The circuit described here is a Touch Switch. Besides being an autonomous device, it can be also used as a building block that you can add to other designs.
The Touch Switch detects the resistance of your finger by using a pair of pads. When touching the pads a relay turns immediately on. By touching again, the relay turns-off. When the finger is removed from the pads, the circuit remains at its last state. No matters for how much time the pads are touched, the circuit will always work.
The circuit uses NAND CMOS gates, having high input impedances. When a small current passes through a high resistance, creates a fairly significant voltage drop, which is exploited to drive a relay. There are 4 NAND gates and a J-K Flip Flop. U1A is used as an inverting amplifier and the other 3 NAND gates are used as a Schmitt trigger.
Normally, U1A, and U1B have logic-0 and logic-1 levels at their inputs, respectively. When the pads are touched, a small current passes through R2 and creates a significant voltage drop at its leads. This voltage changes U1A's inputs at a logic-1 state. At the same time, the voltage at U1A's output becomes logic-0, a current passes through D1 and charges C3. After some fractions of a second, C3 becomes fully charged, U1B's inputs becomes logic-0, and U1B's output becomes logic-1. Logic-1 at U1B's output triggers U1C and U1D R-S latch, which in turn, triggers U2A Flip -Flop. Q1 goes to saturation and the K1 relay is turned on.
When the finger is removed from the pads, C3 gradually discharges through R4 and NAND gates return to their initial states. However, U2A does not return to its initial state and the relay remains energized. This is due to the fact that J and K inputs of U2A are connected directly to logic-1, thus making the J-K Flip-Flop to act as a T-Flip Flop and toggling at the rising edge of every clock pulse. U2A will change its state after a new clock pulse arrives, and this will only happens by touching the pads again.
C3 is actually used in order to provide a small delay. This is useful for preventing the circuit from triggering due to noise-spikes (instant voltage spikes due to noise) at the pads. Q1 is used to provide sufficient current for the relay coil. R5 is used for adjusting current flowing through D2, which is a simple LED indicator. The D2 LED lights whenever the switch is on. D3 is used to prevent Q1 from being damaged due to the reverse current induced from relay's coil inductance.
The prototype needs to be powered from 5V. It absorbs about 1mA at its idle state (when relay is off) and uses a small DPDT 5V-relay. After some minor modifications, the circuit can be also powered from any voltage source in the range of 3 to 15V. Modifications include altering the value of R5 and replacing the 5V-relay with another one, having appropriate voltage specifications. Using the PCB provided below, you will be able to build the Touch-Switch in no-time.
Touch switch Printed Circuit Board details