The only thing in it that could possibly prevent overcharging is D3 if it is a Zener of around 15 volts. Whenever solar cell or windmill voltage exceeds battery voltage plus three diode drops, current flows into the battery via D1, D2 and D4.
D3 can steal that current to prevent overcharging.
Without pin numbers we have to assume your 555 is wired as a free running oscillator.
From its datasheet at http://www.basicanalogcircuits.com/Session_4_files/LM555.pdf
looks as if it'll oscillate at a few hundred hz.
View attachment 197352
Can you figure out its duty cycle from that datasheet? You really ought to learn the 555, it's a fabulously handy device.
Your circuit uses the 555 just to switch that un-named transistor on and off a few hundred times per second.
Alex described that,
but
his choice of words "Open" and "Closed' are not typically what we use in electronics.
That transistor is just a switch.
A switch that is "Open" will not pass current, unlike a water faucet that is open. A switch that is "Closed" will pass current, again unlike a faucet.
To avoid that confusion we use ON and OFF or CONDUCTING and NOT CONDUCTING when speaking of electronic switches,
we reserve OPEN and CLOSED for mechanical switches and relay contacts.
SO ----- Alex's otherwise excellent description
might lead you to the wrong conclusion.
To your question - the 555 only drives that transistor switch.
As Alex said, when that transistor switch
conducts the 47 uf gets charged to supply voltage.
When that transistor turns
off, R5 pulls capacitor's left side up to supply voltage AND
its right side rises by an equal amount.* Therefore it will exceed supply voltage, reverse biasing D1 and overcoming battery voltage pushing charge into the battery.
* (That's the secret behind all charge pumps. )
So the purpose of the 555 is NOT to prevent overcharging, D3 should do that. Check its part # and see if it's not about a 15 volt zener .
The purpose of the 555 is to make the 47uf capacitor pump charge into the battery especially when solar cell or windmill voltage isn't enough to do that by itself.
Here's an example at 8 volts, maybe a cloudy windless day:
View attachment 197354
Write KVL around the current loop both ways and you'll see the circuit can make almost 16 volts from an 8 volt supply.
Let's say that 500 times a second we charge 47 uf to 8 volts then dump it into the battery. How much current is that ?
47 uf X 8 volts = 376 microcoulombs per cycle X 500 cycles per second = 0.188 amps
Not Bad, eh ? It'll trickle charge on a cloudy day !
Actually you get not quite that much current because the capacitor can't completely discharge, and KVL will tell you by how much. Ahh the power of Kirchoff !
We call that circuit a "Bucket and Ladle", as if we were filling a bucket (the battery) with a ladle(the capacitor) from a reservoir below(the solar panel or mill) by repeated lifting of charge .
Electronics guys use word pictures like that to remember concepts. You should work through the circuit using your circuit analysis skills to cement it in your mind.
Good luck with your project. Develop the habit of working circuits in your head, pretend you're inside them to understand the mechanism, then apply math to put numbers on it.
@Tom.G Can you polish this scraggly sprout of an explanation ?
old jim
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