modifying a circuit design for a touch switch

Google knows how !!

First hit of 791,000.

thanks for this link. But what i am looking for is a touch switch that stays on longer than

thanks for any suggestions

makay

I would forget about the 555. It is mainly there to create delays. Also, you don't really need a switch that stays in contact until the next time you hit it. A simple electronic button will do the job. What you need is a memory that remembers the last time you hit the switch was for opening or closing the light. Because you want a simple circuit, I would use a FLIPFLOP to make the memory. If you look on your link, there are some circuit using the FLIPFLOP. If you wanna know more about it, check wikipedia flipflop jk. I beleive using a flipflop is the easiest way to do it.

On a sunny day (Sat, 10 May 2008 05:45:18 -0700 (PDT)) it happened snipped-for-privacy@friendstore.com wrote in :

C1 C2 touch here ------------------- copper ============================================== thin PCB ------- -------- copper | | | | |osc | -------- | out | |comp |

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D thin PCB

touchit.

hi thanks for these suggestions! i am sorry but i'm not very experienced in electronics and have problems understanding the diagram and the description. if you could clarify what you mean (for stupid) that would be great! the object which this touch switch needs to go into has got a metal plug-socket anyway so i would not mind using this as the touch plate but if it would work without it ...that would be fine as well. one more important thing is that it works with three AAA rechargeable batteries, so 3.6v.

thanks again for all contributions m

On a sunny day (Sat, 10 May 2008 10:26:27 -0700 (PDT)) it happened snipped-for-privacy@friendstore.com wrote in :

You are welcome.

Well, the 'PIC" is a Microchip micro controller. As you perhaps have no experience with those, it would be better to look for an other solution.

Not you fault, the terms refer to programming techniques for those micro controllers. So you'd have to program one....

Oh, PIC will work on 3 V upwards.

Here is a more generic description, avoiding micro controllers:

optionally your finger | oscillator ---- capacitor ----- capacitor ---- detector ---- D flipflop ---- load A

The oscillator generates a high frequency (MHz), that is normally passed via the 2 capacitors in series to the detector, causing a voltage at the detector. As the capacitors are very small in value, but sufficiently big to pass the RF, touching point 'A' short the RF via the much bigger capacitance of your body to ground. The detector voltage will drop, and the D flip flop will trigger, switching the load on or off.

Up to you now to design the circuit :-) The oscillator could be a Schmit trigger NAND gate, the D flipflop a 74HC74, detector a diode + cap + resistor (time constant). The capacitors can of course be small capacitors of a few pF with some kV voltage spec, does not need to be PCB oriented.

Hope this helps.

=3D=3D

CB

you=3D

-- load

tance

Great! Thanks a lot I'll try to work it out from here... although i'm sure i'll come up with more questions. Thanks again for helping me out on a sunny day like this... m

load

But your idea depends how good you are grounded? But even if you are grounded, I don't think that the influence of the finger is sufficient for grounding the RF. You have at least adjust the sensitivity of the detector and adjusting it if the ambient changes (humidity, temperature).

I've found a nice description how a touch sensor could be built:

Take a look at the chapter "Finger Capacitance". This works with floating fingers, too.

I've build one of these sensors:

Looks like the capacitance changes about 10% when touching the sensor, so a very simple sigma-delta ADC should work. With the integrated analog comparator of some microcontroller you don't need any more active external components. The microcontroller can filter ambient changes and can detect impulses from touching the sensor.

A more expensive solution (but $3 is not that expensive) is available from Analog Devices:

The AD7142 can measure 14 capacitors. With some clever software and PCB layout, you can even implement sliders, wheels and fine resolution touchpads.

On a sunny day (Sat, 10 May 2008 21:41:59 +0200) it happened Frank Buss wrote in :

No it does not. You capacitance to the 'environment' is easily a few hundred pF, or more.

Well, I have seen this system used in TV sets.

No, very wide operating range.

Well, that is a slightly different circuit. And 10mm thick glass is not what I was talking about.

Yes, different setup.

The circuit I am referring to uses the body - environment capacitance to _short_ out a RF signal. So if the series caps are for example 22 pF, then 100 pF body - environment capacitance will significantly attenuate the output of the detector. Anything bigger is OK too.

Sure.