understanding garden solar light circuit

Thread Starter

tab a

Joined Jan 14, 2015
36
I'm playing around with some old garden solar lights, and 'think' I understand most of this circuit, but do not understand the purpose of D2. Could someone en'light'en me? :rolleyes:

Thanks.

solar garden pcb232.jpg
 

AnalogKid

Joined Aug 1, 2013
8,226
And, D2/R3 provide positive feedback to latch the regulator ON once it starts. This prevents the LED light output from being detected by the CDS and shutting off the circuit.

BTW, your diagram would be much more understandable if all of your ground symbols pointed down, and were a correct ground symbol.

ak
 

ian field

Joined Oct 27, 2012
6,539
I'm playing around with some old garden solar lights, and 'think' I understand most of this circuit, but do not understand the purpose of D2. Could someone en'light'en me? :rolleyes:

Thanks.

View attachment 78799
All the solar garden lights I've seen to date used a single nickel cell, which mandates some form of oscillator/inverter which uses a flyback inductor to generate the 3.4V for a white LED.

It tends to be the older ones that use an LDR - the cadmium sulphide is toxic and not RoHS compliant, the modern ones sense charging current from the solar panel to shut it down in daylight.

The discrete component variety are getting rare too - these days its usually a black-blob chip or sometimes a 4 lead chip about the same size as TO92.
 

takao21203

Joined Apr 28, 2012
3,696
None of these "solar garden lights" is any good

The output of the tiny solar cell is so little they would only be effective in Africa
The small battery gives a few hours when new but that becomes less and less pretty quick
The light output isnt much useful for anything
Typically the whole thing after a year shows signs of corrosion
None of these junk properties warrant to use massive metal components for its construction, which indeed could last centuries
 

ian field

Joined Oct 27, 2012
6,539
+1 I haven't seen one in a long time, a decade at least.
I found the wreckage of one built with discretes, on the footpath behind the flats.

It was also a rarity in having 2 Ni-Cd cells - which were still in the holder.

That was before I was brave enough to make my own lithium chargers for the e-cigarette - Ni-Cd are the only other sealed rechargeable that can handle the current draw.
 

wayneh

Joined Sep 9, 2010
16,126
Thanks. As usual, makes sense once someone explains it! :)
Well, I was in a hurry and stated the obvious. But if that is ALL it is doing, then removing it would make little difference, just a small current leak. I suspect it does more than that, something about biasing Q2 when Q1 is conducting, but I couldn't quite figure that out. Take it out and see what happens!
 
Top