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2 January 2011
Author: Giorgos Lazaridis
How Touch Buttons work

The Resistance and AC Hum touch switch

I was fascinated with the look and feel of the touch button when i first turned on my brand new TFT monitor

Mechanical buttons and switches cannot become obsolete for various reasons. First of all because of the unmatched reliability, and also for the tactile feeling, which stimulates the finger's touch feeling. But they can never be as elegant and flexible as a touch button. There are many different technologies to make a touch button, others applied on surfaces, others on LCD displays, others behind glasses, others on metallic surfaces etc etc etc. The most common ways are the resistance, the capacitive and the inductive touch buttons. In this theory page, i will explain in details the following types of touch switches (which are actually the most popular and easy to make):

• The Resistance touch switch
• The AC hum touch switches
• The Capacitive touch switch

• The Resistance touch switch

The resistance touch switch is based on the fact that human tissues (like the skin), have a great amount of water and salt, something that makes it conductive. Look at the following drawing:

This is the most basic type of touch switch, in its simplest form. The operation is simple. A transistor is used as a switch. The RB resistor keeps the base to ground when no electrode is touched, so it does not float. The RP resistor is for protection, in case that the electrodes are short-circuited, and it prevents an over current that will probably toast the transistor. If one of the electrodes is touched, nothing will happen. But if both electrodes are touched together, then a small amount of current will flow through the skin to the base of the transistor. The transistor will then go from cut-off to saturation, and current will flow from the CE region to the LOAD.

There are numerous different ways to implement a resistance touch switch, with transistors, with 555, with 741, with CMOS and many more. The idea is always the same though: Two electrodes are used for the touch plates. Current flow from the human skin from one electrode to the other, which finally stimulates an amplifier or another current sensitive part. This method is straight forward and no more info is needed.

Disadvantages: Not very flexible in design, two electrodes required

The AC hum touch switches

My body induces 800 mVolts 50Hz. I'm a living generator! Morpheus would agree, right Neo?

I was not sure if i should mention this type of touch switch. I finally decided to include them, because it seems than many people tend to confuse the AC Hum touch sensors with the capacitance touch sensors. The AC hum sensors are based on the fact that -again- the human tissues and skin are conductive, and also that AC lines produce a magnetic field. Any conductive material placed inside this magnetic field, will produce a slight potential between the item itself and the earth. The same happens of course with the human body.

An AC Hum touch sensor uses a high gain amplifier such as a darlington pair to amplify this parasitic voltage. When 50 Hz (or 60 for some countries) are detected, it will arm the load. Here is a basic circuit with such a sensor:

The amplified (and inverted) AC signal that comes from the darlington pair is smoothed on the capacitor C. This capacitor acts as a basic frequency to voltage converter. The PNP transistor acts as a switch for the load. If a slight voltage is applied to the sensor plate (upon finger touch for example), the overall voltage across the capacitor will be reduced, because it is not powered any more with DC current, but from the amplified 50 (or 60) Hz AC current. The PNP transistor will then sense this change and the CE contact will become conductive, providing current to the LOAD.

This design has a few problems. First of all, in order to operate, there must be power lines near by. And i do not mean right above the body, but in the near. In a room for example there are plenty of power lines. But if no light is turned on and no appliance is operating, then no current flows within these power lines, thus no field is generated. So, if you plan to make a light switch with this, then keep in mind that most probably, the light has to be already turned on for the switch to operate (!!!).

And something else... What do you think that will happen, if you walk bare-foot on the cement? Right! A huge amount of the voltage will be grounded to earth through your feet. It is most likely, that the remaining potential will not be able to trigger the base of the darlington transistor, unless you have some serious amplifier over there.

Result: This is a nice educative circuit to study and learn, but in most cases completely improper to use as a switch. Maybe could fit to other applications.

Advantages: Simple circuit, with one single sensor electrode only
Disadvantages: Unreliable in operation - Require power line near by