A watched meter never boils
December 19, 2010
I would love to be able to have continuous readouts from my electricity meter in my home. Such a tedious job is time consuming and tedious. So I need a robot to do that for me.
I've seen projects use digital cameras and optical character recognition software, but to me that seems such overkill. Also, I do not have any cameras to spare or the skills to program OCR into my software. I do however have in my lab:
- IR emitting LED
- IR photo transistor
- piece of heat shrink tube
- LM358 opamp
- lots of AA batteries and holders
and the usual experimental equipment including a brand new oscilloscope, one like this!
The wheel has a silver rim with one black spot in it. That rim is my target. The black spot should be detectable. Even from a distance: I cannot open the meter. It is sealed by the power company. I need to observe it through the little window.
This is the 3 mm IR LED. The potmeter regulates the current. It proved crucial. Too much light and the detector is blinded. Too little and there is more noise than signal. A current meter showed that 8 mA is just fine for my purposes.
The detector needs to be very directionally sensitive. A lens would be perfect, but complicated to set up. Luckily a photo transistor is so sensitive that you can afford to stuff it in the end of a tube. My transistor and tube are 5 mm wide. The tube is 20 mm long. They are not in this photo.
So now I know what it takes for the optical set-up to work. See the video or a demonstration.