#750 Pushbutton board

Hello all, I have a question about this board.

I am using a lighted momentary pushbutton with this board and am running into an issue. The switch has three pins with the led cathode and one side of the switch tied together. On the (greatly) simplified schematic it appears the switch inputs are isolated but they do not seem to like being tied to ground. I have tried both variations with tying an input to ground and the output either latches on correctly but will not turn off or the output will not function at all. So, my question after the lengthy explanation is: can the board be used with this type of switch?


First, the answer to your basic question: unfortunately, no. If you want to use the circuit with an indicator LED, the LED cannot share a terminal with the switch.

Second, be careful about your approach. The simplified schematic specifically shows the button terminals not connected to ground. You characterize them as “isolated”, which is not correct in the sense that they are obviously connected to something useful. Either way, shorting a node that you do not expect to be grounded to ground is a good way to destroy stuff.

- Jan

Hi Jan,

Thanks for the reply. I think the solution will be to drive an optoisolator and power the led with a couple AA batteries. Easier than building in a separate supply for just an led running at 5mA.

Thanks for the warning, always worth hearing no matter how much experience one has. I had taken resistance, impedance, and voltage readings at the switch inputs with the output in both states and felt it was safe to do so. If I had any doubts I would have included some build out resistance.

Thanks for the help!