By Ritu Panday
How this simple mains electronic touch switch circuit works
The indicated 4 diodes form the basic bridge diode network, the thyristor is used for switching the mains 220V AC for the load, while the IC M668 is used for processing the ON/OFF latching actions whenever the touch switch is touched.
The bridge network rectifies the AC into DC through R1 which limits the AC current to safe level for the circuit, and VD5 regulates the DC suitably. The final outcome is a rectified, stabilized 6V DC which is applied to the touch circuit for the operations.
The touch plate is connected with a current limiting network using R7/R8 so that no shock sensation is felt by the user while putting finger on this touch pad.
The various pinout functions of the IC can be learned from the following points:
The supply positive is applied to pin#8 and ground to pin#1 (negative)
The touch signal on the touch pad is sent to pin#2, and the logic is transformed into an ON or OFF at the output pin#7.
This signal from pin#7 subsequently drives the SCR and the connected load into either ON or OFF states.
C3 makes sure that the SCR is not false triggered due to multiple pulses in response to an improper or inadequate touching on the touch pad.
R4 and C2 forms an oscillator stage for enabling the required processing of the signals within the IC.
A synchronization signal from R2/R5 is divided internal through pin#5 of the IC.
Pin#4 of the IC has a very crucial and interesting function. When connected with the positive line or Vcc, the IC enables the output to alternately toggle ON/OFF, allowing the light or the load to switch ON and OFF alternately in response to every touch on the touch pad.
However when pin#4 is connected to the ground or the negative line Vss, it transforms the IC into a 4 stage dimmer circuit.
Meaning in this position every touch on the touch pad causes the load ( a lamp for example) to reduce or increase its intensity sequentially, in a gradually dimming or gradually brightening manner ( and OFF at the ends).
If you have any questions regarding the functioning of the above discussed mains touch switch circuit please write them down through the comment box...
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