Touch Switch

TOUCH SWITCH

Presented by,

A.Charumathi.

Contents

• What is a touch switch?

• Types

• Circuit A

• Circuit B

• Circuit C

• Applications

• Conclusion

What is a touch switch?• A touch switch is an

electronic device that enables us to control a circuit by simply touching a sensor.

• This is done by the concept of touching a plate (or two plates separated by a small gap) and turning a circuit ON or OFF.

• A TOUCH PLATE is a high impedance device (or high impedance circuit) as the effect of a finger will be detected by the circuit connected to the plate.

• If the circuit has two plates, it will be registering the resistance of your finger. If the circuit has 4 plates, it will use two to turn the circuit ON and two to turn the circuit OFF. 

Types• Touch a set of pads and the project turns on. When the finger

is removed, the circuit turns off. The finger can touch the pads for any length of time. We also include the feature where the circuit extends the ON period, so the circuit stays on for a length of time after the finger is removed. This is shown in Circuits A.

• Touch a set of pads fairly quickly and the project turns on. Touch the pads again for a short period of time and the circuit turns off. This is called the "Flip-Flop" effect. If the finger is kept on the pads, the circuit will turn on-off-on-off at a rate of about once per second. This is shown in Circuits B.

• Touch one set of pads to turn the circuit on and another set of pads to turn the circuit off. This is shown in Circuits C.  

Circuit A

• It is called a "super-Alpha pair" and is actually identical to a single transistor with a very high gain. Putting a finger on the touch pads turns the top transistor ON and this transistor turns on the bottom transistor. When the finger is removed, the circuit consumes less than a microamp

High impedence input?

• The output will go low and your finger will not be able to create a HIGH-LOW voltage change.

• The input impedance of a gate can be considered to be very high (greater than 10M).

• When the "super-alpha" pair is connected to the touch switch, the voltage on the "output" of the touch pad will not rise above 1.3v.

• This is due to the base-emitter junctions of the two transistors.

• The output of the super-alpha pair will be low. When a finger is placed on the touch pads, the output of the super-alpha pair will rise. 

Using 555 Timer

• The 555 consumes 8mA, at all times when the supply is connected. The circuit turns on the LED when the finger is applied and pin 7 becomes "open circuit." This allows the 10u to charge via the 100k resistor and when pin 6 detects a HIGH, the LED turns off. The finger should be removed before this occurs.

Circuit B

• The following circuits show a "flip-Flop" effect. The circuit changes state, each time the touch pads are touched.  

Using Gates

Using IC

Improved Circuit

Circuit C

• These circuits have two touch plates. One touch plate turns the circuit on and the other plate turns the circuit off.

Using 555 Timer

Using IC

Touch Switch in projects

• Touch buttons consist of a small carbon block mounted in silicon rubber and when the button is pressed, the carbon block touches the pad and reduces the resistance between the two interleaved tracks.

• When a finger is applied, the resistance between the lines drops to between 150k and 850k, depending on the pressure and moisture in the finger.

Applications

• Doorknob Alarm

• Relays

• Capacitance to Digital converter technology….etc.

Doorknob Alarm

Entire circuit

Conclusion

• It is very simple .

• It is cheap.

• It takes only 100ns to operate the circuit.

• It is used in various applications due to its quicker response.