Mosfet Touch Sensor - Drawback?

[breaktru]

Has anyone experienced using a touch contact / Mosfet with a damp finger?
This happened to me twice so far, my finger was damp when I fired the touch area and the PV would not stop firing until I pulled the battery.
I dried off my finger and the top of the touch area and it worked as designed.

Perhaps I need to tweak the Gate resistor value.

 

[Dalton63841]

I don't know alot about MOSFET's (wish I did), but perhaps the wetness stayed behind and was enough to complete the circuit?

 

[WillyB]

Well wet is wet, the circuit is complete. With the contact points like yours that are almost touching I doubt if changing your resistor values will have an effect. The design of the SaberTouch's touch pad makes good sense. There's almost no way for some errant liquid to complete the circuit. Why don't you adapt yours like that? Just one contact surrounded by a non-conductive material, at least a 3/8" circle and the body itself is the other contact.

You could try lower values as yours and many others I've seen are far too large, which impacts performance (mostly in a single cell mod) and battery life.

 

[breaktru]

I guess using this on a rainy or damp day is out of the question.
I should rephrase, damp hands and not wet.
Like I said in a previous post, This mosfet will not work with any value under 20meg. I chose the 100meg as that was what I had on hand.

 

[AttyPops]

There's a thread around here from scubabatdan and others about using capacitive touch sensor circuits. They use the body's capacitance to check for change. What I was wondering, does a drop or 2 of water have enough capacitance, or does it "filter it out". So it may be more reliable than the resistive capacitance we usually use.

Such circuits are common in remote controls. I had one I tore apart for the button... the circuit board had those little squiggles for capacitive touch sensor pads. And, even more importantly, I think that THEY CAN BE COVERED WITH NON-CONDUCTIVE MEMBRANES and still work. Hmmmmmm.

Then again, a membrane covering (conductive on the underside, but raised off) would work... but then you're just getting into a different version of a standard button/switch. I'd check the capacitive stuff out on a breadboard for fun.....

P.S. Microchip makes a PIC with Capacitive Touch circuit stuff built in.... almost time for that MCU e-cig..... lol.... I'm not 100% there yet tho. Still looking to keep it simple.

 

[MickeyRat]

Such circuits are common in remote controls. I had one I tore apart for the button... the circuit board had those little squiggles for capacitive touch sensor pads. And, even more importantly, I think that THEY CAN BE COVERED WITH NON-CONDUCTIVE MEMBRANES and still work. Hmmmmmm.

I've seen those squiggles inside keyboards where you don't actually get near them and I don't think they are capacitive. With my original ME training, they look like strain guages to me. When you push on them, the squiggle is deformed slightly and that changes it's resistance which actuates the switch. So, if I'm right, it's actually a type of mechanical switch.

Of course, if someone comes on here and tells you I'm crazy, listen to them.

 

[bstedh]

I've seen those squiggles inside keyboards where you don't actually get near them and I don't think they are capacitive. With my original ME training, they look like strain guages to me. When you push on them, the squiggle is deformed slightly and that changes it's resistance which actuates the switch. So, if I'm right, it's actually a type of mechanical switch.

Of course, if someone comes on here and tells you I'm crazy, listen to them.

They are a mechanical switch with inherent resistance. The squiggles on the board are shorted together by a contact on the back side of the actuator that the key is attached to. They are sometimes touching with high resistance and when you press on them the resistance decreases. Of course different manufacturers use different aproaches.

 

[AttyPops]

OK, my bad, but irrelevant. I looked at it again and it had two electrodes. Resistive. lol. I suppose you could implement a capacitive pad with 1 electrode and a squiggly pad... or just a plain round one. nm. Not the point tho.

The idea here is to discuss accidental activation and the fact that a pad using a capacitive switch may be better, or could be easier to seal. Resistive circuits require electrical contact. Capacitive do not, they only have one electrode, and I hear (but am not sure about the specifics) that it can even be put behind an insulator...like glass or plastic.

Anyway... they go way back... you know... those touch sensitive lamps. They've been more common the last 5 years or so as mechanical button substitutes and I think one type of touch screen.

The question remains: What sets em off "accidentally" and how is it better/worse than resistive?

 

[MickeyRat]

They are a mechanical switch with inherent resistance. The squiggles on the board are shorted together by a contact on the back side of the actuator that the key is attached to. They are sometimes touching with high resistance and when you press on them the resistance decreases. Of course different manufacturers use different aproaches.

I might be misunderstanding you but, that's not what I saw when I did a post mortem on a keyboard. There was no electrical contact between the back side of the key and the squiggle. In engineering they use something similar to measure how much metal deforms under a given force. It's called a strain gauge. It's the deformation of the squiggle that causes the change in resistance not electrical contact. In the case of the keyboard I was looking at, the squiggles were in plastic that was thin and flexible enough to roll into a cylinder when it was taken out of the casing. Of course, as you said different manufacturers...

 

[MadmanMacguyver]

I have a mod using spread resistive contacts...you would have to be vaping in a heavy thunderstorm to set it off...yet I have no problems with activating the mod when I want...or rather my friend who is using it...lol I would have to send him a replacement so he can have something to use while I read off the model mosfet I used though...I got it from frys if thats any help...it was NTE...I'll look for the box later but bear in mind I set it up for really low voltage and it operated another component(secret) to provide power to the regulator in the mod.... when I get my final design done and patent the circuit design I will make it open source for modders though if someone wants to use it for profit they will need to ask me...I dont really want to profit from someone else using it I just dont want the chinoi or BT or such going hey this is neat...we're gonna use it and patent it ourselves...maybe 6 more months...

but if you spread out the contacts really wide like on the other side of a box mod where your hand is you reduce risk of liquid setting off the mod ...

I hope that helps you guys...

 

[breaktru]

but if you spread out the contacts really wide like on the other side of a box mod where your hand is you reduce risk of liquid setting off the mod ...

I hope that helps you guys...

Nah, wouldn't be cool looking:

Just have to keep my hands dry.
A note: I can fire this with just touching the Center post (Gate) and don't need to bridge to the Neg outside case.

 

[bstedh]

I might be misunderstanding you but, that's not what I saw when I did a post mortem on a keyboard. There was no electrical contact between the back side of the key and the squiggle. In engineering they use something similar to measure how much metal deforms under a given force. It's called a strain gauge. It's the deformation of the squiggle that causes the change in resistance not electrical contact. In the case of the keyboard I was looking at, the squiggles were in plastic that was thin and flexible enough to roll into a cylinder when it was taken out of the casing. Of course, as you said different manufacturers...

I used to work in a plant that made switches like these and we printed with conductive ink on thick mylar. Not saying yours is this way just that most of the ones I have repaired are.