Photo Cell Help

I am very limited in my electronic ability, and need help with this idea. I want to power a 700ma led light with a 12 volt battery. I have a nice solar panel to charge the battery during the day. My question is can I use a relay in conjunction with a photo cell that is in the normally closed position using power during the day to hold the relay open (plenty of solar power) then in the dark it closes and powers only the led light. I hope I make sense.

hi

You could use a low cost Light Dependent Resistor [LDR] to drive a simple circuit that could operate a relay.

E.

http://www.google.co.uk/search?q=li...5DemX1AWanoCIDg&ved=0CDQQsAQ&biw=1359&bih=914

**broken link removed**

I like the OP's suggestion better.

You could put a diode between the solar panel + output and the battery, then the solar panel + output will only be high during daylight. Then it can drive a relay directly, to hold the relay closed (LED OFF) during daylight. Some small relays only need <25mA to drive the coil and the contacts should be ok for 1 amp.

You could put a diode between the solar panel + output and the battery, then the solar panel + output will only be high during daylight. Then it can drive a relay directly, to hold the relay closed (LED OFF) during daylight. Some small relays only need <25mA to drive the coil and the contacts should be ok for 1 amp.

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hi Roman,
That sounds OK to me.

A 12V relay should get the job done.

E.

ericgibbs said:

hi Roman,
That sounds OK to me.

A 12V relay should get the job done.

E.

Click to expand...

Sounds feasible, but I also need the solar panel to keep the battery charged, would a diode work to keep from back feeding the relay. Never mind I see it. Diode is to the battery. I am gona try it

Well, I was able to get a 12V relay at RS, thought I had it, I ran into a small problem. As long as the sun was hitting the solar panel it generated enough power to activate the coil on the relay. but when shade hit it, it deactivated. Any suggestions?

How big is the solar panel, and what current does the relay draw?

You said your light that runs all night is 700mA, so nightly use is about 7-10 Ah, so I assumed the battery was large about 15-20 Ah, and the solar panel could supply 10Ah average each day so can supply at least 2-3A for 4-5 hours peak solar, ie is a 40W solar panel.

Solar Panel information
STC @ 1000 W.m-2-AM 1.5-CELL T
Model Type SX5M
Max Sys. Oper. Volt 30
Min. Bypass Diode 1p N/A
Series Fuse 1 A

P max 4.50 W
Voc 20.5 V
Isc .30 A
Vpmax 16.5 V
Ipmax 0.27 A
T 47

Will Produce
Pmax 3.20 W
Ipmax 0.22 A

Battery Information

12V - 7.0 Amp Hour
Rechargable

Relay information

nominal 12 V DC
maximum 15-6 V DC
nominal coil current 60mA
Pickup/dropout voltage 9.6 / 0.6 v DC

Excellent, thank you that is the information that was needed.

Your solar panel will provide about 0.28A into a 12v battery, so at (say) 12.5v will supply 3.5W peak power in full sun. It is quite small.

You should check your local "insolation" figures, but going form a normal "hopeful" figure of maybe 4 hours peak sun per day, that will supply 0.28A * 4 or 1.12 Ah per day of energy, and charging efficiency of a 12v SLA battery is about 1:1.70 or 59%, so you will get about 0.66 Ah of energy into your battery each day, assuming good weather.

If you LED light takes 700mA from the 12v battery (as you said in post #1) then the light can run for 0.66 / 0.700 or 0.94 hours per night. Actually less, because that would mean deep discharge and possible battery damage.

Also, if your relay takes 60mA that is very high in comparison to your solar panel's 280mA peak output!

Looks like you need to re-evaluate your design goals in terms of light sizing and how many hours the light needs to work for. My figures above are rough and don't account for things like bad weather where your battery may not charge much at all for a few days!

It would also help if you say what the light is used for, and provide more info on the full application.

In the photo you can see what I am after, its connected to the lawnmower battery in the photo. The feeder motor also runs off a 12v battery (battery info above), what I was really after was the possibility of using the solar power during the day to keep the light off, and also charge the battery. But I see from above that the battery for one is to small. I think now my only option is to use a lawnmower battery and a simple photocell circuit to turn the light off and on.

OK, so it is some type of automatic feeder machanism?

Your battery of 12v 7Ah is a reasonable match to your solar panel that makes 280mA in peak sun. Or at least, the battery is NOT too small.

The problem is that your solar panel does not make enough power to run a 700mA light all night. Even if everything is perfect it will only run that light for less than 1 hour per night.

Solutions would be to get a much larger solar panel, or a much lower current light.

What exactly does the light do? Does it draw the animals to the feeder? Does it have to be on full brightness all night? How much light do you need?

If you can get away with less light you could use something like 3 high intensity white LEDs, I have some 20000 mCd white LEDs here, and even 1 LED running form 25mA gives enough light to walk around the house and do simple tasks. 3 of those LEDs in series will only draw 25mA (not 700mA!) and may be bright enough to do your task.

As a general rule for solar lighting, you light should draw about 10 times less current than your solar panel peak output current, if you want the light to run all night.