I have three illuminated momentary buttons with a separate anode and cathode for the led.

enter image description here (source of image)

I want a circuit that illuminates the LED of the button that was most recently pushed.

My initial idea has been to use three SR Latches created using NOR gates. Connect the S or Q of each latch to the reset of the two other latches. But then this creates the opportunity for HIGH on both S and R which is disallowed. Right?

I imagine there are other ways to accomplish this without latches too. Any help is greatly appreciated.

  • \$\begingroup\$ single chip solution - small micro controller. \$\endgroup\$ – JIm Dearden May 7 '15 at 18:51
  • \$\begingroup\$ If you disallow the possibility of multiple switches being pressed simultaneously you can use Ignacio's circuit. Otherwise a micro would be simple, or you could capacitively couple to the inputs of Ignacio's circuit. \$\endgroup\$ – Spehro Pefhany May 7 '15 at 19:04

The circuit you're looking for is called a "flip-flap-flop". It is essentially a 3-state (rather than 2-state) flip-flop. It can be implemented using a single 74xx27. It is the circuit on the right below:

flip-flop and flip-flap-flop

  • \$\begingroup\$ Thank you that seems exactly the type of solution I was going for. \$\endgroup\$ – Graham.Fraser May 7 '15 at 19:01

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