Solar panel as darkness detector

Kermit2

Joined Feb 5, 2010
4,162
saying it doesn't work and wanting someone to explain your error to you is not enough information. what solar panel? What battery? What transistor? what LED? We are not there, so you must describe using many more details.
 

MikeML

Joined Oct 2, 2009
5,444
It is a very primitive circuit; devised by a newbie. There are many reasons why it might not work. It has no tolerance for normal component tolerances, but the wrong solar panel specs is highest on my list. For it to work, the open circuit solar panel voltage in bright sun would have to be upwards from 15V.
 

ErnieM

Joined Apr 24, 2011
8,196
This circuit sends a current back into the solar cell. Generally this is a bad thing to do, so many panels come with a built in diode to prevent this.

An NPN transistor sink should work with all panels. It still has the drawback of no one having the slightest idea of how dark is dark enough to trip the circuit.
 

Thread Starter

mrel

Joined Jan 20, 2009
179
saying it doesn't work and wanting someone to explain your error to you is not enough information. what solar panel? What battery? What transistor? what LED? We are not there, so you must describe using many more details.
(1) Solar panel 12 volts 10 watts.
(2) battery12volts
(3) transistor pn2905a
(4) led 3volts
mrel
 

alfacliff

Joined Dec 13, 2013
2,458
how about using the usual solar panel circuit with the series diode and just measuring the voltage on the panel side of the diode? should go down in darkness.
 

Bernard

Joined Aug 7, 2008
5,788
If the panel has a built in diode, ckt. will not work. Try connecting the LED- 1k8 R across the diode, anode to cathode, with battery attached. If LED glows, ckt might work.
 

MikeML

Joined Oct 2, 2009
5,444
Here is how I would do this:

I'm using the base to emitter junction of the 2n3055 NPN transistor to replace the diode. This works even if the panel has the reverse protection diode built inside. The 2n3055 has a max base current rating of 7A, so the 1A that the panel can supply is well within this rating.

R1 makes it so that the panel has to have more than scattered light on it before the LED turns on.

The simulation shows the LED current I(D3) blue trace and the battery charging current I(V2) green trace as a function of the solar panel current I(D1) shown along the X-axis of the plot. Note that I am showing only what happens as the sun first comes up; the panel current goes from 0 to 100mA. The final panel current might go as high as 10/12 = 0.83A.

Note that when the panel current is less than 10mA, all available current goes into the battery and none goes into the LED.

From 10mA to 43mA, the battery charging current stays constant at ~12mA as the LED current increases from 0mA to ~28mA, so the LED gets brighter.

Above 43mA, the LED current stays constant (full brightness), and the all of the remaining current goes into the battery.

When the panel is dark, no current leaks from the battery.

221.gif
 

ian field

Joined Oct 27, 2012
6,539
Hello
found this this circuit Solar panel darknes detector on the internet i build this circuit on a bread board.
It don't work see attach file, can any one tell me what wrong with circuit.
mrel
You can get it ready made real cheap.

Solar garden lights have a light sensor that switches off the LED during daylight.

They used to use an LDR light sensor, but I think the Cadmium-sulphide cells have run into RoHS issues (cadmium is extremely toxic!).

These days, most sense the solar cell charging current to disable the LED drive circuit.

If you want to go the DIY route, you should seek out a simple 2 transistor Schmidt trigger circuit so it switches cleanly instead of flickering a lot at dawn and dusk. Unfortunately they're not that easy to find on the web - my searches found any number of non-Scmidt light sensors, but very few proper job!

If you don't have any luck finding a Schmidt circuit - search; "Philips ee8" or "Philips ee20". there's a Dutch hosted website with a large repository of manuals for the Philips Electronic Engineer sets. Most manuals have the schematics in the back, the basic Schmidt configuration is used in a variety of projects.

http://ee.old.no/library/
 

Thread Starter

mrel

Joined Jan 20, 2009
179
Original circuit I post in jpg the circuit supposed charge battery by 12 volt solar panel and as the sunset the Led in circuit suppose to light up because solar panel not get any sun light to charge the battery then battery take over and light Led.
Problem after build this circuit Led don't light up when sunset.
mrel
 

wayneh

Joined Sep 9, 2010
17,153
Try placing a jumper around the PV panel, as if shorting it. (Or you can remove the panel and replace it with a wire.) If that does not make the LED light, then you have a problem with your build of the circuit.

If the LED then lights, I would suspect that you may have a PV panel with a blocking diode built in. This has been raised at least twice before. Can't you answer this question?
 

MikeML

Joined Oct 2, 2009
5,444
Ok, I misunderstood. You want the led to turn on when the sun is not shining. This circuit steals ~10mA out of the battery to light the LED when the panel is dark (panel produces less than 300uA). The LED goes out when the panel produces more than 400uA. The power wasted in R1 is about 4mW in the dark and about 5mW when the panel is illuminated...

83.gif
 

wayneh

Joined Sep 9, 2010
17,153
Yeah, the solar lights that come on at dusk do not use this approach. I've never seen one with anything on the PV side of the blocking diode.
 

MikeML

Joined Oct 2, 2009
5,444
Yeah, the solar lights that come on at dusk do not use this approach. I've never seen one with anything on the PV side of the blocking diode.
Maybe the Panels used on the little yard lights dont have the internal diode D1 and it is the reverse leakage of the Panel itself that supplies base current to Q1. The larger panels (especially the ones sold for automotive trickle charging) do have D1. D3 is there just to reverse bias the Vbe junction of Q1, and could be a Schottky to minimize loss when the sun is shining...
 

wayneh

Joined Sep 9, 2010
17,153
Every little light I've seen has a blocking diode. I've been expecting them to build it into the little controller chip but so far they all still have a separate diode.

But I need to look again at one without the Cds cell. I think I've seen some but can't recall how they shut themselves off.
 
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