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Utility meter

General idea

In my house I use way to much power (40 MWh per year), and I wanted to figure out why. On my power meter I have a blinking light that blinks 10 000 times per kWh. Thus, enter nRF52 with SAADC and some photo sensitive diodes. The setup is shown below:

Setup

The general idea is to detect the blinking light with an photo-sensitive diode, and use a second diode for ambient light cancellation. The pull-up resistors in the SAADC are used to bias the diodes, which prevents static current since the pull-up's are disconnected when SAADC is not sampling.

Equipment

  • nRF52832 DK PCA 10040
  • Photo sensitive diode (I'm using TEPT 5600, which is actually a phototransistor, but that's details)

Algorithm

The best way to understand the algorithm is to read the code, because it may have changed, but in broad strokes

  • Wait for RTC tick
  • Sample differential voltage with SAADC
  • Check if its a blink
//Remove median value, helps with offset
if(results[0] < min){
    min = results[0];
}
if(results[0] > max){
    max = results[0];
}
results[0] -= (max + min)/2;

//Detected a blink, basically a zero cross detection
last_blink_status = blink_status;
if(results[0] <  0 - adc_hysteresis){
    blink_status = 0;
}else if(results[0] > 0 + adc_hysteresis){
	blink_status = 1;
}
if(last_blink_status ==1 && blink_status == 0){
	blink_counter++;
}
  • Count blinks over 4 seconds, and calculate the kW
  • Transmit an advertizing packet with the kW result embedded in the payload.

Status

✅ Get low power sampling with SAADC working

✅ Add light sensor, and see if I can detect the blinking from the utility meter

✅ Add low-power beacon based on https://github.com/NordicSemiconductor/solar_sensor_beacon

✅ Setup a nRF52 to capture the packets https://github.com/wulffern/nrf52-utilitycapture . At this point my laptop is reading out from the nRF52 capture memory, and plotting the kWh over a day, but I need to migrate to a Raspberry Pi or similar

❌ Try it on a Raspberry Pi to use as a gateway

❌ Add nRFCloud functionallity

Results (so far)

Power meter Result

To run

Ideally it should be

make
make SERIAL=<debugger serial> flash

But rarely that will suffice, you probably have to at least change SDKPATH in the Makefile

The debugger serial can be found with

nfjprog -f nrf52 --ids

Excuses

I'm not a professional embedded programmer (i.e I don't get paid to program embedded code), so I'm sorry if you find these examples a bit messy.

Any concrete criticism/improvment requests are welcome. Send me a private message on devzone.nordicsemi.com

Thanks to

  • Hans E. for help with the Solar Sensor Beacon Packet format
  • Håkon A., Jon Gunnar S. and the Nordic support team for answering questions.

Disclamer

This is not an official Nordic Semiconductor release, it's examples I've compiled in my "offline" time.