Computing stuff tied to the physical world

Time for a new mousetrap

In Hardware on Oct 1, 2011 at 00:01

It’s been a while. In December 2008, I posted about my setup for detecting the rotation of my electricity and gas meters. This was before the JeeNode even existed. It was built with an RBBB plus RFM12B with voltage dividers.

That setup has been running ever since, probably sending over a million packets around the airwaves here at JeeLabs. But it’s getting a bit old – and it’s still that dangling mess of wires. Besides, I’d really like to get a more up-to-date setup going: the current unit is still using the old v1 RF12 protocol, so it needs a specially-built receiver to pick up the packets. Then again, I don’t want to replace it before there is a sufficiently stable alternative.

First thing I did quite a while back was to get an extra electricity meter, if only to avoid the constant runs up and down the stairs. That way I can hook up a scope, and try things at my leisure:

DSC 2659

I also wanted to experiment with different sensors, so I tried the GP2S700HCP, which is a tiny SMD device:

DSC 2660

Unfortunately, the range is a bit low -it’s specified as 0.5 .. 5.5 mm. That translates to a fairly weak signal:

Screen shot 2011 06 29 at 02 16 43

About 150 mV swing on a 1.8 V signal, it seems. Hm… I might go for the venerable CNY70 after all. That’s the sensor most people seem to be using. I was hoping to find something smaller, because it would be easier to create a mounting solution for it.

Even better would be to try and detect the irregularities around the entire Ferraris wheel, and perhaps also to detect variations on the digits of the gas meter.

More tinkering needed…

  1. Speaking of mice, I once tried to mount the sensor of an optical mouse to my ferraris meter. That would make it possible to monitor the movement of the wheel itself instead of looking for the black or red line. Unfortunately I could not find a good replacement lens to change the focal distance.

    • Oooh, that’s a cunning idea. Only trouble is mice can be a bit hit or miss with shiny surfaces – You wouldn’t want to have one of those “whizz across screen” moments translated into power consumption!

    • I’ve been wanting to do the same, and held back for exactly the same reason. A well-focused image is probably needed to be able to detect more than the black marker.

  2. Ive used a QRD1114 to detect passes of the “black zone” on our ferraris-meter, signalstrength (difference between peak compared to the rest of the readings) was more or less the same as you report and extremely sensitive to the positioning of the QRD. I can even see effects of temperature on the signalstrength.

    cheers Cor

  3. Hello,

    I’m also interesting by such measurement, but no test have been done at this time by myself. I guest that a side to side emitter and sensor are poor because of the front glass reflectivity. Better result probably could reach with an pair of tilted emitter and sensors.

    Just a schematic in order to illustrate :

    Hope it’s useful.

    • Are you sure about that? Reflectance is very low at 90°, and gets higher as you tilt further away from the surface normal vector.

  4. I had a similar go with our mechanical meter. I used a piezo sensor to pick up the ‘tick’ as it went around. Sort-of worked but I couldn’t get rid of surrounding noise.

  5. I built the same thing with a CNY70, but I added a schmitt-trigger to the circuit, giving me a clean 0/5v signal (could be 3.3V to offcourse) I would suggest however to build it with 2 CNY’s, detecting the rotation direction of the disc. I’ve recently installed solar panels and the meter is turning in both directions now, making my readings totally unreliable now.

  6. Hi,

    You’re right, the reflectance is low at normal incidence (~ 4% with n=1.5). But, in other hand the interesting signal is probably a small part of the transmitted light beam. There is several losses for this signal a) between source and the well b) at the surface of the whell (reflectivity) c) between whell and sensor

    I don’t really know if my assumptions are in line with the reality ?

  7. I just did 2 experiments on my kWh meter: CNY70: U=5V, Rdiode=150 Rcollector=10k => Ucollector= reflected=4V / black spot=4.5V TRCT5000: same values: => Ucollector = 0.25V / 2.5V The TRCT5000 has a specified distance range of 15mm whereas the CNY70 only goes to 5mm. Looks like both are useable but the TRCT500 is much more sensitive.

    • Interesting indeed – but what’s a TRCT500 / TRCT5000?

      Update – never mind, found it – TCRT5000. I’ll try it out, thanks.

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