Now that 5mm White LED’s with high light output are economically available, they can be a good solution for various ‘background’ lighting, both indoor and outdoor.
Many commercial implementations are very crude (if you’re lucky, there will be just a single resistor between supply and the LED’s; one “lantern” I saw had pairs of white LED’s directly connected across a 6V SLA battery) so I’ve ended up with a few circuits for my use. All are capable of operation on a nominal 12V supply, and will work from 10V to 20V (and more if additional heatsinking is provided).
The first circuit is a simple 20mA current limiter for powering 3 white LED’s . Almost any NPN transistors can be used:If you’ve got lots of PNP transistors instead, or want ground referenced LED’s, use this version:If you only want the LED’s to come on when it’s dark, add a CDS photocell as shown; you might need to change the value of the 1M resistor for different light levels. Note that the LED’s come on gradually as it gets darker!
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