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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions will be paid for at standard rates. All submissions should include full name, address & phone number.

PICAXE-based wireless electricity monitor

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This circuit uses PICAXE08M2 microcontrollers and low cost 433MHz ASK transmitter and receiver modules to provide a wireless link between the meter box and a remote display inside the house.

Modern (electronic) watt-hour meters have a LED indicator that usually pulses at a rate of 1000 times per kilowatt-hour (kWh), ie, once per watt-hour. A phototransistor is positioned in front of the LED and a PICAXE08M2 (IC1) calculates the time between flashes, using the pause function and interrupt capability of the chip; an interrupt is generated each time input P3 goes low.

The duration in tens of milliseconds is calculated and is transmitted using a 433MHz ASK transmitter (Jaycar ZW3100 or similar), along with a station identifier to ensure that the receiver does not respond to other transmitters. Each transmit packet is indicated by a single flash from LED1. (A light dependent resistor may also be used if a suitable phototransistor cannot be easily obtained).

The receiver comprises a 433MHz module (Jaycar ZW3102 or similar) that feeds the pulse data to a PIC­AXE08M (IC2). The average power, corresponding to the interval measurement, is calculated by dividing 3600 (the number of seconds in an hour) by the time between pulses in seconds; one second between pulses = 3600W; 10 seconds = 360W etc.

The calculated values in kWh, watts and the pulse time are displayed on a standard 16-character by 2-line LCD (Jaycar QP5517 or Futurlec LCD 16x2). IC3, an 8-bit I2C port expander (available from Futurlec), is used to provide sufficient outputs for the PICAXE08M2 to drive the LCD display. The three address lines of the port expander (A0, A1 & A2) are tied to the +5V line. Two 4.7kΩ pull-up resistors are provided for the I2C clock and data lines. LED2 flashes whenever a data packet is received.

For testing and timing calibration purposes, a third PICAXE08M2 was configured to provide pulses from one to 50 seconds in 0.5-second increments into pin 3 of the meter box transmitter module. Adjusting the pause routine and making some allowances for the transmit time has provided a reasonably accurate result. For example, resolution is about 6W for readings of 3600W, changing to better than 1W for readings below 360W.

Power for the meter box unit is derived from three 1.5V alkaline AA cells while a regulated 5V DC plugpack supplies the receiver module.

The resistors associated with the 3.5mm stereo socket in each circuit provide the standard PICAXE programming interface.

Because the metal meter box is an effective shield, a short antenna for the transmitter module is inadequate. To obtain adequate range, an external antenna was used, comprising a 2.5m length of coax cable with the screen stripped for about 170mm at the far end. The core insulation was left in place and the open screen end was covered with a small heatshrink sleeve. To hold the antenna up, a used hard disk drive magnet was strapped to the base of the antenna with a cable tie and this was then attached to the top rail of a nearby steel fence.

The phototransistor was secured with a bracket made from a small piece of thermoplastic that hooks over the meter body. A small blob of Blu-Tack® helps to keep it in place. The bracket should not obscure the meter’s LCD or the optical in/out port and care must be taken so as to not disturb any of the electrical equipment in the meter box.

During the development of this project, it was apparent that an increase in load could cause the monitor to show a lower power reading. It was soon realised that my rooftop solar system was generating at the time and the mains meter pulse does not distinguish between import or export power (the metering is a net arrangement).

Under these conditions, a small increase in demand will reduce the exported energy. A large increase in demand can invert the export to import. The inability of this monitor to determine power flow direction is a drawback, however it is possible to identify individual appliance loading by switching the item off or on for a short period and observing the meter monitor.

Of particular interest is that I found that the minimum background load overnight was around 100W (refrigerator not running). This was due to numerous items still operating or on standby, including TVs, personal video recorder, USB hard drives, bedside clocks, night light, broadband modem, wireless router, network attached storage, fire detectors, microwave and wall oven clocks, electric gar­age door and watering system transformers. These background loads are operating continuously, with an estimated annual cost of over $200.

Phillip Webb,

Hope Valley, SA.

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