Controlled Switch Model - Electricity and Electronic Circuits Step by Step

A controlled switch is a circuit model where a control signal chooses whether a branch is open or closed, so the lamp ledger can be reduced to path present or path absent. Here HIGH and LOW are ideal states, not a full transistor model; real switches have thresholds, resistance, leakage, delay, bounce, load limits, and measured uncertainty.

A transistor-like control is modeled here only as an ideal switch. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

A controlled switch is still a switch

In this book, a transistor-like control is modeled only as an ideal switch state. Semiconductor gain and biasing are deferred.

\text{control model} \Rightarrow \text{open or closed switch}

LOW leaves the path open

With the control switch open, the graph has no complete path through the lamp, so the lamp is off.

\text{LOW} \Rightarrow \text{open} \Rightarrow \text{lamp off}

HIGH completes the path

With the control switch closed, the graph has a complete path through the lamp, so the lamp is on.

\text{HIGH} \Rightarrow \text{closed} \Rightarrow \text{lamp on}

The control table has two rows

The control signal chooses the switch state. The diagram shows the HIGH row, but the rule also includes the LOW row.

\begin{array}{c|c|c}\text{control}&\text{switch}&\text{lamp}\\\text{LOW}&\text{open}&\text{off}\\\text{HIGH}&\text{closed}&\text{on}\end{array}