Wattson Power Monitor

The Wattson Power Monitor is a device that monitors the AC current usage of three wires. It is used to monitor the usage of certain circuits in a building with the aim of pinpointing faults or inefficiencies in the electrical system (such as airconditioners being left on overnight). When these faults are rectified the user can drastically reduce their electricity bill.

My role in this project was to write the firmware for the Wattson Power Monitor. The basic requirements were as follows:

• The device was to run on a battery and have a battery life of over three years.
• The device should continuously monitor the current of three wires using current transformers (CTs).
• The device should upload the total charge used in dAh (deci amp hours) and the peak current draw (in amps) every 30 minutes using a Sigfox module to the Sigfox network.

Below you can see a photo of the Wattson's PCB. The microcontroller used is a PIC18F which is powered by a 20Ah primary lithium battery. The current transformers outputs are rectified and shunted through a resistor (a capacitor in parrellel with this resistor allows us to sample the circuit far less often, thereby saving power, and still achieve satisfatcory accuracy). The microcontroller samples the current transformers output curcuit 10 times a second.

For one of the initial tests we hooked three Wattsons up to the office's distribution board to see whether their readings matched each other well and whether they matched the expected calculated values.

As mentioned in the requirements the Wattson was to run on a battery for over three years. As you cant wait three years to verify the device will last this long we had to make sure that the device was using the expected amount of current. We used the Atmel Power debugger to profile the device's power usage. This tool samples the current at a very high frequency allowing it to see all of the peaks and troughs in the current usage ultimately giving us a very good idea of the device's power usage. The final, conservative, estimate was that the device would last for at least 3.3 years.