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I made a circuit based on this post, Alternating between two LEDS using BJTs.

It seems to work in the simulation just fine but when I'm connecting it with real components, I get funny behavior. Here's my circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

In my real world circuit, when the switch is closed, the green LED comes on just fine and the red is off, just like it's supposed to do.

schematic

simulate this circuit

When the switch is open, both the green and red LEDs and both are faint. I tried adding a pull down resistor at the base of Q13 and it does nothing.

Any ideas? It's definitely hooked up right on my bread board.

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I'm working with the spare parts I have laying around and I have it working now with this:

schematic

simulate this circuit – Schematic created using CircuitLab

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It appears your red LED is not very bright because 10K should be enough to light the red LED fully. Either that or something is wrong with the wiring or parts (Q14 damaged or reversed emitter-collector could do it).

To keep the the green LED from showing dimly when the red LED is on, you can put a resistor across it. 1K to 1.5K should do the job. That will steal a bit of current away from it when it is 'on', but probably not enough to even notice. Use 10K for the base resistor not the 1K in your top schematic. You can use 10K for R43 as well.

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  • \$\begingroup\$ Spehro, thanks for the feedback. I've tested each transistor individually and they both work like they should. R43 and R44 are both 10k now and putting a 1k across D19 didn't do anything. So I'm still SOL. I think I'm going to try changing out Q13 and Q14 with MOSFETs and play around with that. \$\endgroup\$ – Craigfoo Mar 28 '15 at 17:55

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