Tricky Circuit

A window comparator

Detect the safe middle

A comparator compares voltages and makes a high/low decision. A window comparator combines two comparisons to report whether an input lies between a lower and upper limit. Window width is their difference. Here output is high only strictly inside the interval.

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EXPERIMENT BENCH

Change the circuit settings, predict the readings, then check a challenge. These are learning models, with the assumptions explained in each lesson.

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How it works

Ideal 0 V/5 V decisions, no delay, noise or memory. Equality at either boundary is deliberately outside. Two limits do not automatically give hysteresis, where the previous state changes a switching decision.

Vlow and Vhigh are lower and upper thresholds. Output Vout = 5 V only when Vlow < Vin < Vhigh; otherwise 0 V. The window width is Vhigh − Vlow. Crossing a boundary changes the decision immediately in this model.

Difficulty

  1. Build · Observe

    Power on with input 2.5 V, lower limit 1 V and upper limit 4 V. Obtain 5 V.

    Need a hint?

    Vin=2.5 V; Vlow=1 V; Vhigh=4 V → 5 V

  2. Explore · Explore

    Keep limits 1 V and 4 V. Put input exactly at 1 V. Obtain 0 V.

    Need a hint?

    Vin=Vlow=1 V; Vhigh=4 V → 0 V

  3. Solve it · Solve

    Keep limits 1 V and 4 V. Put input exactly at 4 V. Obtain 0 V.

    Need a hint?

    Vin=Vhigh=4 V; Vlow=1 V → 0 V

Extra reading and circuit recipes

A window comparator

“Too low” and “too high” can both be unwanted. A window detector marks the middle range, like checking whether a sensor value falls within a chosen tolerance.

Vlow < Vin < Vhigh → Vout = 5 V; Vin ≤ Vlow ∨ Vin ≥ Vhigh → Vout = 0 V

Two comparisons must both be true to select the middle: above the low limit and below the high limit. The boundary convention is a teaching choice, not a hardware guarantee.

A useful connection

The same output can mean many different inputs. A high result says inside, not exactly where.

Beyond the model

Sweeping upward or downward gives the same result here. The circuit has no stored state.

A useful trap to avoid

A window is an interval test. A control deadband, a range with no corrective action, is another idea; neither automatically creates hysteresis.

Input is exactly equal to the upper limit. This model outputs…

  1. 5 V
  2. 2.5 V
  3. 0 V
Read more

The chosen test uses strict inequalities, so equality is outside.

Inside the window

Use input 2.5 V with limits 1 V and 4 V.

Both comparisons are true: output 5 V.

At the boundary

Use input 4 V with limits 1 V and 4 V.

Equality is excluded: output 0 V.

Try another change

Sweep input 0.5, 1, 1.5, 3.5, 4 and 4.5 V. Predict low, low, high, high, low, low; reverse the sweep and compare.

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