Where can I buy a touch-sensitive(ipod-like) on/off switch?

I need a switch with a low resistance and that can handle about 20V and 10A. But I don't want to go with the normal rocker or pushbutton switch to do the job. I want one is touch sensitive. Turns on when you touch it. Turns off when you touch it again. It needs to be about 20mm x 20mm x 20mm. Or about that size, give or take 10mm in any direction. Just like a normal rocker switch.

I know they have these things but I just can't find them on google. I have something like it on my Logitech V500's scrollwheel: Just touch the back of the area and the mouse scrolls down indefinitely. Touch the front and it scrolls up indefinitely. Slide your finger around it to do normal scrolling.

While you might be looking at a capacitance switch, you will often find a tactile switch underneath with supporting electronics.

Switching 20 amps in a small package will probably require a solid state switch.

Daniel

The capacitance switch that Gadget Lover mentioned might be the best way to go. Working examples can be seen in the touch lamps which tended to be popular in the early eighties. You touched the metal lampstand to cycle the light on and off. The smarter ones even dimmed the lights based on either the multiple touches or how long you left your hand on the lamp stand.

You can also see other examples of this switch in some elevator pushbuttons. Typically the metal type that doesn't depress or move.

Finally, how about gutting an old laptop for the mouse pad? Alternatively, you could buy a scroll pad and gut that.

Steelwolf said:

The capacitance switch that Gadget Lover mentioned might be the best way to go. Working examples can be seen in the touch lamps which tended to be popular in the early eighties. You touched the metal lampstand to cycle the light on and off.

Click to expand...

It takes "skin" to make these work so gloves are out.

I haven't seen any modern ones but was in a store several years ago when a woman brought one in the she said did not work.
They plugged it in. Tap, tap, tap, tap On, low, medium, high, off.
Several people did it. Then she tried. Tap, tap, tap, tap. Nothing.
Either she had a very low capacitance or it was too high.

explain in more detail what you are trying to switch (AC/DC etc).. i think you need to have a capacitance switch.. i bet there is an IC that has that functionality built in.. and has a simple toggle output.. high/low TTL.. that can be fet to an FET to turn what ever you want on/off.

-awr

oh.. ps.. the ipod scroll wheel is no loger capacitance from what i understand they switched (bad pun) to using pressure-sensitive pads... those work with gloves!

-awr

ps2... http://www.st.com/stonline/books/pdf/docs/7964.pdf is a tiny 'latch' ckt that will toggle the output.. it's about the size of a grain of rice... a pressure sensing switch with this chip will 'toggle' output you can use to drive the gate of an FET and toggle hundreds of amps of you needed.


-awr

I do recall seeing a product demonstrated once. I'll have to try and see if I can dig it up again from amongst all the other junk in the office. I think it was called a membrane switch. Basically just a sheet of plastic with "pads" located at specified points. IIRC, the PCB it sits on has corresponding points where there are some exposed traces. So I guess it works on pressure. A light touch would contact the pads on the plastic sheet with the corresponding traces, thus closing the circuit. But you need to couple this up to a relay to switch more than a few microamps.

Hi there,

Simple on/off touch switches are not hard to construct yourself. There are many
ways to go about this, but the key is to use a high input impedance device such
as a MOSFET or a CMOS ic gate.
If you provide three copper pads on a circuit board the operator then puts their
finger across two of the contacts. Normally this wouldnt short the contacts
very well, but for high input Z devices this will look like a short.
Connecting the center pad to the input of the device (cmos gate) and one outer
pad to +5v and the other pad to ground, when the user shorts input to +5 the gate
input sees a logical "high" on it's input. When the user shorts input to ground, the
gate input sees a logical "low" on it's input.
Sometimes a cmos latch ic will help to hold the state to keep the light (or whatever)
on or off, depending on the last shorted contacts.
Using a MOSFET transistor, once the gate is shorted one way or the other it
may very well stay in that state for a long period of time meaning you can get
away without any additional latching circuitry.

As a side note, one light i've contructed (several years ago) had a touch switch
on it. As you press harder the resistance decreases across the contacts, so you
can use this feature to dim the light. On mine, the harder you pressed the more
brighter the light would get up to maximum brightness.
The contacts are the same: simply two pads of copper clad on a pc board.
The finger shorts the contacts and that reduces the resistance across them.
When the finger is removed the resistance goes back up to several hundred megohms
or more.

In some cases you'll also need a low leakage cap across the contacts to keep
noise from triggering the circuit. I've also used a 20 megohm resistor as pull up.


Take care,
Al

Hi FNinjaP90,

Have a look at the touch sensor IC's from Qprox.

They have a simple, single switch IC that can you could use to drive a MOSFET. By itself it can't handle 20V / 10A, so you'd need to drive the MOSFET to handle the high current and voltage. The upside is that it creates a dielectric sensing field around the "switch" material. This means that a finger entering the field will trigger the switch. So gloved hands are not a problem.

pb

Hi there Pb! How ya been?

Looks great, did you check the price or availability at Digkey?

Take care,
Al

i just checked for a single-button model $1.97 qty 1 at digikey... 9300 instock.

-awr

We were looking into using an ipod type switch for the GatLight. However it is a bit complicated to program... maybe in a futrue version....

they have just whatcha want it think.. sliding touch pad.

-awr

I want a switch like this for a light we're planning to build. I just want it to light up a bulb with a simple on/off operation. Nothing fancy needed. It must be able to tolerate about 25VDC @ 8A. No matter how simple it is, I don't want to do any assembly on the switch. I just want to find a product that has the touch sensor and the electronics premade and assembled.

the links to the sensors above are for pre-fab sensors.. you would probably need to use the likes of an FET to do the actual switching but i think they actually toggle themselves.. worst case you need to add a toggle circuit.

-awr

Hi awr,

Actually, the links above are for the IC's themselves as opposed to pre-fab sensor circuits. With the QT110 you would need to add a 10nf capacitor and a sensor pad. The sensor pad can be virtually any metal object. On at least one hobby oriented website, I saw that someone soldered a penny to the sensor leads.

The IC itself can only handle an absolute maximum of 6.5v and 20mA on any pin. This means that you will need to add a FET to handle the 20V 10A switching. The good news is that the IC can be set into toggle mode by simply connecting pins 3 and 4 to ground.

As far as availability, awr is correct. The IC's cost $1.97 each at Digikey. Currently, Digikey shows 75 available of the QT110-IS which I would recommend over the QT110-D since it has a much wider operating temperature range.

FNinjaP90..

If you are looking for a complete, drop-in solution instead of doing a small amount of soldering (3 components and a strip of metal) then contact one of the many design firms listed on the Qprox site or any of the other industrial touch sensor manufacturers and ask for a quote on a custom item. You'll get what you want, but it likely won't be cheap.

pb

very neat info about the toggle mode.. yup i forgot it's just the 'IC'.. i can't wait to try one of these things out.

=-awr