In the figure we present a touch sensor that operates by detecting the 60 Hz snore of the power grid. This snoring appears in the form of a rectangular signal at the output of the first inverter (pin 3), being then rectified and applied to the input of the second inverter. The resistor R4 and C1 form a network that filters this signal, providing a constant output on pin 4 of the second inverter. The signal can be used to activate a power step with a relay and a transistor, as in the previous circuits of that article, in which case high power loads can be controlled. Remember that this circuit must have a power supply completely isolated from the power grid. Likewise, the controlled circuit must be completely isolated, whether by relay, optocoupler or other resource. The power supply will depend on the circuit that must be activated. We observed that the circuit consumption is extremely low in the standby condition, not being greater than 1 mA even with a 12 V voltage. Greater sensitivity can be obtained with the increase in R1. The maximum recommended value is of the order of 44 M ohms, obtained with two 22 M resistors in series. The sensor consists of a metal plate that must be touched with your fingers. This flat iron must be connected to the circuit using a short wire. If the wire is long (more than 40 cm) it must be shielded with the mesh properly grounded.
