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$5 Power Consumption Meter

Oct 6, 2015 at 5:46 AM
Edited Oct 7, 2015 at 9:54 PM
I've been seeing a lot of talk lately about power consumption in the discussions so here is a dead easy $5 project for anyone who has a digital power meter installed and wants to see their KwH power consumption. It provides a serial data output which you can develop further for your own use. As it stands it displays the readings constantly on the Arduino IDE Gui serial monitor.

Digital meters have a flashing LED indicating the consumption wattage per hour. This project uses a $2 Arduino Nano microcomputer (or Arduino Uno $5) and a $2 Photosensitive Module to detect the light pulses. The Arduino measures the frequency of these light pulses and then converts them into Kwh consumption and displays the result repetitively onto the Arduino IDE Gui terminal. You can of course process the data further to input to your Pi computer etc. Here's the details.

First order the Nano or Uno from eBay not forgetting the USB cable if you require it. Arduino hardware and software is Open Source so all makes of Arduino are legitimate hence the very cheap China versions which is what I used. Also order the Photosensitive Arduino module (so called), this is the one which has a flat faced photocell poking out front with variable pot and diff amp chip on the board with 3 pins out the back, VCC, Gnd and Dout. Search these - atmega328 Nano or atmega328 Uno and Arduino Photosensitive Light Sensor Module. Depending on how keen you are you might want to also order some jumper leads etc. You will also need a 220 ohm resistor and 100nf cap to ground (both non-critical) as a spike filter to attach photocell Dout to pin D8 on Arduino to give rock solid performance.

If you are not familiar with Arduino just go onto website and download the Arduino IDE and install on your PC, Pi or Linux. With this you will write programs and upload them onto the Nano or Uno. Get familiar with it and try out the examples.
Once you've played around with that for a bit you can start on the project. You need to extend the Arduino library to include the Frequency Measurement utilities so download the Zip from and add them to Arduino IDE via Sketch>Include Library>Add Zip Library and select the one you've just downloaded.

Then connect up everything from Nano/Uno to Light module +5v, Gnd, and Do which goes to pin D8 on Nano/Uno via a 220 ohm resistor and connect a 100nf cap to ground from D8 as well. Temporarily sticky tape or bluetac the photo fingy over the flashing light on the electric meter and adjust variable pot so the green led on the photo board flashes in sympathy - shut out all other light otherwise you'll get misreading's - take your time over this to get it just right. Most meters are 1000 flashes per Kw hour but some are 800 (Sprint) or maybe something else so set this in the 'pulserate' constant in the software. Now paste the software below into the Arduino Gui and upload it to the board. Open the Arduino Serial Monitor and Voila! you should start seeing power readings. You can adjust the const sample rate in the software to suit.

/* Thanks to
  • Energy Meter - Github/infinityab - This example code is in the public domain.
  • Connect Dout from Photocell via 220 ohm resistor to pin D8 on Arduino Nano/Uno
  • and connect D8 via 100nf capacitor to ground - this is to filter out any spikes due to
  • likely wire length from sensor module to Arduino. Connect Photocell module to 5v and Gnd.
    #include <FreqMeasure.h>
    void setup() {
    pinMode(13, OUTPUT);
    double sum=0;
    int count=0;
    int OnOff = 0;
    const int pulserate = 1000; // meter pulses per Kw hour for your meter
    const int sample = 3; // sample is how many samples to take to be averaged out
    int timeConst = pulserate * 3.6 / 1000;
    void loop() {
    if (FreqMeasure.available()) {
    // average several readings together
    sum +=;
    count += 1;
    if ( OnOff > 0) { // light the Nano/Uno LED for each pulse duty cycle received
    digitalWrite(13, HIGH);
    OnOff = 0;
    } else
    digitalWrite(13, LOW);
    OnOff = 1;
    if (count > sample) {
    float frequency = FreqMeasure.countToFrequency(sum / count);
    frequency *= timeConst;
    Serial.println(String(frequency) + "Kw");
    sum = 0;
    count = 0;
Where to go from here? There are a number of ways of exporting the data, serial transfer, I2C, or add a wifi module ($4) and web server software to the Nano/Uno thus permitting access via the internal web by Pi, phone, tablet or PC. This is the method I shall be trying over the next week or so I can use the data for auto power management on my solar scheduler.
  • Note that some meters (mine) start displaying the power surplus as well when the solar excess power is greater than the consumption so some additional nifty software is required to track this and indicate deficit or surplus power consumption.
    Another Note - you'll will have to indent the code to pretty up again