Solar lighthouse garden ornament - A design that could never work.

About 12 months ago, the long and suffering Mrs Doz bought me a solar powered garden ornament, in the shape of a lighthouse. 

It's got a white 5mm LED, which is situated inside a rotating silver reflector. Quite a nice touch, and more effective than just a simple flashing LED... except it never worked properly. 

There's a push button switch on the lamp housing, which switches the unit on. After dark, the LED illuminates. Once. 

The following day nothing happened. 

It's not worked since. 

So I disassembled the lamp house to see what the fault was. It consists of two parts, the top part consisting of the solar panels, a charging circuit and LED driver, and a lower part with the motor and it's gearbox, and a 600mAH, AA NiMH cell. 

I popped the cover off the motor unit in an attempt to find out what the problem is.... and spot it straight away. The motor is connected via the push button switch, straight across the battery. As soon as it's switched on it will start turning, and eventually discharge the battery. The solar cells aren't big enough to keep the battery charged up enough. What a silly design. 

I was hoping that the LED driver in the top would be able to provide a suitable switched supply to the motor in a similar way to the LED, but alas no, it's got a small flyback converter to provide ~2.1V to drive the LED. 

So, a plan is hatched... 

A separate solar cell, and battery connected by a wire to the motor unit. 

Recently we had to replace another solar light in the garden, after it was damaged over winter. This has yielded a nice solar panel and it's fitted with a single AA NiMH cell, excellent. The battery is tested, and looks to be in reasonable condition. The control electronics in this enclosure are removed (again, it's a flyback converter)

So we need a circuit to charge the battery during the day, and switch on the motor at night. 

The solar cell charges the battery via D1, a BAT43 small signal Schottky diode (a strange choice, but it's rated to 200mA, which is more than the charging current capable of being delivered by the solar cell, and it'll have lower forward voltage drop than a normal silicon diode), which prevents the battery discharging into the solar cell at night. When the cell goes dark, Q1 loses it's base bias via R1 &R2, and switches off, which causes the voltage at it's collector to rise. This in turn switches on the base of Q2, which causes it to conduct and switches on the motor. 

After a bit of experimentation on some breadboard to get the values of R1 & R2 right (this effects the threshold where the unit switches the motor on), it's built up on a bit of perfboard, and fitted inside the enclosure.

After sealing everything up with some liquid rubber, it's taken outside ...

Now just to wait for nightfall...