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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Schmitt Trigger circuits.
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nathanxl



Joined: Apr 24, 2012
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Location: Wa

PostPosted: Tue Dec 18, 2012 10:13 pm    Post subject: Schmitt Trigger circuits. Reply with quote  Mark this post and the followings unread

Hi.
I have a particular need and I think I require the use of a schmitt trigger circuit to do what I need, hope you guys can guide me a little.

I have a piezo trigger fixed to a kick drum going straight into a CD 4040 counter. The 4040 outputs go through an R2R DAC to a VCO, so everytime the kick drum is hit the 4040 outputs shift and the VCO pitch is altered.

The problem I have encountered is that when the drummer kicks hard the counter goes through multiple cycles and often finishes where it started, hence not changing the VCO pitch at all.
I gather that I need to somehow "debounce" or attenuate the triggering so the 4040 clock in only recieves one pulse per kick drum hit and I think a schmitt trigger is designed to do this... no?

So this is where Im at. Im sufficiently mystified as to what circuit design I need to use to fullfill this requirement and would love you to assist me if you can.
What I do know is that I would like its sensitivity to be adjustable, perhaps with a pot, because Id like to use this with other things/materials other than booming kick drums in the future

I have a 40106 nearby the 4040 on the perfboard that has some ins/out free and the circuit is running on 9VDC.

Thanks in advance.
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Cynosure
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PostPosted: Wed Dec 19, 2012 6:56 am    Post subject: Reply with quote  Mark this post and the followings unread

It is difficult to know without a schematic to follow, but from your description a schmitt trigger would probably make one long sustained pulse.

You might want to use a highpass filter instead so you get a short gate pulse. Try sending the pulse past a 100k resistor to ground, through a 10k resistor, and then through a 4148 diode. That will change a sustained signal into a short gate pulse.
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PHOBoS



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PostPosted: Wed Dec 19, 2012 7:29 am    Post subject: Reply with quote  Mark this post and the followings unread

and don't forget to use some protection diodes. piezo's can produce quite a voltage spike.

I think this could be helpfull.

and http://electro-music.com/forum/topic-47085.html

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analog_backlash



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PostPosted: Wed Dec 19, 2012 9:46 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah, so that's what all the diodes are for! I've been playing around with this today using a JFET preamp and 2 x 40106 inverters, but all the while I could have been blowing them up... I haven't got any schottkys with a high enough voltage spec, it turns out.

Anyway, I got something 'working' but after reading these posts, I'm not sure that it's such a good idea. I'll carry on thinking...

Gary
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nathanxl



Joined: Apr 24, 2012
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PostPosted: Thu Dec 20, 2012 7:14 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Cyno and Phobo.
That link is good and will give it a go tomorrow, will head out to find some schottkies! Hope to find ones of the correct voltage.

Mr Backlash... Can you share the scheme for your working circuit?

And here I am thinking there is a Lunetta answer to every electronic question!
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analog_backlash



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PostPosted: Thu Dec 20, 2012 1:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi,

I didn't post it before, partially because of the diodes issue, but mainly because it's not sensitive enough. I think if I used an op-amp instead of the 2N3904 (see below) I could get it to work much better, but I didn't know if you had any room for that. No doubt there are much better ways to do it than this, but this is my stab at it (so far). I've tried it out on a digital counter and it does seem to give one count per "hit" (once I stopped the breadboard bouncing about at the same time!). You might need to play with the values of the resistor and capacitor connected to the input of the first inverter, to adjust the pulse length.

By the way, the BAT41s that I've used only have a Vrrm of 100V, so according to PHOBoS' link, these at least 100V short of safe! The sensitivity is better without the diodes, but you could potentially do damage without them, so it seems best to stick with that idea.

If I come up with anything better, I'll post it.

Gary


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elektrouwe



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PostPosted: Thu Dec 20, 2012 4:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

I think the above circuit is a bit of overkill: a standard piezo disk can generate lots of energy when hit directly. So the only thing to do is limit the maximum positive voltage to power supply voltage and avoid that the negative peak after the hit can reach the CMOS. Rsens can be made much smaller for big disks to reduce sensivity or much bigger (10M) for smaller disks - a potentiometer/trimmer could make sense.


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analog_backlash



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PostPosted: Thu Dec 20, 2012 4:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

elektrouwe wrote:
I think the above circuit is a bit of overkill


Yes, you're almost certainly right! I wasn't really happy with my original circuit and then I made it more complicated by adding the protection diodes. I know that I've seen loads of circuits in the past that didn't include the schottkys etc. (including a simple drum pad circuit that I built many years ago). Actually, I'll dig that circuit out and see how that was designed...

Gary
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nathanxl



Joined: Apr 24, 2012
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PostPosted: Fri Dec 21, 2012 5:04 am    Post subject: Reply with quote  Mark this post and the followings unread

Wow, you guys are great.
Youre right, I dont have a lot of room to play with and was actually originally planning on using the leftover ins/outs on my 40106 for a VCO.

Backlash, your scheme looks tops and elektrouwe's simplified one looks good too.

Today while I was in the shed, I tried out the circuit in Phobos link, with some BAT 45 schottkys rated at 50v and it seemed to work just fine. I wacked the drum sooo hard and it only clocked over once and it was even pretty sensitive with gentler hits... I tried out a 1m pot where the resistor is on the schem but it didnt seem to make any difference.

I did a gig over the weekend with the piezo going straight into the 4040 and it survived OK, so these extra components will increase its chance of survival I assume. I have another gig in a week so will run this simple circuit through its paces to make sure, may even tape a spare 4040 to the lid just incase.

I think this is some good information that perhaps, while not strictly Lunetta, uses discrete components to drive Lunetta circuits. should I change the name of this thread to make it easier to find in the future?

"Piezo Trigger debouncing for clocking Lunetta circuits" ?
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richardc64



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PostPosted: Fri Dec 21, 2012 1:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

The multiple triggers advancing the counter are caused by the piezo output being so spike-y. A cap in parallel with Rgnd, in elektrouwe's circuit, for example, would smooth the burst of pulses into one pulse per hit. Cap could be small, .001 - .01uF.
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analog_backlash



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PostPosted: Sat Dec 22, 2012 5:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

Just got round to trying out elektrouwe's circuit with richardc64's added capacitor (I used a 10nF) and it works very reliably, but I wanted to up the sensitivity a bit. I tried adding a JFET preamp, but now it's giving me 2 pulses every time I hit it! I'm sure there's an obvious reason for this, but I haven't worked it out yet. I shouldn't start doing these things late at night...

Gary
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jonasx26



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PostPosted: Sun Dec 23, 2012 5:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

I would suggest adding a series resistor to limit the current when the clamping diodes are conducting as well.
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PHOBoS



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PostPosted: Mon Dec 24, 2012 6:46 am    Post subject: Reply with quote  Mark this post and the followings unread

Normally a current limiting resistor would be advisable or even required. But from what I understand piezo's hardly provide any current, so it's not necessary in this case.
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