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My circuit does not work as I thought it should and I'm really confused now.

What I want? Two LEDs which will pulse to audio LEFT and RIGHT channel separately. And same for display BACKLIGHT, but there is going to be SPDT switch, which will switch between direct voltage or voltage from transistor - in order to react to sound.

So far I have this:

enter image description here .

1.) It works when I don't use power supply of GameBoy but external for the circuit.
This is what it looks like

2.) LEDs with no transistor will light UP when I use GameBoy power supply.

3.) Somebody told me before to put ground from audio to actual ground, not to emitter, but it works like that. When I tried to connect it to ground it stopped working. But still don't know why it works with audio ground on emitter.

4.) It is really weird, but when I switch the switch, it works as expected, BUT it switch LEDs not BACKLIGHT.

I really can't find any logical explanation in my head. I started with electronics few days ago, so there must be things I cannot see.

Thanks in advance!

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  • \$\begingroup\$ Tried to edit your picture in line. Can you please 1. Add your images directly instead of links? 2. Break up your images. Schematic is king here. Welcome to EE.SE! \$\endgroup\$ – winny Oct 28 '18 at 21:11
  • \$\begingroup\$ I don't know the gameboy voltage levels but I would put LED's on the collector side. Not the emitter, the emitter goes to ground. Google images "npn transistor led" will show you that in 90% of the cases. \$\endgroup\$ – Oldfart Oct 28 '18 at 21:21
  • \$\begingroup\$ The Audio Ground/Gameboy Ground should go to the negativfe side of the LED power supply. \$\endgroup\$ – Peter Bennett Oct 28 '18 at 21:36
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    \$\begingroup\$ This looks like the usual problem of people using npn transistors as a high-side switch. Try moving the LEDs and resistors to the collector of the transistor instead and see if that works. \$\endgroup\$ – Hearth Oct 28 '18 at 21:54
  • \$\begingroup\$ Yep, moving the LED did work.. But don't understand why. Anyways, thank you a lot. \$\endgroup\$ – Radim Lycka Oct 28 '18 at 22:15

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