Computing stuff tied to the physical world

Picking up magnetic energy

This is the start of a new project. The goal is to detect when an AC mains appliance is turned on and send out wireless packets when this is the case. An important requirement in this project is to do so without direct (galvanic) connection to AC mains. In fact, we don’t want any external power source – no batteries, no wires, nothing. Just a little gadget that sits next to the appliance in some way, and reports its activity over wireless – forever.

We’ll call it the “Micro Power Snitch”.

Powering up a micro-controller and sending out wireless packets requires energy. We’re obviously going to need to “harvest” this energy from somewhere. In this project, the goal will be to convert magnetic energy into a tiny little power source, just enough to drive an LPC810 µC with attached RFM69 wireless radio.

The device to make this happen is called a current transformer (CT), such as this one:

DSC 4947

Think of it as a special kind of transformer, such as this diagram, courtesy of Wikipedia:

763px Transformer3d col3 svg

… except that one of the windings is a single “loop”, and the other is about 1000..2000 turns, inside that blue hump at the top. The ring itself is made of ferrite material, which captures the magnetic field. The ring is actually two halves, so the whole device can be opened and clipped around an existing wire without having to cut or break that wire.

When we put the CT around a (single!) phase of a power cord, and there is current going through it, then the two wires coming out of the CT will show a signal which looks like this:


The voltage is cycling at the same rate as mains power, i.e. 50 Hz, and as you can see, this one alternates between about +5V to -5V, in a somewhat odd shape. It’s clearly not a sine wave. The shape of this signal will not change when more power is being consumed by the AC appliance, only its amplitude.

Note that we have yet to determine whether there’s enough energy in here to drive a µC and a radio module, but this is essentially the power source we’re going to have to work off.

As you will see, this is an extremely weak power source. We’ll need to use lots of tricks to even stand a chance of reliably running off this energy supplier.

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