Solar powered Light

[Hero999]

I had an idea when looking at the Joule Thief circuit in the other thread. I thought, it should be fairly easy to convert it into a solar powered light by adding a few components. When light shines on the solar cells, the battery charges and when it's dark the Joule Thief circuit fires up, turning on the light.

The solar cells can be 0.45V 100mA with an AAA 1000mAh NiMH cell.

The operation is the same as a normal Joule Thief, except that when the voltage generated by the cells is over 1.2V Q1 is permanently turned on via R4 and D3 taking Q2's base to 0V, thus shutting down the oscillator. D2 prevents B1 from discharging back into the solar cells, ideally it must be a Schotky for the low voltage drop to achieve high efficiency. Yes I know the 1N5817 is overrated but the voltage drop will be only 0.3V at 100mA.

This circuit is the easiest way of doing it with minimum parts; expect to find this kind of circuit inside a cheap Chinese solar light. There's no hysteresis so the light will flcker or brighten up slowly before turning on properly. There will be some dead-band i.e. when the solar cells' voltage is above between 1.2V and 1.5V the light will be off but the battery won't be charge until the cells' voltage exceeds 1.5V.

I haven't built this so no promises. If any of you builds this, please let me know how well it works.

[audioguru]

I have many solar garden lights. Most use an LDR and extra tranaiator to turn the light on and off. Most use a single battery cell.
But they are not bright enough to light up anything, just bright enough to be seen in the dark.

[Hero999]

Your lights must either be good quality or perhaps the underpaid Chinese engineer didn't think of using the solar panel as a light sensor?

I have a couple of solar garden lights. They both use the solar panel as a sensor like my circuit. I opened one of them up to see the shoddy soldering. I didn't reverse engineer the circuit but I will now to see if it's similar to my circuit.