I had several goals in mind for the project:

• Light is active only at night
• Must have manual on/off switch
• Only active if there are people present
• Light source not visible when the light is off

I chose an Arduino Uno as the controller.  I have a few of them around and have wanted to find an appropriate project to give them a try.

I hardwired a segment of LED strip to a 12V wall wart and taped them inside the clock to get a sense for how many LEDs I would need to achieve the desired effect.  I used a bit of tape to hold them in place during testing.  Two strips of 9 LEDs each did the trick.  One is placed on the top of the door frame, facing down.  This nicely illuminates the clock face.  The second is on a cross member just below the clock face.  This one faces inward and cast a great light on the mechanism and pendulum.

I knew I would not be able to control the LED light strips directly.   Each group of 3 LEDs draw 20mA under full brightness.  The configuration I settled on has 18 or 6 segments of 3.  The total current draw will be 6 x 20mA or 120mA total.  Arduinos have a max. draw of 40mA per pin so I’ll have to use a transistor with a higher current capacity.  I had a few TIP120‘s in the parts bin so I went with those.  I’m hoping that I can run them without a heatsink.

A simple photo-resistor will determine the ambient light in the room.  I’ll use this to determine when it is ‘night’ and enable the lights.

For the ‘people sensor’ I’m using a PIR motion sensor.  It will run off the +5V rail of the Arduino and provides a simple digital high/low signal to indicate motion.

I’m planning on mounting the on/off button out of sight so I needed a big target to make it easy to find by touch.  I choose a momentary arcade style button.  The control will simply toggle the lights on/off when pressed.  I opted not to have a power switch to turn it off completely; I’ll just unplug the wall wart for the few times I’ll want to disable it.

The first step will be to breadboard the parts so that I can start on the software.  I’ll cover that next time.