I was disappointed to find that the HC-SR04 schematic does not have part numbers for the receiver amplifier IC or the RS-232 driver used to increase the drive voltage to the transmitter. It seems to me that if you’re going to provide the schematic and board layout file in Eagle format, you should have a parts list also.
I’ve moved beyond simple distance measurement to doing some experiments with doppler signal processing to use ultrasonic transducers for motion sensing. I’m range-limited when driving the transmitter with 3.3V and I’m only using a very simple amplifier on the receiver. Rather than try to implement the HC-SR04 combination of buffer, bandpass filter, and comparator I’m considering buying one of the HC-SR04 boards and desoldering the MCU. I could then send signals for the driver and receive the comparator output for processing on the Teensy 4.0 I’m using to get the doppler data.
In the meantime, I guess I can plug the amplifier circuit into LTSpice to check out the characteristics of the bandpass filter to make sure it’s not too narrow to block doppler-shifted return data.
I did a quick LTSpice analysis of the bandpass filter section of the HC-SR04. It seems to show a very broad peak with the maximum at about 32KHz and about two or three dB down at 40KHz and 24KHz. I’m assuming that the transducers are 40KHz unit, and this filter doesn’t seem well optimized for that frequency. My question now are:
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Are the transducers really operating at 40KHz?
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Are the components on the board the same as those shown in the schematic?
I’ll be able to answer the questions myself when I get my boards. However, my curiosity wasn’t sufficient to warrant fast shipping, so I won’t have answers for a week or so—unless someone in the forum has taken a close look at this module.
What schematic? The HC-SR04 was originally designed and made in China; nothing to do with Sparkfun.
Various people have reverse engineered the HC-SR04. Look here https://uglyduck.vajn.icu/ep/archive/20 … cator.html and here: http://www.pcserviceselectronics.co.uk/ … hp#circuit
The design has been modified several times, presumably for cheaper production. An original version of the HC-SR04 used the MAX232 driver (or clone) to increase the voltage to the transducer, but later ones don’t, and substitute an unlabeled buffer IC.
I’m talking about the PDF schematic and Eagle files for the Sparkfun version with the QUIIC interface. Unfortunately, both that version and the earlier version are called “HC-SR04”. I’ve seen several other schematics on the web. Most seem to have a bandpass filter better optimized for 40KHz transducers. The schematic in your first link is also poorly optimized according to the web page author—who actually went to the trouble to model the circuit. The schematic in your second link seems to have a much higher Q, but the peak of the bandpass is at about 18KHz.
The SparkFun schematic seems to split the difference between your two examples, but the lack of optimization argues for either a very lazy designer or that the web site schematic is different from the actual hardware.
This part isn’t actually designed by or built by SparkFun so we don’t have a whole lot of data on it. (It’s one of a handful of products we carry that are built for us by a third party) If you want more information, you might contact the manufacturer, [NOA Labs, to see what they can provide.
Looking at the board it looks like the amplifier section is a standard LM324 Quad OP-Amp and as jremington mentioned the driver appears to be a variant of the MAX232. I’m not sure what frequency these operate at, unless NOA Labs can tell you, you’d need to probe the output transducer with a scope to find out.](https://www.noa-labs.com/)
Yet another sloppy third party design? But, it isn’t claimed to work any better than the much cheaper version lacking the Qwiic connectors.
your first link is also poorly optimized according to the web page author
That was the first available schematic. As you will see in the comments on that page, I modeled the input bandpass filter and pointed out the frequency mismatch problem to the author.
He was happy to update his page and take credit for the “discovery”, though!