I must have a case of bad Google-fu today, since I was completely convinced that this should be a common subject. I even seem to recall having read something about this in the past. No luck today, though..
The country house got some new electric radiators, and they're half-fancy. Not the oil-filled variants from 20-10 years back but modern filament-to-air with a fast(ish) switching on and off (on the order of many seconds seconds to minutes). A faint(ish) red LED indicates the state of the regulation. Two 1 cm high green 7-segments displays the set temperature. Red LED is 0.5 cm to the left of left 7-segment.
I'd like to detect/monitor/log the LED state.
What would be your suggestions for a detector that can be temporarily mounted on the outside of the radiator?
I'll work out the mechanics but it'll probably be something that just hangs over the top of the radiator holding the detector in front of the LED. Intention is NOT to have it permanently attached and logging, but rather temporary for a few days during different temperature/weather conditions.
If your suggested detector has some special requirements on how it is connected to an AVR, or some special circuitry is required then please tell/hint about that too.
I'm looking for the least ambitious detector/solution. Digital would be best, but analogue not rejected if it has to come to that. (The idea of placing a camera over the whole display and hiring Cliff to code for the image analysis/processing has crossed my mind, but been rejected. ;-)
The complete gadget will probably be battery-powered. I might pull out a dusty USBstick (i.e. AT90USB1287) to do the job. Or I might go for radio/wireless and use e.g. a Raven. I reckon I will need to sample the LED on the order of one timer per second to get a reasonable accuracy. The resolution of the saved data (i.e. how often do I actually want to save) might be on the order of 5 to 15 minutes.
I'll spend some time with the bio-optics and carbon-based neck-mounted computer tonight to get a better grip on the approximate frequency of the regulation. Yeah, I know it won't be a fixed frequency, but I'd like to get a grip on how fast it switches on and off under reasonably normal circumstances. Just to get a better grip on what sampling and logging frequency makes sense.
I'll work all that out eventually, but I'd like suggestions on the detector.
Photo below shows display. Window is just below 40 mm wide. (As an internationally standardized item of size-comparison I've included a small PCB with a 12-pin 100 mil connector.) Red LED appears brighter in photo than in real life.