I have a P.I.R. motion detector (from Ebay) that turns on six white LEDs. If I remove one or more of the LEDs the remaining LEDs still glow. If I try to replace one of the original white LEDs with a common red LED, the red LED glows and the other five white LEDS do not. The answer is bound to illuminate my ignorance, but I can take it. What is the answer?
Boom-tish, etc.
On Sat, 29 Nov 2008 23:26:13 GMT, "L.A.T." put finger to keyboard and composed:
Are they all connected directly in parallel, without load sharing resistors? If so, then the red LED probably has a lower forward voltage drop, hence it shunts the white ones.
- Franc Zabkar
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Most common red LEDs have a typical voltage drop of about 2.0 volts. Most common white LEDs have a typical voltage drop of about 3.5 volts. Depending on the circuit configuration (I'm assuming the LEDs are in parallel fed from a common series voltage dropping resistor), the red LED having a lower voltage drop than the white ones is not leaving enough voltage across the white LEDs for them to turn on, hence no light emission from them. With the red LED out of the circuit, measure the voltage across the white LEDs. Then replace one of the white LEDs with the red one and re-read the voltage across the LEDs. If the voltage has dropped considerably (see typical figures above), there's your answer.
Cheers, Alan
snip
Yes If so, then the red LED probably has a lower forward
Yes
Thank you
snip "> With the red LED out of the circuit, measure the voltage across the white
That is indeed the answer. Thank you
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