Replacing a white LED in a night-light with a red LED

[L.A.T.]

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?

 

[Franc Zabkar]

On Sat, 29 Nov 2008 23:26:13 GMT, "L.A.T." <tonyt@sci.net.au> put
finger to keyboard and composed:

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?

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
--
Please remove one 'i' from my address when replying by email.

 

[Alan Rutlidge]

"L.A.T." <tonyt@sci.net.au> wrote in message
newsckYk.1978$Et1.402@news-server.bigpond.net.au...

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?

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

 

[Jeßus]

On Sat, 29 Nov 2008 23:26:13 +0000, L.A.T. wrote:

The answer is bound to illuminate my ignorance,

Boom-tish, etc.

 

[L.A.T.]

snip

Are they all connected directly in parallel, without load sharing
resistors?
Yes

If so, then the red LED probably has a lower forward

voltage drop, hence it shunts the white ones.

Yes
Thank you

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[L.A.T.]

snip
"> 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

That is indeed the answer.

Thank you