switching on a circuit with minimal force.

hi. does anyone know of a way of switching a circuit on with minimal force? i have tried capacitative touch on an attiny but i want it to run on batteries and they would run down with the constant sampling and the capacitative capabilities are vastly reduced without a proper grounding. so yeah basically id love to be able to 'switch on' my circuit but with something more dainty than a push button. could a couple of closely spaced conductors connected to a sensitive transistor be bridged by a finger perhaps? forgive my rudimentary electronics knowledge please. thanks. dan

That "minimal force" will be applied by a human? If so, you can have 2 spaced conductor (but close enough) that one have to touch with his finger, closing the circuit.

Mercury switch?

and a circuit that immediately latched (somehow?) - so that mercury disconnection didn't switch it if again?

Hi, What is the application, why does it need to be done with minimal force.

Tom...... :)

Capacative can be very low power. I've seen capacative sensors that run on a coin cell for months and do useful work. Gounding? That doesn't sound right.

hi, thanks everyone for the reply's. i am making something which i want to be aesthetically pleasing so i dont want any visible buttons. using capacitative touch would have been perfect but for the quoted issues. minimal force because i want the interaction to be as subtle as possible. MorganS, the issues with grounding are explained here ive tried it out myself and it works beautifully when plugged into my computer but as soon as i run off a battery the sensing drops. this can be remedied slightly by adding a few more megaohm resistors but the sensing plate doesnt work you have to touch the wire.. now sure of the science behind it....i am interested in your claim about low power capacitative sensing though. is this on an arduino? any information on how this works? thanks

The problem with capacitive sensors in portables, wearables or other battery powered devices is the lack of a common ground/earth connection, required to close the external capacitive circuit loop (Csensed in the picture of your link).

Your circuit should work from battery as soon as you put an finger on its Gnd line, too. If so, add another plate (piece of foil) to the Gnd line, then the sensitive area is over and between both plates.

The sensor senses capacitance between touch area and [u]circuit[/u] ground. It calibrates on power-up to a "just off" state. Your added capacitance sets it off.

If possible, add a piece of e.g. tinfoil around the touch sensor. Connect that to circuit ground. Keep ~5mm of space between sensor and tinfoil. Leo..

ok this is working, thanks very much folks! the only other problem now is the power issue. i am planning on using a cr2032 3v coin cell battery for this project. any ideas how long it would last without the led powered on? i know there are low power libraries out there but i believe that they work by switching off for a specified duration and then on again which wouldn't work when the sensor needs to be consistently responsive. again any help would be very much appreciated. thanks.

Not long then. CR2032 only has 225mAH capacity, example http://www.dipmicro.com/store/BAT-CR2032 Was there a circuit posted somewhere that I missed?

i know there are low power libraries out there but i believe that they work by switching off for a specified duration and then on again which wouldn't work when the sensor needs to be consistently responsive.

Yes it would work and be responsive if you turned it on for 10mS and off for 400mS. It would respond in under half a second and only use 1/40 th of the power.

Play about with the on / off times and see what compromise suits you.

Grumpy_Mike:
only use 1/40 th of the power.

Which in this part of the world would be called “40 times less”. I’m still trying to understand how you can multiply by a number greater than one and end up with a smaller result. But it sounds cooler.

I'm still trying to understand how you can multiply by a number greater than one and end up with a smaller result.

Who said anything about multiplying?

I saw somewhere that the response time (power consumtion) of some touch sensor chips can be programmed. Leo..