This simple circuit can be used to activate whatever you like, for example, by connecting it to the microcontroller, relays, secret alarms, or robot applications or just turning on LED1 which lights up as long as you touch the metal plate. The circuit consists of voltage dividers R1 and R2, one Schmitt trigger/inverter gate from a 40106 IC, a small capacitor to keep strong RF at bay, and LED1 with current limiting resistor R3. The metal plate is connected via a wire to R1. R1 and R2 together form a voltage divider. Since the current from your body is very small it’s understood that R2 has a high value like 10 Megohm to maximize the voltage over R2 so it can be detected by input pin 1 of gate IC1.A. R1 has been added to prevent electrostatic discharge (ESD) energy from damaging the inverter gate input. ESD may occur when you have been charged with an amount of electrostatic energy by walking on a carpet with rubber soles.
You can increase the sensitivity of the detector by experimenting with lower values for R1 e.g. 1 kΩ and a smaller metal plate. The value of pull-up resistor R3 is calculated such that the current through LED1 is below its maximum continuous rated value. Most regular LEDs are 20 mA types. The circuit still works if you remove LED1 and just have the pull-up resistor R3 connected to output pin 2 and then connect a microcontroller input pin directly to pin 2. Do check however that the microcontroller has a weak pull-up (i.e. to +VDD) at its port line.
The CD40106B device consists of six Schmitt-Trigger inputs. Each circuit functions as an inverter with Schmitt-Trigger input. The trigger switches at different points for positive- and negative-going signals. The difference between the positive-going voltage (VP) and the negative-going voltage (VN) is defined as hysteresis voltage (VH). The CD40106B device is supplied in ceramic packaging (J) as well as standard packaging (D, N, NS, PW). All CD40106B devices are rated for –55°C to +125°C ambient temperature operation.