MainsFrequency

Project MainsFrequency
File:Whyunoimage.jpg
A simple mains frequency counter
Status	Initializing
Contact	bertrik
Last Update	2018-04-08

Introduction

This page is about creating a simple frequency counter for mains power and publish the frequency over MQTT.

It's based on the Arduino platform, using an ESP8266 to do the wifi/network/MQTT stuff.

The frequency measurement principle is just to count the number of mains cycles in a fixed period. To get a resolution of 0.01 Hz, the period is 100 seconds.

To keep the measurement circuit relatively safe, only a small part of the electronics is actually connected to mains.

Hardware

The plan is to use an ESP8266 because it can easily publish the measured value over wifi/MQTT.

I'm looking at various ways of actually getting the mains signal into the microcontroller. The simplest way appears to be an optocoupler in series with R, C and a diode anti-parallel over the optocoupler's LED.

• could be a transformer, like shown here. Safe but bulky.
• could be a capacitive/resistive dropper with an optocoupler, like the circuit in this post
• could be this thing from aliexpress

Software

See the github page.

The working principle is that we count the number of cycles in a 100 second period, this should nominally be 5000. A cycle count is done every second and the result is put in a circular buffer of 100 bins. The average of these 100 bins then provides the frequency over the past 100 seconds.

The accuracy of the frequency count depends on the accuracy of the crystal (among other things). To get 0.01 Hz error at 50 Hz, we need an time reference with at most 0.01 / 50 = 200 ppm frequency deviation. This is probably doable with the built-in crystal on a typical ESP8266 board (like a Wemos D1 mini).

Other projects

• http://www.palebluedot.nl/jml/projects/arduino/43-measure-mains-power-frequency
• https://engmousaalkaabi.blogspot.nl/2016/11/ac-220v-frequency-counter-using-arduino.html