Pullups and Decisions

Input Controls Output

a switch can invert a signal

A controlled switch can make a high input produce a low output in this ideal pull-up model. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Open input gives high output

When the controlled switch is open, the output is 12 volts. The controlled switch is only the circuit role of a transistor here; gain, saturation, base or gate current, and CMOS details are deferred.

open input⇒Vout=12 V\text{open input} \Rightarrow V_{\text{out}} = 12\ \text{V}
Input openAn open controlled switch leaves the output high.12 V6 ohmopen+-12 VHIGH

Closed input gives low output

When the controlled switch closes, the output becomes 0 volts.

closed input⇒Vout=0 V\text{closed input} \Rightarrow V_{\text{out}} = 0\ \text{V}
Input closedA closed controlled switch pulls the output low.12 V6 ohmclosed2 A+-0 V+-12 VLOW