Circuit knowledge

By replacing the common-emitter resistor in a conventional Schmitt by a zener diode,the hysteresis normally associated with these circuits is eliminated. 



One cheap IC, ul.914, can be used as an extremely simple and effective Schmitt trigger suitable for many applications. Hysterisis of the circuit is about 0.1 Volt. This may be varied by altering the values of R1 and R2. 



An LED and phototransistor optical coupler system may be used to lengthen a three microsecond pulse to 55 milliseconds by using the values shown in the diagram. This allows complete isolation between input pulse circuits and the lengthener to be obtained.