switch the brightness of an LED

I want to use LEDs in a motor vehicle, but they must be dimmed to about 10% of their daytime brightness at night to avoid blinding the driver. I need a simple, cheap circuit to do this.

My thinking was to use a light dependent resistor (ldr) to switch a transistor (tr) that then added a resistor (R2) in parallel with the LED. This resistor would reduce the current in the existing resistor R1 (as it is in series with it) thereby dropping the current available to the LED. But would it also change the voltage substantially at the resistor/LED junction so that the LED no longer lights?

+V--\\-----------------\\ | | ldr R1 | | | --R2--+ | / | +---Tr\\ LED | \\ | | | | 0V---/-------/--------/

Remember this is in a motor vehicle so there is plenty of spare power and heat sinking available.

thanx, bye.

Reply to
bgennette
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If you're using a uC to turn the LEDs on and off, can you just use PWM at 10% duty cycle? Hook the photocell to one of the uC's ADC inputs.

Reply to
DJ Delorie

Already run out of I/Os (and code space)

bye.

Reply to
bgennette

Maybe try a phototransistor.. D from BC

Reply to
D from BC

Buy a bigger micro.

Reply to
MooseFET

DJ Delorie wrote:

bgennette@ >Already run out of I/Os (and code space)

The *bigger uC* idea sounds apt.

Alternately:

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Reply to
JeffM

I have 1 ADC input and 1 LED output (PWMed) in less than 2K (out of a

16K AVR). I can't find any reasonable uC with less than 8K. How small can you get?
Reply to
linnix

I have 1 ADC input and 1 LED output (PWMed) in less than 2K (out of a

16K AVR). I can't find any reasonable uC with less than 8K. How small can you get?
Reply to
linnix

This would work better.

Set RA for minimum LED current and RA//RB for maximum LED current.

-o-----o-------o----o---- | | | | .-. .-. .-. .-. LDR | | | | RB | | | | RA | | | | | | | | '-' '-' '-' '-' | | | | | o--+ | | | | | | | | |/ | |< | o---| +--| | | |> |\\ | | | | | .-. .-. +----o | | | | | | | | | V LED '-' '-' - | | | --o-----o------------o----

(created by AACircuit v1.28.6 beta 04/19/05

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John B
Reply to
John B

forget resistors to do this stuff... use PWM to make them *appear* to be dim... your LDR idea is not the way to go - you have lights on,use this to signal to a PIC or similar to switch them

Reply to
feebo

Why not hook into the lights circuit? Thats way you will know if it is dark without a sensor ! PWM an LED from a simple micro.

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Reply to
Marra

How about paralleling a CDs cell across R1? Set the R1 value for night time luminosity.

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs (among other things)

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void _-void-_ in the obvious place

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Reply to
Boris Mohar

here's another way.

// LED input ---------->|----. | park lights ----[R1]--+ | 0V ----[R2]--' that may look screwy on your screen, use a fixed pitch font. I've never seen a LED damaged by being reverse biased at a low voltage (as may be possible with this circuit) but if you're worried add a diode in series with the LED.

Bye. Jasen

Reply to
Jasen

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