The dimmer portion of the timer/dimmer started life as this lens assembly that I bought at a carboot sale. The lens has a strange fitting that I can't use, but the motors and gears had to be useful for something!
This shows the arrangement I've come up with. It will never win any awards for workmanship, but it does exactly what I was aiming for. The gearing in the motor means that it takes nearly two minutes to turn the variable resistor (see picture below) through its ten turns, which is ideal for bringing the light levels up and down in the birdbox.
I needed to switch off the motor at the ends of the resistor's movement and the middle and right-hand gears do this. The right-hand gear turns just once in the two minutes, and the pin on its rim operates the microswitches to switch off the motor. The central gear is needed to slow the motion down, and is spring leaded so that it can be pulled away from the other gears to allow adjustments of the wheel positions.
The variable resistor will control a separate 12v supply to white LEDs in the birdbox.
The electronic timer (another carboot purchase) provides independent control of four relays. These are used in pairs to operate the motor in clockwise and anticlockwise directions. The black box is the 12v power supply for the dimmer mechanism.
Once the units are mounted on a wooden board I will be carrying out the first trial with the lights in the birdbox, hopefully before the end of the week.