What Circuit Do I Need

Projects General Guidance
1

Rank amateur here, hoping someone can point me in the right direction.

I need some kind of switching circuit, here are my parameters:

  1. I need a voltage (12v) running to either fan A or fan B (either way one fan is always on).
  2. When a signal is sent down line A (5v) I need fan A to turn on, and fan B to turn off.
  3. When a signal is sent down line B (5v) I need fan B to turn on, and fan A to turn off.
  4. BUT when the signal stops on either lines, I need that last fan that was switched on to stay on.

I cannot control the incoming signal. It's part of an existing arduino project that I can't change it's software, I just sort of want to 'tap into the pins' with a custom shield with my circuit. The signal down lines A and B is always either/or, never at the same time.

Is there a way to do this with ordinary components from radioshack? (without another microcontroller)

Right now the only way I can think of is to program another microcontroller, like an attiny, to control a pair of fets. That would be easy enough for me to execute but requires mail order.

2

Couple of relays and transistors to sink the coil current, diode across the coil.
Set/Reset flip flop, with Q and Q/ outputs controlling the transistor.

May need to invert the incoming signals so that inactive state ("when the signal stops") is a high level, not a low level.
Use a 4 x 2-input NAND gate. Wire 2 as the flip flop, wire 2 as signal inverters to feed the flip flop input.

3

Ah, I knew it was simply a matter of googling the right terms. Thank you, CrossRoads.