Following on from a recent teardown of a low cost appliance energy meter, I’ve done a bit more hacking of the device. As you may recall, I identified that one of the pins on the meter’s chip (EOUT) output a train of pulses proportional to the energy consumed. Some tests with a multimeter seemed to confirm this because the average voltage of EOUT changed with the appliance wattage.

I was hesitant to connect my logic analyser to the meter to measure the EOUT pin because of potential differences in voltage levels. To resolve, this I quickly whipped up a small board with a 4N25 opto-isolator to provide some voltage isolation between the internals of the energy meter and my logic analyser.

Again, I will repeat the obligatory warnings prevail. Do not:

• Attempt or copy any of this if you do not fully understand or appreciate the hazards of mains power
• Open the meter whilst connected to mains power
• Perform any measurements whilst the meter’s case is open
• Connect another mains powered device to measure the chip. The energy meter’s power supply is not isolated from active, neutral or earth.

The circuitry that allows for isolation of voltage levels is based around a 4N25 opto-isolator. An opto-isolator  allows for a signal to be transferred using light, this provides an airgap which provides electrical isolation.

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