here's my project.

i need solar powered security light for a spot way back on my property. was looking at the battery only solar units but i dont see any that give more than about 30watts of light using halogen (power wasting).

my idea is to build a hybrid using car battery, 14-20v solar panel, inverter, and standard 120v security light (perhaps a sodium unit, etc). but i dont want to have the inverter powered on all the time. i was thinking using some sort of photo cell to sense darkness and then have a relay switch on the 12v to the inverter...

any suggestions?

 

As i understand you want to activate a lamp at night, right?

You can use an LDR, a few resistors, a transistor and a relay to make a dark detector. Check the circuit at the following website. You can replace the phototransistor by an LDR.

http://www.electronics-lab.com/projects/sensors/022/

 

You could use a simple CdS cell to sense ambient light. It's basically a photoresistor. Radio Shack sells an assortment of 5 in a package for a couple bucks.

A car battery would not be a good choice. You need a deep-cycle type battery. They're frequently sold as boat or "marine" batteries.

A solar panel sufficiently large to charge a battery that's powering an inverter with a sodium vapor light as a load might have a pretty hefty price tag.

You'll need enough amp-hour capacity in the battery so that running the light during darkness will leave the batteries at about 70% capacity. Discharging even a deep-cycle battery further than that will shorten it's life a great deal.

 

let me clarify further.

i think a standard car battery will work ok. the security light itself has motion sense and wont go on until motion is detected. the "on" time will only be about 5min after motion ceases. deep cycles want long use and charge cycles.

as for the solar panel. it will only be some mA worth or current going into battery during daylight hours. this is enough to keep the battery fresh. solar panel is current limited with simple resistor. using such low ingres current is how i keep my car batteries fresh, and my tractor battery over winter months.

the sodium bulb lamps draw about 1/4amp 120vac but output huge amounts of lumens....

i will use a cheap inverter and solar panel (from habor freight), car battery (wal mart), and a sodium fixture with motion sensor (home depot). thus, the fixture simply plugs into the inverter. the other components will be in a shed.

my dilemma is how to power on/off the inverter after dusk. no reason to have the inverter powered on during daylight hours, etc.

 

my dilemma is how to power on/off the inverter after dusk. no reason to have the inverter powered on during daylight hours, etc.

You have to build a simple dark detector circuit as the one i told you in my previous post and power it continuously by the battery. Also use a relay to connect and disconnect your inverter to the battery. You can search google if you like for more dark detector circuits. I suggest you to design the dark detector circuit to draw as less as current possible from the battery when it will be on stand-by.

 

You have to build a simple dark detector circuit as the one i told you in my previous post and power it continuously by the battery. Also use a relay to connect and disconnect your inverter to the battery. You can search google if you like for more dark detector circuits. I suggest you to design the dark detector circuit to draw as less as current possible from the battery when it will be on stand-by.

i looked at that schematic. doesnt the load typically go on the collector side of a npn transistor?

 

You need a PIR sensor to detect motion if you decide to roll your own.

So basically you have all the components but just need to design the power source? Do not forget a charge controller. Also when spec'ing a solar panel you usually look at the wattage/current output in addition to the voltage. How much usable solar light so you have in your area? What is the quiescent power draw of the inverter?

 

You need a PIR sensor to detect motion if you decide to roll your own.

So basically you have all the components but just need to design the power source? Do not forget a charge controller. Also when spec'ing a solar panel you usually look at the wattage/current output in addition to the voltage. How much usable solar light so you have in your area? What is the quiescent power draw of the inverter?

not even a power source. i'm just looking for a good way to power on the inverter once it gets dark. this switch has to use the 12v battery for power, so obviously it needs to consume less energy (on and off states) than what the solar panel is putting into the battery during daylight hours, otherwise the battery will eventually die.

i'm thinking a circuit like mik3 linked to, but using a solid state relay instead of a coil based relay.