This cheap touch switch circuit is the touch switch is updated from the original version. Which use IC to control the operation cause difficut for the electronics beginner. So this circuit is thought to are suitable because uses TR purely. When was broken also a simple repair. and More cost-effective equipment. But The application is easy namely when we want to relay connected A and B contract. But when we want to relay release We also touch on the B and C.
The working principle
When we touch at point A and B will be has some current from point B through our finger to point A. Which base of TR2 this current will makes TR2 conduct current between pin C and E. This current will pass R-820 ohms in to pin B of TR3 cause TR3 conduct current as well.
Figure 1 the cheap touch switch circuit using transistor
And this current will pass from pin BE of TR4. When TR4 has current flow through pin E-B will make it also conduct between pin E-C. Thus relay so derived the current and there is LED glow to indicate to we know once that the relay is running now.
While at pin C of TR4 will has higher voltage nearly equal this point voltage that through R 4.7M to pin B of TR2 cause current flow continuously even we will release the touch point. The relay will remain active. When we want to make the relay stop working. We also touch points B and C. The some current will flow through the our fingers from the point B to point C.
And go to pin B of TR1, cause TR1 conduct, thus current pass through a R-4.7M instead will in pin B of TR2 , Returned to the ground via pin CE of TR1, making TR2, TR3, TR4 not conduct current, stop conduct the relay so release contact apart. Until the touches point new AB.
How to builds
This circuit use the 9V power supply and all resistors use size ¼ W, Relay size 9V rate of flow resistance of the relay depends on the needs of applications. But usually use the 1-3 A. D1 is for reverse voltage protection relay to the circuit. The C-10uF 16V makes supply voltage is smooth. and C-0.01uF, C-22uF prevents noise from the air that passes through the fingers when touched. You can see the PCB layout in Figure 2. and the components layout as Figure 3 which if you are lover of electronic will build it easily sure.
Figure 3 the components layout and wiring of the cheap touch switch.
The components list
TR1-TR3______C1815 or C828____45V 100mA NPN Transistor
TR4_____________CS9012____________45V 800mA PNP Transistor
LED1____________LED as you need
C-10uF 16V_______Electrolytic capacitor
C-0.22uF 50V_____Electrolytic capacitor
C-0.01uF 50V_____Ceramic capacitors
R-470K________1/4w +5% Resistors