View Full Version : Does this circuit look OK?

02-14-2010, 12:11 AM
I recently tried this touch switch circuit:


Using a couple of 2n2222 I had lying around. Although I got a response from the first transistor, I could not get the second one to switch and light the LED. I was trying to implement with a 6v supply rather than 9v - does the circuit look OK?

I have two foil plates that act as the contacts.

Any advice would be appreciated.


P.S. Can this kind of switch be implemented with just an opamp configured as a comparitor? I would think so, but so far I've not had a lot of luck getting it to work.

02-14-2010, 12:15 AM
You're stepping into a frustrating world if you are trying to implement a touch switch like you linked above. That circuit depends on about a billion things lining up correctly to work.

I would recommend looking at a commercial touch solution if you want something dependable. Here is one by Atmel:

Note that I have never used that product but I had a sales rep tell me they are awesome for whatever that is worth. It does look like a neat product.

02-14-2010, 01:05 AM
That is a strange circuit. Just what do you mean by "touch circuit" and what part do the foil plates play?

02-14-2010, 01:22 AM
AS it's designed the bas current for the first transistor is supplied by bridging the gap on the contacts which are attached to metal plates. The second transistor conducts once the first starts. If the resistance of your finger is too high then it won't work. It's going to bounce like crazy if it does work. I would avoid that design if possible.

02-14-2010, 01:43 AM
Shobley hi: I have used this circuit many times and works very well. its variable voltage is a lot more forgiving.


This circuit uses a 555 timer as the bases of the touch switch. When the plate is touched the 555 timer is triggered and the output on pin 3 goes high turning on the LED and the buzzer for a certain period of time. The time that the LED and the buzzer is on is based on the values of the capacitor and resistor connected to pin 6 & 7. The 10M resistor on pin 2 causes the the circuit to be very sensitive to the touch.

your circuit may have a problem in as much that you are reducing the voltage to 6 volts, as that resistor is current limiting to the led that should be changed to about 220ohm for 6v, it may not be turning on due to the 470 ohm.

The output on this circuit can also go through transistors for a higher out load. you can also omit the buzzer.

This is another circuit that works and has a higher output load.


This next circuit uses a schmitt trigger to switch on and off the circuit.
It turns on an output when the Touch-Plate is touched very briefly and turns off the output when the plate is touched for a slightly longer period of time.


Hope this helps you out.

02-14-2010, 09:57 AM
The touch concept here seems very similar to momentary switches actuating relays that make a seal in circuit.

02-14-2010, 12:57 PM
The circuit is called a darlington pair and it is a very common driver stage. The darlington pair is a simple amp that acts like a single transistor with high input impedance, low output impedance, plus it's cheap and compact. The current gain is pretty high about ~B1*B2. Generally, you see another amplifier stage as the input to a darlington pair and the darlington driving a load. In your case an LED but it's common to see a small motor.

I'm guessing your circuit is not wired correctly or you do not have enough potential to activate (or saturate) the second transistor's base to emitter junction. The base-emitter-base-emitter has to drop 2(6.5V) for the silicon 2N2222. That is, you have to supply sufficient base current to turn the circuit on. Check your biasing.

I'm not sure what you're trying to accomplish but the one shot above (4mem8) is a good idea if you want to hold state for some period of time after a switch is closed

03-08-2010, 02:23 PM
Hi shobley, Can you turn on the LED by shorting the two touch pads?