We’re now going to build a contactless, AC mains detector circuit. Before we start looking at the circuit, let’s try to understand the difference between DC and AC.
We know that the battery we are using gives us 3V across the positive and negative terminals. If you plot the voltage against time, it would look something like the above – a steady potential difference of 3V across the terminals. When we connect a circuit to this battery, current will start flowing in only one direction and this is called direct current or DC.
If we were to plot the mains voltage then it would look something like the above. The neutral or common terminal will have a potential of 0 with respect to the earth, while the live terminal will have positive and negative cycles. When a circuit is connected to it, current will start flowing back and forth as seen in this simulation. This is said to be alternating current or AC.
When current flows through a wire, it generates a magnetic field. If the current changes, as is the case with alternating current, then the magnetic field produced will change as well.
Conversely, if we place a wire into this changing magnetic field, it will induce a voltage across the ends of the wire and if we connect this wire to a circuit, it will cause current to flow through it. The amount of voltage and current depends on the strength of the magnetic field and the properties of the two coils. Generally, a stronger magnetic field will result in a higher induced voltage. This is the principle used in transformers and we will make use of it in this circuit.
The magnetic field produced by ordinary household wires carrying alternating current is very weak and this is further reduced by the insulation. We can coil a jumper wire to increase the magnetic coupling which will increase the induced voltage.
This voltage is still very small and insufficient to drive an LED so we will connect the wire to the base of a transistor to amplify it.
We know that the BC547 has a gain of approximately 200 and this is not going to be enough to produce an output current that can drive an LED. We add another transistor to amplify this further, giving us the circuit as shown. The output of this is fed to the final transistor that can be used to control an LED or some other component.
Let’s build the circuit and verify its operation by placing the coiled wire close to a wire carrying AC mains.
This circuit does a good job in detecting alternating current but it is also susceptible to noise and stray magnetic fields. If you want to get the same performance as a commercial product, then you would need to add some sort of shielding to reduce the impact of stray magnetic fields. Now let’s move on to design some basic alarm circuits.