Power consumption of various devices is becoming a concern. Various commercial meters of different price, capabilities, and quality are now appearing on the market. Their function is usually integration of the power consumed by an attached appliance, and calculating its cost. Some such devices are fairly cheap; the FH9999 one was the cheapest one available at the moment, so it was selected for experiments.
Many such meters are built around a common chip, used also in residential power consumption meters. One of such chips is ADE7755, used in this device. It measures the mains voltage and current through the shunt resistor, integrates the power consumed by the load, and outputs a pulse for each power unit. The number of pulses per unit is configurable by selector inputs; a high-speed output, with low amount of power per pulse, is used in this device. (Power meters, which can contain mechanical counters, can use lower speed pulses where one pulse means more power.) The datasheet of the ADE7755 is available. The schematics was reverse-engineered over the course of one evening.
The metering chip outputs pulses from pin 22. The pulses are active-H, about 4-4.5V high, and about 18 microseconds wide. These are connected via a three-pin connector to the display board, where the microcontroller that handles all the other functionality is located.
As knowing the time-power dependence may be crucial for optimizing the power consumption, a way to log the power over time is needed. This meter however indicates only the total power, omitting the time dependence. A way to output the immediate values is therefore desired. The easiest way is to output optoisolated pulses directly from the metering chip.
The output characteristics in the given device was checked with a digital oscilloscope. Three options were considered; a no-fancy direct connection of the optocoupler to the output connector pin (which pulled down the pulse voltage too much to let the display board react to the pulses), a connection to the chip's output pin with a dedicated series resistor (which worked well), and a prettyfied option with a blue LED that indicates the pulses (which was chosen at the end; everything is better with LEDs and blinkenlights in general). This one however required an amplified signal, as the LED current needs to be high, to make the flashes better observable as they are fairly short. A common NPN transistor was chosen for this task. The transistor switches both the optocoupler and the LED.
A 3.5mm audio jack was chosen as the output connector, due to its size, cost and availability.
Two holes were drilled into the case; one for the LED, one for the connector. Both the LED and the connector were affixed to the analog board in a way not requiring further attaching to the case, which helped to maintain the modular design and easy disassembly. The LED is attached to the board via its own leads, which proven to be stiff enough to hold it in position; the LED's top does not protrude much from the casing, which reduces the eventual axial forces at the pins and prevents them from bending and sinking the LED inside the case. The connector is attached to the board by a hot-melt adhesive..
A pulse counter attached to a host computer has to be added as an additional device. A multichannel counter is a possibility. As the cost of the device is very low, it may even be economical to take several of the meters and harvest the meter boards, deploying them as a multichannel power meter for measuring every power line from the circuitbreaker box.
Inside, both boards
Analog board, top side
Analog board, bottom side
Added circuit, whole board, top
Added circuit, whole board, bottom
Added circuit, connector detail
Detail of added circuit
Connector mount detail, top
Holes, modded case, inside
Holes, modded case, outside
LED and connector, inside
LED and connector, inside
Modded meter, front side
Additional LED and output connector