Single Probe Touch Switch – B1P16

It’s not always convenient to have two probe points for a touch switch so we’re now going to build a circuit that can be used as a trigger for a touch alarm or doorbell circuit.

Single probe touch switch - schematic

Here’s what the circuit looks like and we have a new symbol so let’s talk about that first. It’s no secret that there are a lot of radio frequencies around us and we tend to pick up a bit of this in the form of small voltages or potentials. An oscilloscope is a device that can be used to analyse waves and signals.

Stray AC mains voltage

Here’s what it measures when I simply touch the oscilloscope probes – a 50Hz signal. 50Hz is the frequency of the AC mains and that’s the signal that is being picked up here.

Stray AC mains voltage

This signal is not very strong but it is enough to trigger a transistor if the circuit is connected correctly.

Remember that current needs a path to return to the source and a circuit provides this path. If we want to utilize this stray Ac mains signal then we would need to provide a return path for it in our circuit. That’s what the additional symbol is for. It tells you that you need to connect the negative terminal of the battery to some ground terminal around your house. This could be a metal water pipe or a metal chassis of your computer etc.

Mains reference - ground potential

I’ve plugged this USB cable into an AC mains charger and I’ve wrapped a wire across the end which I’ll be using as a return path.

Single probe touch switch - simulation

We have used the other components before so let’s take a look at the simulation. The circuit uses the stray AC mains signal as a trigger so we can replace the probe with a switch connected to the positive terminal for the simulation. Q1 is the NPN transistor while Q2 is the PNP transistor. When we touch the probe, there will a small positive signal that will switch Q1 ON. When this happens, the capacitor will start charging and it will also switch ON Q2 and the LED. When this positive signal is removed, Q1 switches OFF, but since the capacitor was charged previously, it starts discharging. Once Q2 is completely discharged, the base of Q2 is pulled up to its emitter voltage which switches it OFF. The cycle repeats when the probe is touched again.

Single probe touch switch - layout

Let’s use the breadboard layout to build the circuit. Remember to connect a link which will be used as the earth’s reference point.

Single probe touch switch - demo

You should be able to trigger the circuit by simply touching the probe point with a finger.Let’s move on to the next project.