Replacing a piezo sensor with a switch

Thread Starter

Midge

Joined Jun 4, 2015
13
I'm trying to modify a battery operated electronic toy drum for a child, who for medical reasons, can't hit the top of the drum hard enough to activate the piezo sensor fitted to it.

If I disconnect the wires to the piezo sensor, and put my meter across it, striking the drum generates a spike of 2-9v depending on how hard you do it, so I figured I might just be able to replace it with a battery and disability-friendly switch, to simulate the device. However, if I connect the sensor back up to its circuit, I see -0.6v across it, rising only slightly to -0.4v when the drum is struck. The circuit is I assume some sort of microcontroller - in reality just an unidentifiable blob.

I'm not that familiar with these sorts of devices so just wondered if anyone could tell me how it would operate and how I can rig the switch up?

Thanks!
 

RichardO

Joined May 4, 2013
2,273
You might be able to use the circuit more or less as it is. Replacing the piezo with 2 or more in series or maybe a single large piezo is worth a try.
 

Thread Starter

Midge

Joined Jun 4, 2015
13
You might be able to use the circuit more or less as it is. Replacing the piezo with 2 or more in series or maybe a single large piezo is worth a try.
Thanks - I'll look into that as an option, but it might still not be sensitive enough. With some effort I know the child can press a very sensitive switch but can't tap like you and I would - and the drum does all sorts of things pretty much on it's own so the child can still get something out of it.
 

Alec_t

Joined Sep 17, 2013
10,355
You could try this :-Toy-trigger.PNG
A lever-operated microswitch needs very little pressure to actuate.
 

GS3

Joined Sep 21, 2007
408
If I disconnect the wires to the piezo sensor, and put my meter across it, striking the drum generates a spike of 2-9v depending on how hard you do it, so I figured I might just be able to replace it with a battery and disability-friendly switch, to simulate the device. However, if I connect the sensor back up to its circuit, I see -0.6v across it, rising only slightly to -0.4v when the drum is struck. The circuit is I assume some sort of microcontroller - in reality just an unidentifiable blob.
A piezo sensor by itself has a very high output impedance plus it blocks DC. To replace it with anything you might need a capacitor to block DC. Other than that it is difficult for me to suggest anything specific if I cannot see the part of the circuit it is attached to. If it is some sort of amplifier then you could add a preamp which should not be difficult to do.
 

Thread Starter

Midge

Joined Jun 4, 2015
13
You could try this :-View attachment 93182
A lever-operated microswitch needs very little pressure to actuate.
Thanks - I'll give that a go. You're correct about the microswitches - I tend to use a disability friendly switch called a Jelly Bean for this sort of purpose, and it's built around a miniature lever microswitch and really sensitive.
 

Thread Starter

Midge

Joined Jun 4, 2015
13
A piezo sensor by itself has a very high output impedance plus it blocks DC. To replace it with anything you might need a capacitor to block DC. Other than that it is difficult for me to suggest anything specific if I cannot see the part of the circuit it is attached to. If it is some sort of amplifier then you could add a preamp which should not be difficult to do.
Thanks -I'll take another look at the PCB and see if I can make sense of it. There are about half a dozen surface mount components as well as the round black blob, but not holding out much hope!
 

ian field

Joined Oct 27, 2012
6,543
I'm trying to modify a battery operated electronic toy drum for a child, who for medical reasons, can't hit the top of the drum hard enough to activate the piezo sensor fitted to it.

If I disconnect the wires to the piezo sensor, and put my meter across it, striking the drum generates a spike of 2-9v depending on how hard you do it, so I figured I might just be able to replace it with a battery and disability-friendly switch, to simulate the device. However, if I connect the sensor back up to its circuit, I see -0.6v across it, rising only slightly to -0.4v when the drum is struck. The circuit is I assume some sort of microcontroller - in reality just an unidentifiable blob.

I'm not that familiar with these sorts of devices so just wondered if anyone could tell me how it would operate and how I can rig the switch up?

Thanks!
The piezo component produces a voltage when you strike it - a switch doesn't.

You could probably amplify it a bit for the blob-chip. An electret MIC capsule contains only the electret piezo element and a special JFET, with a bit of surgery you can rescue the JFET, this will amplify the piezo disc big time.

Maybe a 2k2 drain load resistor to the existing +V and a 1nF capacitor to couple it into where the original sensor was connected.

A while back I constructed a circuit along these lines that made it possible to compare the microphonic qualities of various capacitors - I followed the JFET with a high gain amplifier, but the signal was there to be amplified.
 

Thread Starter

Midge

Joined Jun 4, 2015
13
Sorry for the tardy response to this guys. I kept picking it up and trying different things - but the nearest I got was triggering it, where it would then hang and need to be powered off to reset.

There comes a time when curiosity, determination, and patience just run out - so I had a dig about and found a similar toy new for just £12 and modified that - which was annoyingly easy by comparison.

Thanks again for the suggestions. At least I know a bit more about these devices than I did!
 
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