The AC power line indicator presented here has a complete galvanic isolation from the grid. The indicator is an LED that lights up when a current flows, although the current can be measured more accurately with an AC voltmeter set to its mV range. The detector is a transformer taken from an old cell phone charger. The value of the secondary isn’t important because we only make use of the primary winding only (normally 115 V in the US). The (extension) cable through which the current has to be detected should have an as short as possible section of its outer insulation removed. The wires should then be moved apart. The blue wire should be placed on top of the transformer and the brown wire underneath, or the other way round. The brown and blue isolation shouldn’t be removed, so there is no danger of the AC line voltage becoming exposed. If there is a green/yellow wire as well, this can be placed on either side of the transformer. The brown and blue wires should be in parallel with the windings on the transformer. The secondary winding(s) should be left open circuit so that they don’t attenuate the measured signal.
In our prototype, we found that an alternating 60 Hz voltage of about 2 mV was induced when a 30-watt soldering iron was connected to the extension lead. With higher-powered devices, the measured voltage rises proportionally. Since it is unlikely that the iron core of the transformer will ever become saturated, the relationship between the measured voltage and the current flow should be fairly linear.
The transformer output signal is amplified by a differential amplifier built around T1 and T2. If you wish, you can connect an AC voltmeter across the collectors of T1 and T2 to get an indication of the size of the current. The rest of the circuit takes care of lighting up the LED when a current flows through the (extension) cable. The measured signal is amplified again by T3 and then T4 is used to drive the LED with a 60 Hz square wave. A 9 V battery is suitable for the power supply.
When a capacitor is connected in parallel with the primary winding of the transformer it can make the circuit less sensitive to frequencies other than 60 Hz. Ideally, the circuit should resonate at exactly 60 Hz. This will make the circuit most sensitive. The capacitor should be chosen such that the measured signal across the collectors of T1 and T2 is at a maximum for a certain current flow. However, the capacitor isn’t vital and the circuit still works well when just the transformer is used. When a low-current type is used for the LED, R13 can be increased to 1.2 kΩ (≈ 5 mA max. for D1).