It’s not very often I bring you a circuit – but this is quite fun to play with and simple. I don’t claim to have come up with the original design – but then the original was only 3 components….
Here we see a circuit that a first glance should do very little. the circuit is a little misleading in that actually we’re looking at 2 or 3 white LEDS in series such that the turn-on voltage is higher than the battery voltage. Imagine (in conventional terms which of course is wrong) juice coming out of the + of the battery, through half of the coil, through the LED – except no – the battery is not high enough voltage so essentially nothing happening there and in any case, the second (rightmost) transistor is turned on so you’d still not get any LED lighting up – you’d just drain the battery and probably blow the transistor.
Ok, but that doesn’t happen because the FIRST transistor is holding the second one OFF due to light from the solar cell generating a voltage into the base of the first transistor.
Of course that’s all wrong – in fact, the coil, 1k resistor and second transistor form a reliable oscillator when not held off – I might be wrong but I seem to recall it’s called a Colpitts oscillator!
So during the day, the solar cell is charging the battery, the 47k resistor is turning on transistor one which is holding off transistor two. Result, very little current flowing – battery gets charged.
Comes night-time, the solar cell generates nothing, the first transistor turns off – the second transistor starts oscillating like mad. Because there is a coil involved, there is a back voltage caused by the oscillation – which generates pulses of MORE than the battery voltage- and the LEDS light up brightly.
Where this circuit would normally not do too well – and variations of this are common on cheap Chinese solar garden lamps- is that at twilight the LEDS would partly come on – and they end up flashing and all sorts. the 330k resistor provides positive feedback to arrange a snapping action – ensuring the LED does not come on until it’s REALLY dark, then stays on until it’s reasonably light – no dim lighting or half-way house – the only issue being – the current coming back from transistor 2 comprises pulses – which we don’t want – hence the solitary capacitor on the circuit!
Try it – works a treat. The coil, well, I had one of those ferrite rings, maybe an inch diameter, you can see it in the photo below – it’s blue. I simply wrapped thin wire 20 times, brought out the centre-tap – another 20 times… that’s it. Solar cell is around 5” square and generates 6 volts off load – a bigger one would be nicer.. the battery is merely 3 1.2v AA 2500MAH NIHI batteries in series and for LEDS I used some 10mm whites – in total I had 6 of them (2 sets of 3 in series) – lights up part of the garden a TREAT for several hours. I was planning on trying again to night but guess what – it rained overnight!