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**Carl-NC** · 08-23-2023, 04:24 PM

Probably there is nothing lost in dropping the frequency. Faint targets produce very little delta-f, so for the deepest targets you are listening mostly for a break in the threshold.

What is the resonant frequency of your piezos? Did you try the headphones with just one piezo? When you say "the signal across the piezo circuit became non-distinct" do you mean the amplitude collapsed?

**ihatesanity** · 08-24-2023, 10:17 AM

Carl, thank you for the response! The resonant frequency of the piezos is supposedly 3-5kHz. I've hooked them directly to an old HP audio oscillator and easily get a tone from a very low frequency up to the upper limit of my own audible range. Granted, I know what they do when they are attached to the HP oscillator isn't the same as what they will do when attached to the MPP's VCO.

Using just one piezo provides the same result as two piezos in parallel.

I looked into 555 VCO circuits a little more, and I think the correct way of describing what I called "non-distinct" is to say the on-time of the oscillation appears to remain fixed (or at least almost fixed) while the off-time decreases as frequency increases. To this end, at a point, the off-time is very short and the output looks nearly like a constant high signal. Seeing this is what drove me to hypothesize halving the VCO maximum frequency would allow me to hear an audible tone for near targets. Is there a simple way to reduce the on-time of the VCO in the MPP, or would the circuit need some additional components? Again, I don't understand electronics well, but my initial assumption is that decreasing the VCO output on-time would give the piezo a better chance to reset between pulses at higher frequencies.

**Carl-NC** · 08-24-2023, 04:08 PM

OK, your problem is definitely the VCO, not the piezos. A 555 timer used as a VCO has a fixed pulse width set by (in the case of the MPP) R29 & C12. You can adjust R29 to reduce the pulse width to make it work better at the high end, but it may sound quieter at the low end.

Ideally you want a 50% duty cycle through the whole range. I don't know of a way to do that with just the 555, but if you run 2X the frequency and add a flip flop to the output then that will do it.

**ihatesanity** · 08-25-2023, 01:52 AM

Carl, thanks for giving me some direction! I can certainly add a couple flip flops to my next Digikey or Mouser order. Is there a particular type you would recommend?

Thanks again,
Ron

**Carl-NC** · 08-25-2023, 03:19 PM

I expect a D flip flop to be the easiest to use, just feed /Q back to D and clock it. 4013 or 7474 will work.

**ihatesanity** · 09-23-2023, 10:25 PM

I thought I might follow-up on this post since I've done a little bit more experimenting with it, and since I bought a modern oscilloscope allowing me to take screen shots.

First of all, I think Carl's suggestion is the best, and I plan to whip up my own version of the MPP Rev E with a flip-flop in the VCO so the audio output frequency will always have 50% duty cycle.

In the meanwhile, I replaced R29 with a 50k trimmer pot (the wiper is tied to the leg opposite the adjusting screw - the pot is shown with a red circle in the picture below) and I messed around with the three types of 555 timers I have on-hand. I took some oscilloscope screenshots to throw out a little more information someone in my shoes might be looking for. I found all three 555 timers used in the U8 position produce different maximum audio output frequencies when R29 was adjusted to produce a duty cycle very near 100%. I have pictures below, but I found the three different 555 timers have the following approximate maximum "usable" audio output frequencies: