Disappointing Ultrasonic Dog Deterrent (DUDD)

RE: Disappointing Ultrasonic Dog Deterrent (DUDD)

kmikemoo — Fri, 13 Mar 2020 21:46:17 GMT

[mention:43d2c3e8825e4123967cacd2bb7a769b:e9ed411860ed4f2ba0265705b8793d05] The 15kHz makes sense. I hooked up my audio generator last night to half of my H-Bridge - which I think is an A-B Amplifier. I could hear the pitch rise up to around 15kHz to 17kHz according to the scope. Oddly enough, I believe the tone then started to decrease - slightly - as I continued up to around 30kHz. I feel [mention:ef601b47037147bfb9b0b4a3ab63026a:e9ed411860ed4f2ba0265705b8793d05] hit the nail on the head with intermodulation distortion, electrical and mechanical.

It took me a bit to catch on, but I like your microphone suggestion. Just because I think I'm putting 30kHz into the piezo doesn't mean that's what I'm getting out of it. I abandoned my detector because the transmitter still had audible noise. I should pursue it still.

RE: Disappointing Ultrasonic Dog Deterrent (DUDD)

jc2048 — Fri, 13 Mar 2020 10:38:42 GMT

If you want to see how you're torturing the power supply, you could place a small value resistor in series with one of the supply lines and look at the voltage across it with your 'scope. That would then show you the current it's supplying and how it varies. Pick a low value, so that the supply voltage doesn't vary too much, but high enough that you get reasonable resolution on the 'scope. For instance, something like 100mOhm would give you 100mV per amp and not impact the supply voltage too much on a 12V circuit. Try to go for a non-wirewound one if you can. Don't run it for too long if you see a high current and the resistor wattage isn't quite up to the job.

Be careful if you want to measure voltages on a second channel at the same time. One way to arrange that would be to put the resistor in the ground return to the supply like this so that the 'scope grounds were common (that will lead to the current trace being upside-down but, if you're not comfortable with that, 'scopes usually have the means to invert a channel):

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RE: Disappointing Ultrasonic Dog Deterrent (DUDD)

beacon_dave — Fri, 13 Mar 2020 00:13:43 GMT

That cheap tweeter may not be rated for 30kHz operation. Looking at the spec on this one:

https://cpc.farnell.com/pulse/pls00307/tweeter-piezo-horn-188x80mm/dp/LS03583

it suggests 3-20kHz frequency response.

Similar with this one:

3" x 7" Piezo Horn Tweeter3" x 7" Piezo Horn Tweeter

May be an interesting experiment to use a microphone connected to the scope and see what happens as you increase from say 10kHz (still audible I hope) past 20kHz on the input.

RE: Disappointing Ultrasonic Dog Deterrent (DUDD)

jc2048 — Thu, 12 Mar 2020 12:39:16 GMT

Did the original circuit have the collectors of Q3 and Q4 connected to ground, by any chance? As you've drawn it, on the low side of the bridge you'll have a diode path emitter-to-base to whichever 555 output is low.

RE: Disappointing Ultrasonic Dog Deterrent (DUDD)

dubbie — Thu, 12 Mar 2020 10:21:53 GMT

This is an interesting idea which looks like it should work (I'm not analogue electronics expert so who knows!). I've done a fair bit of ultrasonics and never thought of using a simple ultrasonic receiver to check for proper operation. I will have to remember that.

Dubbie

RE: Disappointing Ultrasonic Dog Deterrent (DUDD)

Gough Lui — Thu, 12 Mar 2020 06:47:49 GMT

I suppose this is a good exploration into getting some sort of power into a signal - it can be a little tricky and I'm definitely no expert. That being said, hot transistors normally mean one of a few things - the transistors are not being driven fully hard on/off, the switching transition is happening too slowly so the transistor is spending part of its time acting as a "resistor", the transistors themselves are reaching their current limits (due to internal resistance limitations) or you're somehow trying to switch them very quickly and they just can't cope. It did get me thinking ... they're using the BJTs in a way which MOSFETs may be a better choice if you can find one with the proper gate voltage properties.

I suspect that the original H-bridge style circuit was used to ensure that the waveform on the piezo was a nice true AC waveform. Feeding the piezo directly is likely to make this a pulsed DC waveform instead, so in terms of a speaker cone, one can imagine it going from equilibrium to forwards and back to equilibrium rather than back-and-forth of the equilibrium, which is likely to cause limitations in drive power and possible distortion. I suppose you could try adding a DC-blocking capacitor in series with the piezo to try and transform the pulsed DC into something more like AC, so as to reduce the chances for distortion (noting that the capacitor is likely to result in changes to frequency response). It might also result in a reduction in the current draw but I can't be sure ... haven't played with piezos in a long while.

I wonder whether the noise you were hearing audibly was a result of distortion causing intermodulation of frequency components which have a product in the audible frequency range. It definitely could happen due to physical material properties of the piezo and not necessarily at the electrical level. In fact, this does have some similarity to a mosquito repellent type device that I've seen in my distant past ... but I never actually built it, so nice to see your write-up!

- Gough