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.