Is the below a valid circuit implementation and are there any things to watch out for? It's a simplification of the larger circuit but the crux of it is that I need to sink current into two GPIO input pins. The pins are signified by the 1 and 0.

Also, would a diode on the pins be advisable?

enter image description here

Edit: Here's the non-simplified circuit. The question relates to the 3.3v part of it. In essence, the timer and decade counter sequentially pulse current, 'activating' the transistors one at a time. The keypad is a simple matrix keypad. What I'm wondering if I can do is to put a GPIO pin on the line between the keypad and ground. The intended effect is that, when a button on the keypad is pressed, 2 pins will change state - this signifies the row and column.

Note that some of the resistance and capacitance values are notional at the moment.

enter image description here

  • 1
    \$\begingroup\$ For me at least, I'd want to see the non-simplified circuit. Otherwise you've tailored the question to get the answer you think it is. Also, I can't at all tell what you're doing. \$\endgroup\$
    – Samuel
    Jan 9, 2014 at 18:10
  • \$\begingroup\$ The input connected to Gnd will stay in the low state, no? \$\endgroup\$
    – user28910
    Jan 9, 2014 at 18:41
  • \$\begingroup\$ Your non-simplified circuit has only three "0" points that are supposed to be GPIO as you say, input or output? And where are the "1" GPIO that are only shown in the simplified version? \$\endgroup\$
    – alexan_e
    Jan 9, 2014 at 18:48

1 Answer 1


If you connect a uC GPIO pin directly to ground, like you show at point "1" in your first diagram, it will not be much use for anything.

As an input, it will always read 0 because it is tied directly to ground.

As an output, you accomplish nothing if you drive the output low, and the uC could be damaged if you try to drive the output high.

  • \$\begingroup\$ I see where you're coming from. I'll open a new question with a revised solution for sanity checking. \$\endgroup\$
    – phil-lavin
    Jan 9, 2014 at 19:32

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