The hardware

For the hardware, I decided on an implementation centered around an ATMega88. That AVR has enough flash to put large programs in and the code for the NoveMini probably wouldn't be huge. I could even have settled for a smaller AVR, but the only AVRs I had left were either too small (as in: eight-pin chips) or didn't have an AD-converter, which was necessary for the photo-sensor.

With the design in mind, the schematic is easily explained. In the middle, the ATMega88. Connected to it via series resistors are the 9 LEDs. I didn't feel like doing any fancy stuff like putting them in a matrix or charlieplexing them, hence the direct connections. On the left is the photo-transistor, in a small blob of analog electronic. Quick explanation: the first 47K resistor is used to bias the voltage at the left end of the 10nF capacitor. The capacitor, combined with the resistors to the right of it, forms a high-pass filter. The idea of that is to remove ambient light, including any 50Hz/60Hz influences from e.g. TL-lamps. The resistors and the transistor finally form an amplifier which amplifies the voltage about ten times. The remaining voltage still is minute, but at least it's amplified enough to be detectable by the AD-converter of the ATMega88.

I'm not that good with analog electronics, so the resistor/capacitor values are either decided by trial/error or sheer luck or calculated according to this excellent tutorial about transistor amplifiers. I have no idea if the circuit needs any tuning, though; it has worked OK for me in all conditions, tho'.

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