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# tricky switch problem

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#### jim555

##### New Member
I am having trouble designing a circuit (footswitch modification for some medical equipment) to act as follows:
There are two spst switches, A and B, used to control lights LA and LB. The switches are continuous and toggle between gnd and +5. When the piece of equipment is turned on, both LA and LB must be off, regardless of the state of A and B. (they could have been left in either 0 or 1 state). If A or B is toggled, the respective light should come on and the other light should go off. A second toggle of the same switch should turn its light off and the other light should also remain off. Any help would ge greatly appreciated!!

jim555 said:
I am having trouble designing a circuit (footswitch modification for some medical equipment) to act as follows:
There are two spst switches, A and B, used to control lights LA and LB. The switches are continuous and toggle between gnd and +5. When the piece of equipment is turned on, both LA and LB must be off, regardless of the state of A and B. (they could have been left in either 0 or 1 state). If A or B is toggled, the respective light should come on and the other light should go off. A second toggle of the same switch should turn its light off and the other light should also remain off. Any help would ge greatly appreciated!!

If you can keep your switches and toggling down into the realm of 0-5V, you could use a JK flip-flop setup as a toggler(you might need 2), if your lights run from higher power, then use the output of your toggler to drive transistors to switch the lights.

Try a google search for JK flip flop or "toggle" flip flop to give you some ideas. Sounds like a problem easily solved by flip flops.. oh, and you can easily set it up so that you power up with lights out.

I havnt thought your design out - just trying to

Thanks opticon -

In fact the circuit I have come up with does use JK flipflops, one-shots & some gates. I will attach a drawing. It works in circuitmaker (software) and I am about finished with a breadboard version. It does seem too elaborate, so if you or anyone could simplify it, that would be great. The pulse generator modules in the drawing are implemented with 74123 dual one shots configured so that one fires on the rise and the other on the fall and then the outputs are 'anded'. That seems klugy...

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• circuit_188.jpg
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i vote for an FPGA if you think its too elaborate That is if you have the hardware available to program one

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