Tricky Circuit

Schmitt trigger and two thresholds

A decision that remembers

A Schmitt trigger switches at two thresholds. Hysteresis means that between them the output remembers its previous state. Here it changes high at 3 V and low at 2 V.

Open interactive experiment

EXPERIMENT BENCH

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

Sign in to save progress. Guest progress stays on this device.

Open laboratory in a full window

Why it works

Ideal non-inverting trigger, fixed 0 V/5 V output and inclusive thresholds at 2 V and 3 V. It starts low; power off or reset clears memory. No noise waveform, delay or physical feedback resistors are modeled.

Vin is input voltage and Vout output voltage. Vin ≥ 3 V sets Vout = 5 V; Vin ≤ 2 V sets Vout = 0 V. Between 2 V and 3 V, keep the previous output. The next switching threshold is 2 V when high and 3 V when low. The 1 V gap is the hysteresis band; this is state memory, not a time delay.

Difficulty

  1. Build · Build it

    Power on and raise input to 3.5 V. Switch the output high at 5 V.

    Need a hint?

    Vin = 3.5 V → Vout = 5 V

  2. Explore · Explore the boundary

    First raise input to 3.5 V, then lower it to 2.5 V without resetting or switching power off. Keep the remembered high output.

    Need a hint?

    Vin: 3.5 V → 2.5 V; Vout = 5 V

  3. Solve it · Solve the challenge

    Lower input to 1.5 V. Cross the lower threshold and reset the output to 0 V.

    Need a hint?

    Vin = 1.5 V → Vout = 0 V

Extra reading and circuit recipes

A decision that remembers

Picture a heating controller with one temperature for turning on and another for turning off. Separating those decisions prevents every tiny change near one boundary from reversing the switch. The same idea works with voltage: our two thresholds create a region where history matters.

Vin ≥ 3 V → Vout = 5 V; Vin ≤ 2 V → Vout = 0 V; 2 V < Vin < 3 V → Vout(t) = Vout(t−1)

The active threshold depends on the current output. A high output waits for the low threshold; a low output waits for the high threshold. This prevents small reversals inside the band from undoing a decision.

A useful pattern

At the same input of 2.5 V the output can be 0 V or 5 V. Both are valid: arriving from below 2 V and arriving from above 3 V give different remembered states.

Beyond the model

Positive feedback means returning some output to reinforce the decision. Real Schmitt circuits can use it to make separate thresholds; this model specifies the thresholds directly, rather than calculating a resistor network.

A useful trap to avoid

Hysteresis does not wait a fixed time and does not reject every possible disturbance. A change crossing the opposite threshold still changes the output.

After input 3.5 V made the output high, input falls to 2.5 V. The output…

  1. falls to 0 V
  2. stays at 5 V
  3. becomes 2.5 V
Read more

The input is inside the memory band and has not reached the lower 2 V threshold, so the previous high state remains.

Build it

From a reset state, power on and set input 3.5 V. The output becomes 5 V; its next switching threshold is 2 V.

Vout = 5 V

Explore the boundary

Power on and set input 1.5 V. The output becomes 0 V; its next switching threshold is 3 V.

Vout = 0 V

Change one thing

Compare two journeys to 2.5 V: 1.5 V → 2.5 V and 3.5 V → 2.5 V. Reset between journeys. Record both outputs to reveal the stored history.

ti.com