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I'm building a simple circuit in which a solar panel will charge a 12 V battery during the day and then switch over to battery power during at night to light up a few bulbs. I'm using an LDR which will trigger the NPN at night and switch the relay, as shown in my circuit schematic below. I'm very inexperienced so I'd like if you can tell me what is wrong or what could be done better. I'm also wondering if there is a better approach to activating the relay than using a 9V battery. Thank you.

schematic

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  • \$\begingroup\$ You'll probably need a 6 volt relay - a 25 volt relay won't work reliably, if at all, on 9 volts. \$\endgroup\$ – Peter Bennett Jun 10 '15 at 1:15
  • \$\begingroup\$ The circuit will also depend on what type of battery you are using. If you are using a 12V lead acid battery you will need to take its characteristics into account as this is just the nominal voltage. At lower voltages will the results be desirable for you? \$\endgroup\$ – Sada93 Jun 10 '15 at 4:20
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    \$\begingroup\$ Consider a small solar charge controller so your battery does not overcharge or over-discharge (if lead-acid). The controller combined with EM Fields cct below should work well. Also use LED lighting if you can - much better efficiency. \$\endgroup\$ – Logic Knight Jun 10 '15 at 7:13
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Since you have a PV you won't need an LDR, and since you have a 12V battery you won't need any other battery, just hook up the circuit shown below.

In operation, when it's light outside and the PV is charging the battery, it's also turning on Q1, which will energize K1 and pull COM away from NC, breaking the circuit between the battery and the lamps, turning them off.

Then when nighttime comes and the PV is no longer generating anything, there'll be no current into Q1's base, turning it off. That will de-energize the relay and COM will spring back to NC, closing the circuit between the battery and the lamps, and they'll turn on.

R1 is used to adjust when the lights turn on, and is rotated just enough to cause the lights to turn on when you decide it's dark enough to use them. R2 is there to make sure that if you accidentally rotate the pot too close to the PV's 14V output you won't fry Q1, and K1 is just one of those garden-variety relays with a 400 milliwatt coil and with contacts that can break 12 volts DC with the total lamp current through them. There's noting holy about the coil, though, and if you want to get something more sensitive, (which will allow more current into the battery when it's charging) no problem.

enter image description here

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    \$\begingroup\$ Nice circuit and good use of a PV as a "dark detector"! \$\endgroup\$ – Logic Knight Jun 10 '15 at 7:09
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    \$\begingroup\$ I'm not entirely sure that the 14V PV will stay at 14V given that it is a highly non-linear source. This will definitely depend on the amount of current it is supplying. Will it get enough irradiance over the course of the day to charge the battery enough? Better try it out I guess. \$\endgroup\$ – RYS Jun 10 '15 at 10:05
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    \$\begingroup\$ @RYS: Your "point" is irrelevant in that it addresses a non-issue rather than the OP's query. \$\endgroup\$ – EM Fields Jun 10 '15 at 16:01
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    \$\begingroup\$ It's an issue if your "circuit" doesn't work since you're assuming the panel is going to be at 14v all day. But hey, that's none of my business.. \$\endgroup\$ – RYS Jun 10 '15 at 20:05
  • \$\begingroup\$ @RYS: If it's none of your business, then you ought not to be putting your nose where it doesn't belong. As for the technical issues, I made no assumption that the PV was going to be outputting 14 volts all day long, nor was such an assumption warranted since the issue wasn't even slightly related to the output power capabilities of the PV or the capacity of the battery. Go back and read the original query and you might find that the OP was asking for a way to charge the battery during the day and then to use the stored charge to turn on some lights at night. End of story. \$\endgroup\$ – EM Fields Jun 10 '15 at 20:17

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