There are many types of meters used to measure electrical power consumption. The modern meters are electronic and feed consumption data
directly back to the power supplier and usually have a diode light that flashes once for each unit of power used, generally 100 pulses per kWh.
These are the easiest to interrogate since all that is required is a method to detect and count the diode pulses. A simple photo-detector circuit
linked to a 1-wire counter will do this.
Unfortunately if, like me, you have a mechanical meter with a spinning aluminium disc as shown above there is no flashing diode so you need to
introduce your own. If you look closely at the disc you will notice that is is graduated 0 to 100 and that there is a black paint marking at the zero position.
We need a method to produce an electronic pulse each time this marker passes a fixed position.
There are photo-diode plus photo-detector components which will do this but they have a specified focus point and it is difficult to assess the
distance from the front of the glass face to the edge of the spinning disc. I elected to build my own detector using two ball-point pen barrels and
inserting a photo-diode in one and a photo-detector in the other. In this way I could adjust the direction and position of each until a
satisfactory pulse is produced when the zero marker passes. The pen barrels were then glued into a small plastic box which was then attached
to the meter face using 'prestik'. The detector was attached this way so that it could be removed if it became necessary to convince the meter reader
that you had not interfered with the meter.
The circuit I used to connect the photo-diode - photo-detector combination to a 'HobbyBoards" 1-wire counter is shown below.
The variable resistor adjusts the brightness at which the transistor switches on and off. Any general purpose low power transistor can be
used in this circuit. The 10kΩ fixed resistor protects the transistor from excessive base current (which will destroy it) when the variable
resistor is reduced to zero. To make this circuit switch at a suitable brightness you may need to experiment with different values for the
fixed resistor,but it must not be less than 1kΩ.
On the front plate of the meter along with the manufacturer's name, number of phases etc you will see a calibration value. In my case
it is 300 Rev/kWh that is 1 kWh of power usage results in 300 revolutions of the disc so all you need to do is divide the nuber of pulses
produced by the monitor device to get the kWh used. Normally the monitoring system such as the GHowSA I developed for myself will read the counter
every minute and therefore it is easy to determine the power used during that sampling period and produce an energy usage chart such as the one