Electronic touch switch

Introduction

This touch switch is a so-called hum-induced type, the advantage of this type is that you need only a single contact.

This is how it works:

Normally the capacitance of the touch contact to ground is very small.
When you touch the button, you will act as a large capacitance to ground (C_par human),
allowing a small AC current (I) to flow from the mains, (!) through the small parasitic
capacitance of the mains transformer (C_par transformer). and the rectifier (C_par bridge) of the DC supply into your finger.
This current will induce an AC voltage (V_ac) on the input of the inverter (pin 1 of IC 1A), which will produce an AC
square wave on its output (pin 2 of IC 1A).
As an experiment you can try to touch the probe of your oscilloscope: the voltage amplitude is quite high!

Circuit description

I used the touch switch type described above for the on/off switch and the input selection switch of my valve amplifier.

In the practical schematic below you see two practical touch switches, built with two schmitt trigger inverters, two resistors
a diode and a capacitor.

Normally, the input of the first inverter is low because of the 10M pull down resistor, consequently the output of the first
inverter is high.
When the output of the first inverter is high, the input second inverter input is also high because of the 1M pull up resistor.
Altogether, when you do not touch the button, the output of the second inverter is low.

When you touch the button, you will cause a 50Hz square wave on the output of the first inverter:
  • In the low period of the square wave the 100nF capacitor is discharged fast;

  • in the high period the capacitor is charged slowly.

    • In the end the capacitor will be discharged, causing the second inverter output to become high.

    Total front schematic:


    The upper touch switch controls a selector circuit built with a CMOS4017 counter.
    In this circuit, always one of the outputs of the counter is high. When you touch the button,
    the counter will switch to the next output until you reach the last one (output_3 in this case).
    After output_3, Output_4 will activate the Master Reset: the IC is reset; output_1 is high.
    Each output drives a relay and an indication LED through a buffer built with a BC847 transistor (=smd type BC547).
    The buffers and the CMOS logic use one 18 V supply.
    The diodes connected to the relay coils prevent high voltage peaks to destroy the transistors at switchoff.

    The lower touch switch controls an on/off toggle switch built with a CMOS 4027 JK flipflop.
    The J & K inputs are both connected to +18V (logic "1"). In this mode the output will be inverted every clock pulse.
    The RC network built with R4/C3 ensures that the relay is in off mode after the 18V supply is switched on.
    This prevens the amplifier to switch on accidentally due to a mains voltage dip.


    Note: when you use this circuit, do not connect the low voltage power supply ground to the mains earth:
    Cbuf will short circuit the small parasitic current (I) to earth. I encountered this problem when
    connecting my tuner and TV set to the valve amplifier: The shielding of the antenna cable of the tuner/TV is connected to
    both earth and the audio signal ground! In this way the internal ground of my amplifier was also connected to earth.
    To solve the problem I cut the shielding of the antenna cable and re-connected it with a small (330pF) capacitor
    In this way HF signals (>1MHz) can pass but LF signals (50Hz of the mains) are not connected to the shielding earth.

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