Newbie: using photo diode on GPIO for power meter reading

Hi all,

I'm new to the Pi and have only limited soldering and electronics experience from the ancient past- but know how to use google

I'm planning to use my Pi for reading several power meters at my home- either older Ferraris types or ones with blinking LEDs (1000 impulse per kWh).

For the latter I'm thinking of hooking up a simple photo diode plus pullup resistor to an GPIO input. Is that all it takes or do I need an op amp? Or would a photo transistor be better suited (thinking about the transistor for the Ferraris type meter anyway...)

Best,
Andi

I've found a solution!
Using an inverting op amp with offset voltage provided by mini trimmer I'm able to adjust working point of the photo diode. 100nF capacitor between output and inverting input further increases stability.
For control purposes I've also added LED to the breadboard, however driven by 2nd op amp as the LM358 looses output voltage when current is drawn.

I'd love to see any more details on your set up if it is convenient to provide, a few pics or circuit diagram, hand drawn is fine. It sounds like a cool project, over my head at the moment, but I'm learning thanks to folks like you sharing how you accomplish things.

Glad to, please find the diagram attached:

Unbenannt.png (12.36 KiB) Viewed 883 times

RE1 and Pot1 are used to adjust the working point of the photo diode, RE2 and 3 are there to setup minimum input voltage for second amplifier to avoid it opening on floating input.

Cool! Thank you very much.

Hi,

My solution to blinking LED meter readings is at: http://github.com/yfory/power

Released under MIT license.

Yfory,
this is _really_ simple
I wouldn't have thought that resistor/capacitor could be fast and sensitive enough. Are there any special requirements regarding your meter?
kind regards,
Andreas

No special requirements; everything listed is what was used and is working for me.

I am sure there are far more complex and interesting ways to do this, but I like building things that are simple enough for everyone to reproduce. I am using 10m of 4-core (3 wires, 1 spare) telephone cable to an external meter without any fault.

The only "issue" to watch out for is the 90,000 number, which is bound to be more or less on your set up, so add some print commands to the .py code to figure out a reasonable number before you leave it running 24/7. I believe I added a note about that to the documentation.

Yfory,

I have read through your information on Git Hub.

What value LDR did you use in your setup?

I have tried to create my own but the value going to the GPIO is not changing from enough when I cover or uncover the LDR.

An LDR is probably too slow. A photo-transistor is the way to go.

A friend got this working on his meter by connecting the Emitter to ground. The collector to 3.3V (via a 100K trimming potentiometer) and the collector to a GPIO input (via a 2K resistor). Adjusting the potentiometer adjusted the point at which the GPIO would go High/Low on a flash.

Good idea- the transistor may also be sufficient to drive an LED as well as then Input?

[mahjongg]

Unless I'm very mistaken, an LDR would also be fast enough, and much simpler, in principle just the LDR connected to a GPIO with a pullup resistor.
Yes, an LDR might need several seconds to regain its full "dark resistance" of several Meg Ohm, but generally its response time (between say 1K and 50K) is a few dozen milliseconds, for a slow blinking LED it should be fast enough. In any case its worth trying if it works.