How to wire up Piezo touch sensitive switch?

[Ben Whittle]

Hi Everyone,

I could do some advice on wiring up a Piezo touch sensitive switch. I'm in the middle of building a desk PC project and thought it would be a nice touch to have a touch sensitive power switch, so I've bought a nice shiny touch sensitive illuminated Piezo switch and can't figure out how it works. The switch I've bought is an APEM PBAR2AFB000LSG found on RS components - http://uk.rs-online.com/web/p/push-button-switches/6919553/

It has 4 wires, two of which are for the bi-colour LED and the other two for the switch itself. I very quickly figured out that it is not quite as simple as wiring the two switch wires to the power switch header on my motherboard. I thought maybe it needed a slightly higher voltage or a tad more current to pass through it to activate so I wired it up in line with the coil in a relay with a 12v supply and when I touch it nothing happens. If I need to power some sort of circuit to get this to work I have the 5v standby power from the PSU and a separate 12v power supply which will provide constant power too so there are a couple of options. I assume I'm going about this in entirely the wrong way so I thought I'd seek some expert advice, any help would be much appreciated.

Cheers

 

[berkeman]

Hi Everyone,

I could do some advice on wiring up a Piezo touch sensitive switch. I'm in the middle of building a desk PC project and thought it would be a nice touch to have a touch sensitive power switch, so I've bought a nice shiny touch sensitive illuminated Piezo switch and can't figure out how it works. The switch I've bought is an APEM PBAR2AFB000LSG found on RS components - http://uk.rs-online.com/web/p/push-button-switches/6919553/

It has 4 wires, two of which are for the bi-colour LED and the other two for the switch itself. I very quickly figured out that it is not quite as simple as wiring the two switch wires to the power switch header on my motherboard. I thought maybe it needed a slightly higher voltage or a tad more current to pass through it to activate so I wired it up in line with the coil in a relay with a 12v supply and when I touch it nothing happens. If I need to power some sort of circuit to get this to work I have the 5v standby power from the PSU and a separate 12v power supply which will provide constant power too so there are a couple of options. I assume I'm going about this in entirely the wrong way so I thought I'd seek some expert advice, any help would be much appreciated.

Cheers

Welcome to the PF, Ben.

I had to go to APEM.com to get a datasheet for the part:

http://www.apem.com/files/apem/brochures/switch-pushbutton-PBA-ENG.pdf

It's not great at explaining what is going on with the switch, but try connecting a resistance meter (DVM in resistance mode) across the switch wires and push the button. You may see a momentary blip down to 10 Ohms or so when you push the switch (or it may be a constant 10 Ohms when being pushed -- I'm not able to tell from the datasheet).

If this is so, just connect one end of the switch wires to ground, and the other to your 5V or 3.3V power supply (whichever you are using for your logic circuit) through a 10kOhm resistor. The wire connected to the resistor should go from the power supply voltage down close to ground when the button is pushed.

 

[davenn]

It's not great at explaining what is going on with the switch, but try connecting a resistance meter (DVM in resistance mode) across the switch wires and push the button. You may see a momentary blip down to 10 Ohms or so when you push the switch (or it may be a constant 10 Ohms when being pushed -- I'm not able to tell from the datasheet).

yup, I had already read that too, but didn't suggest it because of its lack of clear advice
its a pity they hadn't given a couple of practical application circuits. That would have made things much clearerDave

 

[Ben Whittle]

That's great, thanks for the quick response guys. Checking the resistance with the meter it turns out that the switch isn't quite as sensitive as I expected, I have to apply a bit of force to activate it but the resistance does drop significantly when I press hard enough. It still doesn't seem to fire up my motherboard when connected directly to the header though. I guess I can link one switch wire to the gate of a MOSFET and connect the source and drain pins across the power switch header on the motherboard, with a 5v supply on the other side of the switch? I'm being overly cautious here as its quite an expensive motherboard, I don't want to damage anything by making any daft mistakes. Am I right in thinking I can use a MOSFET in this way and there is no chance of current flowing the wrong way and shorting something?

Cheers.

 

[berkeman]

That's great, thanks for the quick response guys. Checking the resistance with the meter it turns out that the switch isn't quite as sensitive as I expected, I have to apply a bit of force to activate it but the resistance does drop significantly when I press hard enough. It still doesn't seem to fire up my motherboard when connected directly to the header though. I guess I can link one switch wire to the gate of a MOSFET and connect the source and drain pins across the power switch header on the motherboard, with a 5v supply on the other side of the switch? I'm being overly cautious here as its quite an expensive motherboard, I don't want to damage anything by making any daft mistakes. Am I right in thinking I can use a MOSFET in this way and there is no chance of current flowing the wrong way and shorting something?

Cheers.

So you are wanting to use this as a power switch for your motherboard? I'd recommend using a different (real) switch for that purpose. This appears to be for use as a logic input means, not a solid switch. You could follow it with a FET switch circuit of some kind, but then you'd need a separate logic input switch to use for an OFF input into the FET circuit.

Best to just use a real switch for this, instead of the piezo version, IMO. You can still have pretty LED lights and highlights, though...

 

[Ben Whittle]

Cheers for the advice, taking this into account I've managed to get the touch switch to activate a small relay through a MOSFET. This way it is just closing the contacts on the board as an ordinary switch would. Seems over-complicated for a simple power switch but I've paid for it so I figured I might as well use it. Plus it looks really good mounted in the brushed aluminium panel I've made for it :) Thanks guys, this has solved my problem.