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I need a very simple circuit to connect a signal to one of two ways like this:
My com signal is actually a microphone input, so very low voltage, <200mV, how can I make a solid state switch that not interfere with the low voltage signal? So com can go to L1 or L2 with no distortion. Im planning to actuate the switch with TTL voltage.
Two suggestions:
You can use e.g. an analog switch/MUX like the 4053.
If the signal you want to switch is a pure AC signal you might also be able to use a diode that is switched between forward and reversed biased. See for example here in section "Analog Switch". This method is often used with signals in the RF range.
The electronic solutions proposed are fine and will do what you want. However, if you want a more generic and flexible solution I'd consider using a decent quality reed relay. You are not limited with supply rails in this method and the input can be totally floating and isolated from your control voltage.
Here is a decent offering from Pickering relays. It can be driven from TTL and is suitable for switching thermocouple inputs (this means it will be fine for low level audio signals too).
Using a dedicated analog MUX, as stated above, would probably be the best solution. You can, however, build it from discrete components. Following circuit should do the job. Each switch (otherwise known as a transmission gate) consists of two MOSFETs, P- and N-Channel respectively. It is activated by applying VCC to N-FET's gate and by pulling P's gate to GND. Applying voltages in the opposite order will disable the switch. That's what the inverter's doing (it can be realized by two MOSFET's as well).
Note that this is only guaranteed to work when used in the form of an off-the-shelf IC. It may or may not work when bodged together using discrete CMOS
It will function properly - more or less linearly - if the INPUT voltages remain below VCC - Vth, where Vth is the switch-on threshold voltage of the MOSFET's.
