Main on/off switch for amp using a piezo switch - diyAudio
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Old 1st August 2005, 10:01 AM   #1
using_e is offline using_e  Austria
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Unhappy Main on/off switch for amp using a piezo switch

I'm stuck with a quite annoying problem.

I've tried to use Schurter PSE M24 piezo switches for use as a main on/off switch.

Link to switch PSE M24


When I first saw these Schurter piezo switches with their ring LED illumination I couldn't resist. The first problem -->price!
The price I paid for two of these babys, someone else is already doing a complete project for. I payed about $200 for one pair. If it were just the price it would have been my smallest problem but........ getting these switches to do what they are planned for is another thing.

I wanted to use them as main switches for on/off. So my idea was to use a simple JK-FlipFlop (Logic IC 7473), connect the output to a transistor and switch a relais. I've done this already a few times with normal mechanical switches and always no trouble at all. It's just a few minutes work to adjust the two resistors (see picture below) to their optimum value and that's it. But this time ......no! These piezo switches output a variable pulse between 20 to 1000ms depending on how hard you press them. I've spent days to look for the optimum values for the two resistors to find a compromise which works with all impulse lengths but ....nope....no chance!

I'm already loosing my will to carry on so any help on this subject is appreciated.

Thanx
using_e
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Old 1st August 2005, 04:31 PM   #2
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I wonder what have went wrong for you

I have made such on/off switches some years ago... No problems...

The piezo contact has a app. 10-100R resistance when the contact is actuated. And the 7473 accepts a clock signal as short as 20 ns, so it should work......

Have you renembered to tie the J, K and Clear high level ??
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Old 1st August 2005, 04:54 PM   #3
johnnyx is offline johnnyx  United Kingdom
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You're right, they look good. Do these switches have contact bounce, noise etc, or a clean pulse when pressed? Maybe you need to provide some noise immunity for the flip-flop.
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Old 2nd August 2005, 05:16 AM   #4
Enzo is online now Enzo  United States
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I would contact the switch manufacturer for application notes on the product. Applications might even be part of the data sheet.
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Old 2nd August 2005, 03:41 PM   #5
using_e is offline using_e  Austria
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I didn't think up to now to put J, K and CLR on high. Is this really required? Currently they are just not connected. Could this be the cause??? What could happen if they are just floating?

The original layout where I have this idea from also didn't make any reference on putting these contacts on high. I also used this layout already two times with mechanical switches and no problem at all.

The switch has a resistance of <20 Ohm during ON position.
The switch has no bounce and noise should also not bee the cause. I'm currently feeding this test setup from a good lab supply on 24V and a 7805 for the IC.

I will try to put the mentioned contact on high and report back afterwards but unfortunately I will have no time for this before the weekend so I will report back on Saturday.

Thanx
using_e
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Old 2nd August 2005, 04:05 PM   #6
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I've also got some thoughts about this but I'll wait till you report back on the weekend.
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Old 2nd August 2005, 04:26 PM   #7
Eva is offline Eva  Spain
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Quote:
I didn't think up to now to put J, K and CLR on high.
How do you expect a JK type flip-flop toggling on clock transients if you don't tie J and K inputs high? It won't
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Old 2nd August 2005, 04:47 PM   #8
mzzj is offline mzzj  Finland
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Old 3rd August 2005, 06:40 AM   #9
using_e is offline using_e  Austria
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Ok, shame on me, I know I should have thought first before posting my last statement.

It's just that I took this circuit from a homepage a few years ago and it has already worked for me this way two times without any problems (using a mechanical switch). Please don't ask me now how this is possible with J, K and CLR floating I don't know for myself. It just worked until now with this new piezo switch.

Next time it would maybe be better not only to copy the circuit from someones hompeage but also to have a look at the function table of the data sheet of that IC.
Now it seems obvious but I didn't think of looking into the data sheet cause it worked before.

I will put J,K and CLR on high and report back on Saturday.

Thanx alot
using_e
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Old 4th August 2005, 02:44 AM   #10
Enzo is online now Enzo  United States
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Never ever leave an input unterminated on a logic chip.
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