TOUCH SWITCH

Part 1 of project

For complete project go to http://electronicsforengineers.blogspot.com/search/label/TOUCH%20SWITCH%20PROJECT

INTRODUCTION

Touch switch is a device which produces high output(led glow) when its plate is touched. It is basically used for security systems. The actual concept behind touching the plate of touch alarm is its grounding. It glows led only for the time for which it is in contact with the ground.

The main component used in touch switch is 555 timer. The 555 timer has high output for certain time interval which depends upon the triggering of device. This output is used to glow the led.

PRINCIPLE

The touch switch is based on the principle of triggering of 555 timer by a negative pulse which is given by touching its touch-pad. Once triggering is done, then oscillations are produced and circuit starts working. When it stops working after some specific time interval, it will not start again by itself, until a new trigger is given.

COMPONENTS USED

a) Resistance :

Here two resistances are used of 100 k and 4.7 M Ω as shown in circuit diagram. The function of the 4.7 M Ω resistance is to control the sensitivity of the touch switch.

b) Voltage Source :

The 9 V DC battery is used to provide electrical energy to the circuit.

c) 555 Timer :

The 555 timer is the main component used. It is a pulse-generating circuit consisting of 8 pins. Pin 1 and pin 2 act as a touch plate of the touch alarm and the battery is connected to the 8^th pin.

d) Led: Led glows when plate is touched.

continued to TOUCH SWITCH 2

TOUCH SWITCH 2

part 2 of the project

For complete project go to http://electronicsforengineers.blogspot.com/search/label/TOUCH%20SWITCH%20PROJECT

CONSTRUCTION

This circuit uses a 555 timer as the bases of the touch switch. You can learn more about 555 timers in the Learning section on my site. When the plate is touched the 555 timer is triggered and the output on pin 3 goes high turning on the LED and the for a certain period of time. The time that the LED 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 touch.

WORKING

When the plate is touched, 555 timer is triggered and the output on pin 3 goes high which turns the buzzer on for certain period of time.

Now let us see what actually happens in 555 timer when the plate is touched.

As shown in figure, when the plate is touched, actually a negative going pulse is given to the pin 2, due to which the output of the comparator goes high. This transition of the output of comparator leads to the production of pulse. This changes the state of the flip-flop from low to high and hence the output of 555 timer goes high as shown in figure. This output is used to glow led. Output remains high as long as triggering pulse is applied. Due to this reason, the buzzer sounds as long as the plate is touched and it stops immediately after the touch is removed.

When output of the timer is high, it turns off the transistor Q1. Now the resistance Ra is open circuited. Now the voltage at point A is +Vcc which turns the output of comparator 1 high which further changes the output of the flip-flop from high to low state and hence the output of 555 timer becomes low and the led stops glowing..

RESULT

When touch plate is touched then led glows.

ADVANTAGES

• The touch security is cheap in cost and hence can be used for house security at doors.

• It is useful circuit for dusty conditions.

• It is very useful in rainy conditions.

DISADVANTAGES

Since 555 timer works in a range of 5 to 15 V so it can't be used for higher voltages and hence its sensitivity is less and can't be used for security of banks etc. APPLICATIONS

The major applications of touch switch are given below :

a) It is basically used in security systems in banks, homes, and other private sectors and offices.

b) This device can be implied to continuously check out the grounding of other appliances.