# Logic controlled bipolar switch

#### mikep56

Joined Feb 27, 2009
7
Hi All,
I am looking for a simple schematic of an electronic switch. This switch will be driven from a TTL level signal. The output of the switch will be +5 and -5, depending on the input level.
I have used this before, but I do not remember the exact details. I think that it had an NPN and a PNP transistor with the emitters connected, and the +5 fed to the NPN collector through a resistor, and the -5 fed to the PNP collecctor through a resistor. The tied emitters is the switched voltage point.
Thanks for any help on this.
Regards,
mike

#### praondevou

Joined Jul 9, 2011
2,942
Do you mean a level shifter?

1. What's the output voltage for 0V input? -5V?
2. Whhat's the output voltage for 5V input? +5V?

Or is there any reason to call the circuit a "switch"?

#### mikep56

Joined Feb 27, 2009
7
Hi praondevou,
For 0V in the output is -5V, for +5V in the output is +5V. So it level shifts a logic 0 to -5V and a logic 1 to +5V.
I guess I mean a level shifter.

Regards,
Mike

#### SgtWookie

Joined Jul 17, 2007
22,230
I'd posted something like this before; it's pretty simple, just five resistors and a couple of transistors.

As shown, it'll be able to source current from +5v pretty well, but depending on your application, you might require more current sinking down to -5v. If your load is light, this should work fine.

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#### mikep56

Joined Feb 27, 2009
7
Hi SgtWookie,
Thanks for the circuit diagram. It will be used to drive a '4066 analog switch, so the current draw is minimal. As I said in previous posts, it will take a logic 0 and output -5V, and a logic 1 and output +5V; to drive the switch inputs of a '4066.
Thanks again for all the feedback.
Regards,
mike

#### John P

Joined Oct 14, 2008
2,011

#### SgtWookie

Joined Jul 17, 2007
22,230
Ahh, I see. Yes, it'll work fine for that.

If you are switching audio, you may wish to slow the transition down quite a bit to avoid "popping" noises. If the 4066 inputs change slowly, they'll go through a resistive state rather than a fast switch from one state to the other; giving the voltage levels on either side of the switched points more time to equalize.

#### SgtWookie

Joined Jul 17, 2007
22,230
I don't get it. The 4066 takes inputs which are 5v and 0v (nominally) but never a negative input. Am I misunderstanding?

http://members.shaw.ca/roma/4066.pdf
Hi John,
Our OP is probably using the 4066 for switching audio in a preamp or something similar. It's pretty usual for such audio amps to have bipolar supplies. If they simply used ground and +5v for the 4066, the lower half of the waveform would be lost.

They're probably controlling the 4066 using a microcontroller or other logic level device; so they needed an interface between the different logic levels.