Robots - Notes - One Pin Switch and LED Circuit
 

This simple circuit is devised to allow a single microcontroller I/O pin S, for Signal, to operate as an input push button switch or as an output LED.

At 4.9V normal draw is 1.35mA with D1 lit and SW1 open. The table below indicate measurements with pin S in output high, output low, and as input presumably with high z. All measurement were taken with SW1 open (normal) and closed.
 
             
 

SW1 = Open (normal)

SW1 = Closed
TP1 TP2 D1 TP1 TP2 D1
S = Output 0 0 ma 0 ma Off -2.2 ma 2.22 ma Off
S = Output 1 1.35 ma 0 ma On 1.35 ma .04 ma On
S = Input Z 0 ma 0 ma Off 0 ma .7 ma On

Highest draws, 2.2mA, were measured with SW1 closed and pin S configured as output low. Having the button pushed while the pin is set to output is not a normal operating condition but the results seam to be well within the 20mA limit of the pin.
 

Patch panel with two "One Pin" circuits. Each circuit takes Power, Ground, and MPU pin.

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