Simple touch switch circuit using transistor, 4017, 555 IC
A touch switch circuit operates with a touch action of an object or sensible material. It is used in hobby devices to high end consumer electronics for various applications.
Touch switch circuit using transistor
The given touch switch circuit works with an arrangement of two NPN transistors, which can obtain a high current gain for the input base signal.
In the circuit, the emitter of transistor Q2 is connected to the base of the transistor Q1. So the emitter current which is the sum of the input signal (base current) and the amplified signal of Q2(collector current) is again amplified by the Q1. Thus the resultant output has extra sensitivity to the input signal.
The given circuit is a basic design to just operate a single LED with a 3V supply. The circuit can be further modified for additional applications.
Transistor – Q1,Q2- BC547
Resistor – R1 – 1k, R2 – 220Ω
LED – Red
Supply – 3V battery
Piezo touch switch circuit using 4017 IC
In this touch switch, a piezoelectric sensor is used to sense the touch action. It can generate an electrical charge from any change in physical parameters such as pressure, strain, force, acceleration and temperature.
A decade counter IC 4017 is used to toggle the output for successive touch actions.
The clock input of the Decade IC has a positive edge triggering. Thus the output pin holds the state till another edge trigger occurs. The output pin has taken from output 2. It shifts the ON state from output 1 to output 2 on the positive edge trigger. The reset pin 15 of the IC has connected to the pin 4 (output 3 of the decade counter). So, it resets the counter on next triggering. Thus, the ON and OFF state repeats for consecutive touch actions on the sensor. The Output from the IC has connected to a transistor, which drives the Relay.
The sensitivity of the touch can be adjusted by varying the resistor connected across the pin 14.
IC – 4017
Resistor R1 – 1M, R2 – 10k
Transistor – Q1 – BC547
Diode – D1 – 1N4007
Relay – SPST – 12V,200ohms
Supply – 12V
Simple Touch alarm circuit using 555 IC
Here the circuit diagram for a piezo-sensor based touch alarm. Unlike from normal 555 touch plate switch, the piezoelectric touch switches are more sensitive and accurate in action.
The circuit consists of 7555 IC, which is the CMOS version of the 555 IC. The buzzer connected to the output pin 3 gets ON with a triggering pulse at pin 2. The triggering in a 7555 IC occurs when the voltage at the pin 2 drops below 1/3 of the supply voltage. Here on touching the piezoelectric sensor the vibration or pressure generates a proportional voltage, which triggers the circuit since it drops the voltage.
The alarm gets turned OFF after a short time period. Once the circuit triggered, the output will be ON, until the voltage at pin 6 reaches 2/3 of the supply voltage.
The ON time can be adjusted by varying the 1M potentiometer in the circuit. The ON time can be calculated as,
Time period = 1.1 R C.
Here this circuit can obtain a maximum ON time of 55 seconds, by adjusting the resistance to a maximum of 500KΩ.
IC1 – 7555
Resistor – R1 – 1M, R2 – 1M pot
Capacitor – C1 – 100uf, C2 – 100nf