# In search of a simple circuit

Discussion in 'General Electronics Chat' started by onejames, Jul 4, 2011.

1. ### onejames Thread Starter New Member

Jun 12, 2010
16
0
I need to have a led light when an npn transistor is in an off state, and turns off when it is closed.

Could someone point me in the right direction?

Thanks!

2. ### #12 Expert

Nov 30, 2010
18,094
9,676
Put the transistor in parallel with the LED and have it dump the LED current to ground when it is on.

3. ### mbohuntr Senior Member

Apr 6, 2009
431
32
Thanks #12, I whipped up a circuit, but I wanted to see if I was on the right track before I posted. My values are probably off some though.

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4. ### #12 Expert

Nov 30, 2010
18,094
9,676
I'd connect the collector to the point labeled 5 and eliminate R1.
There is no fear of the transistor not saturating enough because LEDs always use more than a volt...maybe even 2 volts to activate.

The way you have this hooked up, the transistor will have to dump half an amp!

5. ### mbohuntr Senior Member

Apr 6, 2009
431
32
Could you draw that? I was simulating a load for the transistor with the 10 ohm. I figured that keeps it under the 800mA limit. I am not really good at BJT calcs other than keeping the base at a fraction of the collector current.

6. ### #12 Expert

Nov 30, 2010
18,094
9,676
I'm too lazy to do a drawing. I haven't figured out anything better than to draw the circuit with a pen, scan it, shrink it, crop it, conver it to a png, and upload it. Just remove the 10 ohm resistor and connect the collector of the transistor to the junction of R2 and the LED. That way, you're simulating a load for the transistor with R2. Then you can change R3 to 4.7k.

The problem with your circuit is that it makes no sense to dump hundreds of milliamps through the transistor when the LED only uses about 8 ma. Dump the 8 ma to ground with the transistor.

7. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
674
I edited your drawing. I added a 10k B-E resistor, because I don't like floating inputs.

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11.3 KB
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8. ### mbohuntr Senior Member

Apr 6, 2009
431
32
Thanks guys, Both James and I got something from this!

9. ### onejames Thread Starter New Member

Jun 12, 2010
16
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Yes thanks! I'll work it into my circuit when I get home .

10. ### onejames Thread Starter New Member

Jun 12, 2010
16
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Works like a charm!