCU. \$\endgroup\$ – Chris Stratton Mar 20 '17 at 17:53
  • \$\begingroup\$ I should have mentioned that I don't have a sensor of my own, and I don't have the stock PCB or any such hardware. \$\endgroup\$ – Bort Mar 20 '17 at 17:56
  • \$\begingroup\$ Also, it appears that the transmitter and receiver are the same device, as I cannot find one for purchase that is labeled as one or the other. \$\endgroup\$ – Bort Mar 20 '17 at 17:59
  • \$\begingroup\$ Probably the first thing you should do is buy a full set to experiment with. They're not exactly expensive. My experience with such is that the transmitter and receiver are not interchangeable - it works quite poorly when they are swapped. Typically they are just labelled ABCD or 1234 and the user is expected to match the connections and install according to the instructions; it's up to you to identify the distinct roles within that scheme. \$\endgroup\$ – Chris Stratton Mar 20 '17 at 17:59
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You've proposed a very intriguing question. Since you've offered up no datasheet, I've taken the liberty of using a freely available datasheet from a comparable bare sensor that can be used in 2 pin mode; That is, the same pin used for power sends back the distance of an object.

This model is the SRF005, and from the photo below you can see that the sensor itself, aside from the helping hardware, only has two pins:

Reading the documentation of the SRF005, Picaxe provides instructions on how it does this: It simply sends a 40kHZ burst out of the sensor, turns the pin into an input pin and measure the time it takes for the burst to be returned. With some basic calibration that can be stored in the micro, the time can be converted to a distance and sent out via SPI or RS232 or similar. See update below

That should answer your question on what the output pulse looks like: a 40kHz pwm signal (10uS wide), and the input should be very similar, allowing for noise distortion.

If you're going to build the circuit yourself, you will need to filter the noise on the input as well as scale the input voltage down to a level acceptable to your micro.

While the sensor I referenced doesn't have a 140Vpp maximum that you reference in your sensor, I suspect that has more to do with detectable distance, as it is an allowable peak, and not a nominal voltage level, I would experiment with different levels, starting with 12V and 24V and see if it has an effect on distance. Murata, the company referenced in your link, also makes a sensor with a 20Vpp square wave max version, if you don't have 140Vpp square waves lying around.

Update

ChrisStratton pointed out that my comment above about the SRF005 using 2 pin mode was erroneous, they are actually 'faking' a 2pin mode using the driver and micro.

However, as for why this works, essentially this transducer is made up of an excitation wire and a ground wire. On the send, you apply a voltage to the excitation wire to send out the physical signal, and on the receive, the echo of the sent ultrasonic signal vibrates the transducer and it generates a voltage on that same excitation wire. This can be done with simple additional circuitry, however TI makes the TDC1000 that does all the heavy lifting for you:

TDC1000 with single transducer

It is important to note though, that you don't know for certain if those sensors you're interested in are actually 'combined use', or a sender and receiver in a single package. As they are shipped in a pack of 4, it's entirely possible that 2 are senders and 2 are receivers, or that they are indeed 4 combined use. Murata makes both types.

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  • \$\begingroup\$ These are not, in fact comparable other than in general concept. The sealed transducers are much less effective than the unsealed ones, so need drivers than can deliver a more powerful pulse. The circuit to examine is instead that of the electronics box that ships in the $10-20 complete systems the pictured transducer comes from. \$\endgroup\$ – Chris Stratton Mar 20 '17 at 20:27
  • \$\begingroup\$ @ChrisStratton I didn't say their specs were comparable, I am implying that they are comparable in the sense that they can perform the same with only 2 wires. \$\endgroup\$ – MDMoore313 Mar 20 '17 at 20:34
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    \$\begingroup\$ In that case, your posting is erroneous. The pin re-use in your link is for the data interface to the driver electronics, which has nothing to do with the question since there aren't any driver electronics in the question. While each transducer has only two pins, they are not used for both transmit and receive, as distinct transceivers are used in each role. Even if the transmitter and receiver were interchangeable (which those asked about are not), they would still in this type of design be distinct instances. So it's not 2 wires, but rather 4. \$\endgroup\$ – Chris Stratton Mar 20 '17 at 21:05
  • \$\begingroup\$ @ChrisStratton I overlooked that, nice catch; I've updated my answer. \$\endgroup\$ – MDMoore313 Mar 21 '17 at 15:01
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The sensor you link to is from an Analog system, notice the article is from 2010.

Most common sonar vehicle sensors today are digital and include electronics in the sensor housing. See this guys efforts with a more modern sensor.

If you want to use the older analog sensors I'd suggest you simply take the sensors off a readily available HC-SR04 and attempt to drive two of your sensors (as a test to see what can be achieved at a lower Tx signal level) ...one as Tx and one as Rx. You'd have to boost the 18 V p-p from the HC-SR04 to 140 V p-p for the TX, but at least you have the sensor logic and distance sensing to first echo built in.

If you want to use just a single sensor then the electronics becomes more involved. You now have to be able to send (using a high voltage drive signal) while clamping your receive amplifier input, then enable the high gain RX channel.

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  • \$\begingroup\$ HC-SR04 electronics will not drive these... was the first thing tried, since it obviously would have been easy if that were the case. Also, it's been documented online that the usual implementation of those is horrible - filter frequency completely off, etc. \$\endgroup\$ – Chris Stratton Mar 20 '17 at 20:26
  • \$\begingroup\$ @ChrisStratton. I didn't suggest they would drive them directly, but I've used multiple of them and they are on frequency sufficient (IMO) to provide a starting point. The difficulty with the automotive sensors is that they have a plastic cover that attenuates both Tx and Rx levels. \$\endgroup\$ – Jack Creasey Mar 20 '17 at 23:05
  • \$\begingroup\$ They work as intended, yes, but it turns out that their receive filter is wildly mis-tuned from their frequency of operation. Of course it isn't particularly high Q, either, which is why it works despite that flaw. Most people would probably interpret "I'd suggest you simply take the sensors off a readily available HC-SR04 and attempt to drive two of your sensors" as a recommendation to drive them directly, so if you meant something else you should probably explain. \$\endgroup\$ – Chris Stratton Mar 20 '17 at 23:12
  • \$\begingroup\$ @ChrisStratton. I did suggest that the OP would need to amplify the Tx signal didn't I? I didn't suggest that it would necessarily fulfill his needs, but it would be worth testing to understand his sensors better. I'd suggest the Rx channel would work without modification, though the distance over which it will work will depend significantly on the Tx power. \$\endgroup\$ – Jack Creasey Mar 20 '17 at 23:36

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