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.
Step 2: Circuit Board
If you wish to make your own printed circuit board then please download the actual files in EAGLE format, or use the attached picture and process it yourself ie. copy it and use whatever pcb method you normally use to manufacture it.
If you are stuck without the facilities to make the board then as a last resort email me and I will see if I can provide you with a PCB (this is dependant on the number of requests I have for the PCBs!).
Step 3: Components and PIC Programming
Should you wish to order the PIC from me, then please request by Email and I will reply with the cost of shipping a pre-programmed PIC.
For you to self programme your own PIC 16F684 the HEX file is here available.
Carefully insert and mount all the components (following the component layout here), start with the low level components such as resistors first. Solder and trim the component legs.
Take care that the components are correctly orientated or components will be damaged. Note the LEDS have a flat side (the cathode!). The PIC should be ideally be mounted in a Dil socket and be sure that it is correctly mounted with the notch at the correct end as the layout picture. Note the polarity markings on the bridge rectifier and the Electrolytic capacitor! double check your component assembly and also the printed circuit board (for bridges/shorts or open circuits or properly soldered joints), before applying power. The circuit and PIC have been proved and tested, so of it fails to work as expected the fault is with the construction or a failed component, the most likely being that a fault exists on the copper side of the PCB, DOUBLE CHECK EVERYTHING (A visual check is the most important check you can make before any other!).
Step 4: Testing
Connect the power source to the board (15-to-18 Volts DC or AC) and expose the photo resistor to a normal bright light, the output LEDS should light up full power (Don't look directly at them, it can hurt your eyes). Next shadow the photo diode with your hand or better still with a piece of black tubing, and the output light should reduce to a very low light. If this test proves OK then you have succeeded, otherwise you need to go back to the previous section and go through the fault seeking order again.
OK your unit works, now its up to you how you use it, for my application, I mounted the board some 1 metre away from the Bedroom clock so that the clock face was evenly illuminated, and it functions very well. If you have other applications you can of course move the Photo diode away from the board via a wired connection and mount it a position better suited for your application.