How can I make simple buttons using humans?

[N.Carsane]

Hi everyone, I have seen a few of these USB boards that you can touch with your fingers and it works as a mouse etc. I think they use some sort of transistors and a potential divider and using this idea i want to make a human operated switch but can't seem to figure out a schematic...
I will appreciate any help! Thanks a lot!

 

[DerStrom8]

Do a google search for "Resistive Touch Switch" or something of that ilk.

**broken link removed**

 

[N.Carsane]

I found the following types :

• The Resistance touch switch

• The AC hum touch switches

• The Capacitive touch switch

Which one would provide a 'clean' switching action and what values of resistors (and capacitors) should i use? and mainly whis NPN transistor should i use? A BC547? or something else?

 

[DerStrom8]

Resistive touch sensors, like the one in the image I posted, use a human finger as a resistor. Paired with another resistor, it creates a voltage divider so that when it is touched, just enough current flows into the base of the NPN transistor to turn it on.

AC hum switches detect the mains hum that your body picks up. You can easily see this hum represented if you touch the probe of an oscilloscope. Your body acts as an antenna and picks up AC signals that are emitted by the mains connections in your house.

Capacitive touch switches are similar to resistive touch sensors, except that they use your body as one plate of a capacitor instead of a resistor.

Any of them can provide a 'clean' switching action provided you have the right circuitry. I suggest you just pick one and research just that style. The benefit of a capacitive touch switch is that it only needs a single plate, whereas resistive touch switches require two (one for each side of the "resistor" -- your finger). However, reliable capacitive touch switches area bit more difficult to build. Simple ones can be built with a 555 timer and a small metal plate (or even a wire), but they can also be set off in the presence of static electricity, electromagnetic fields, etc. Resistive touch switches tend to be the most reliable, even though the touch pad has to have two separate halves. Try googling strictly "resistive touch switch" and you should find dozens and dozens of designs. Pick one and build it! They tend not to be very fragile, so don't be afraid to mess around with the resistor values on your own. Experiment and see which arrangement works the best.

Regards,
Matt

 

[N.Carsane]

Thanks a lot!