EE 252 Laboratory Exercise

Schmidt Trigger


Design a Schmidt Trigger circuit to switch at TTL logic levels, 2.0 and 0.8 volts. Use the output of the Schmidt Trigger to drive a TTL 7400 gate, which in turn drives a 12-volt relay through an interface circuit. The interface circuit will consist of a voltage comparator integrated circuit and a PNP transistor. Thus, this exercise will demonstrate two uses of voltage comparators and the use of a PNP transistor.


It is sometimes useful to detect if a signal goes high and not let the output change again until a signal drops below a lower threshold. This type of circuit exhibits hysteresis and is called a Schmidt Trigger. For a circuit with hysteresis, as the inpu t signal increases, the output does not change until the input reaches a threshold. The output stays in this state until the input drops below another threshold, lower than the first. An example of a circuit with hysteresis is shown below. The voltage comparator is set up with the input signal going into the inverting input of the comparator. When the input signal crosses the threshold, the output of the comparator goes low. This change causes the threshold voltage at the non-inverting input of the c omparator to drop to a lower value. Thus, the input signal must drop back below this lower threshold before the output of the comparator can go back high.

Performance Requirements:

Inverting Schmidt Trigger with thresholds at 0.8 and 2.0 volts.
Use an LED to indicate the state of the comparator output.
The output of the voltage comparator circuit must also drive a 7400 gate which in turn drives a 12-volt relay through a voltage comparator and PNP transistor interface circuit suggested below.


Use a LM311 voltage comparator for both the Schmidt trigger circuit and the interface along with a 2N3906 PNP transistor to drive the relay. The 7400 gate may be any standard 7400 series TTL gate.

Schmidt Trigger CircuitTransfer characteristic with hysteresis

Suggested circuit configuration