A Simple Touch Switch – B1P15

Touch switch circuit - schematic

We’re now going to build a touch switch with a small delay. We’ve previously switched ON a transistor using skin resistance and we’re now going to extend this principle to build a circuit that has a latching effect. We’ve used all the components before so it’s best we jump straight to the simulation.

Touch switch circuit - simulation

We have two probe points and we’re going to replace them with a switch for the simulation. We also have an NPN and PNP transistor along with some resistors and capacitors.

Touch switch circuit - simulation

When the switch is open, the circuit is in the idle state. If we close the switch, a couple of things happen. It switches ON the LED and Q2 as they both now have a path to ground. The capacitor also begins to charge through R3 but no current flows through the Q1 as the switch pulls the collector pin to ground.

Touch switch circuit - simulation

When the switch is opened, the collector of Q1 is no longer pulled to ground and current begins to flow through it. This keeps the base of Q2 at a lower potential as it continues to stay ON. The capacitor now begins to charge through Q1 as it has a lower resistance path to ground. As the capacitor charges, the current flowing through it will start to decrease and this means that the voltage at the base of Q1 drops as well until it reaches a point where Q1 switches OFF. This causes Q2 to switch OFF as well. The capacitor starts discharging through R3 and R5 and once it discharges, the circuit can be triggered again by closing the switch or touching the probe points.

Touch switch circuit - layout

Let’s use the breadboard layout to build the circuit.

Touch switch circuit - demo

We can trigger the circuit by touching both the jumper wires with a finger and you can then watch the latching effect. As observed in the simulation, the capacitor has to discharge before the switch can be triggered again.

In the next post, we will learn how to build a circuit that uses a single probe point.