Very Simple Touch Switch Using 4050 CMOS Buffer

Many efforts has been made to make simpler circuit, and here is one result, a very simple touch switch.  You can build up to 6 similar circuit using only one 4050 chip. If you touch the “on” pad, the output will be on on remains on until you or some one the “off” pad. Here is the circuit schematic diagram:

When the power supply is first applied, the output will be high because C1 capacitor trigger the buffer gate, after the output get high then the input will not depend on the C1 charging anymore because it maintained by the R1 that connect the output to supply voltage level.

When the output is high, although the input is connected to the output via R1, if you touch the “off” pad, then the voltage of the input pin will be low because your skin resistance (below 1Mega Ohm) is much lower than R1 resistance (10 Mega Ohm). Because now the input is low, then the output will we low. Releasing your finger doesn’t switch back the output because now the input is connected to ground by the output via R1.

If you touch the “on” pad when the output is low, the input pin will be high because your skin resistance is much lower than its output grounding via R1. After the input goes high then the output will switch to high and now the output will maintain the input remain high after the touching gone, via R1.