Our bedroom has a large window facing East that is almost always unshuttered, so that the bedroom light is constantly varying during the evening and morning hours. This gave me the notion that any clock illumination should be inversely proportional to the illumination within the bedroom.
This led to creation of the following simple unit, comprising a double row of 4 high brightness white LEDs, whose brightness is controlled by a PIC processor via the input from a photo diode.
The PIC was chosen as the control, as it contains an ADC input for sensing the photo diode, and a PWM channel for light intensity control of the LEDS. This unit satifactorally controls the clock illumination to the desired level.
The unit is simple to build and may have many other uses where a light source needs to be inversely proportional to the ambient light, for instance an illumination source for car instruments.
Step 1: Components list
1 off power supply 15 to 18 volts AC or DC.
8 off high brightness white LEDS 5mm ( or as you wish) Led 1 - 8
1 off GP Photo diode Led 9
1 off PIC 16F684 Micro. IC1
1 Off 1 Amp bridge rectifier.
1 off 7812 12Volt regulator. IC2
1 off 78l05 5 Vlot regulator. IC3
1 off 100 uF 25 VW electrolytic capacitor.
3 off 0.01 capacitors (any small sort will suffice)
1 off 470R 1/8thw resistor R5
1 off 470K 1/8thw resistor R4
1 off 1M 1/8thw resistor R3
2 off 1R 1/8thw resistor R1-2
Wire and solder as required.