.3VDC input to drive LED (automotive)

[Stratocaster]

One last question, being a newbie and all, What transistor should I use? Got a part number?

 

[MikeMl]

2n3904, 2n2222, 2n4124, 2n4401, etc.

 

[Stratocaster]

Mike, is this the circuit you are talking about?

**broken link removed**

Reduce Rc to ~680 and put the LED in series with RC. Hook a 1K resistor from the left end of Rb to +12V (same place as Vs).

OK, After far too long, I had time to test this, and the results are in.

The normal operation of the dash list is as follows. When the car is turned on, the dash lite will light, after a few seconds, it will go off. What I am seeing with the circuit, the LED will be bright, and then dim. I need to to go off. What am I missing?

 

[audioguru]

Maybe you are from a country where you read things backwards.
Your schematic should have the input on the left side and the positive supply at the top.
I corrected it but then I see that the LED is connected backwards.

The transistor that turns on the LED is driven from a voltage divider so it might not turn off.

 

[Stratocaster]

Thanks, that was caught long ago. I messed around with it today, and found that if I replaced RB=10K with a zener diode, the circuit works as required.

Thanks again for reading my thread.