Arduino / embedded-devices - the learning begins

Significance - learning getting underway...
No prior idea of details - here encountered and running with them.

Thing is - find there is / are issues even connecting a "simple" push-button in a digital logic circuit...

[explained shortly]

Find - need to make state of signalling connections to semiconductor logic devices determinate.
Either

• HIGH - typically at the 5V supply voltage
• LOW - typically ground (GND) connection

So find "pull-up" and "pull-up resistor" [Wikipedia] to HIGH supply voltage;
likewise "pull-down" and "pull-down resistor" to GND ground connection might be an alternate strategy (it is an either/or - "pullup" or "pulldown").

No further fundamental explanation offered.

Started with Arduino "example" "DigitalInputPullup.ino"

First picture "built-in "PullUp" on pin 2" is where find this issue is so common that you can instruct the digital connection pin to be "PullUp"'ed - so external circuit off Arduino is very simple and sparse.

Second picture is where commented-out "pinMode(2, INPUT_PULLUP);". Initially LED lighting randomly when button not pressed. Then added in the circuit the "pull-up" resistor connected to on-board supply voltage and the circuit behaved like the first circuit.

The path ahead beckons...

(R. Smith, 18Mar2024)