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what size variable resistor?

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tasker101

New Member
im trying to make a set of 10 LED's that will run on a 9V battery. i want it to brighten up when it gets darker like a lamp post and to be able to turn on/off and off with a 3 way slide switch. what size variable do i need and what kind of LDR to make it work without it turning on in the day?
 

tasker101

New Member
they white superbright LED's with 470r resistors conected to each leg, it will only be turned on for about 3 hours a day so ideally, the battery should last up to 3 weeks.
 

blueroomelectronics

Well-Known Member
Ok so 10 White LEDs in parallel with a 9V battery will draw ~120mA off a fresh 9V battery. Do you plan on recharging the battery everyday?
 

Thunderchild

New Member
you can't just put the LDR in series with the leds you will need an appropriate control circuit, battery life may be extended by using a comparator circuit to totally shut off the current so that your not in the position where the leds are just slightly on using current but have nothing to show for it.
 

tasker101

New Member
how does that circuit work, im tryin to do this project for my mate but im only in year 10 so im fairly new to all this and only understand basics lol
 

audioguru

Well-Known Member
Most Helpful Member
A new 9V alkaline battery will supply 120mA and will last for about 4 hours with the LEDs dimming all the time.
A rechargeable 9V Ni-MH battery will last for about 1 hour for each full charge.

Four AA alkaline battery cells make 6V when new and will supply 120mA for about 18 hours.
 

colin55

Well-Known Member
Here is a circuit that detects light and turns off your LEDs. Simply put the LEDs on the collector of the transistor. You can use either an LDR or photo transistor.
 

Ayaskanta

New Member
I remember my school project where I had to light up many LED's. I used an old car battery :D Works great! :D

For light detection the above circuit is perfect :)
 
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