schmitt trigger

As long as the input voltage to this trigger circuit is less than the lower threshold level, TI is cut off; T2 will be saturated through R2. Consequently, the T2 collector voltage will be low, determined by the T2 saturation voltage (approximately I 00 m V) and the voltage across the positive feedback diode D2. The latter voltage can vary over a limited range as a function of the power supply voltage and the R3 resistance. Both these parameters may be selected between wide limits without upsetting the operation of the circuit.