This circuit allows a 4-to 20-mA current loop to control an isolated SCR drive. IC1A and are one-shots. Q2 detect
s zero crossings of the 120 Vac line, which triggers one-shot IC1B. IC1A causes Ql to discharge C2. When C2 recharges through R2, it triggers IC1A, and the optoisolator and SCR1/SCR2. Triggering of SCR1 and SCR2 is a function of input current, which can control motor speed, light intensity, etc.