Comparator / Operational Amplifier Periodic Pulse Generator

Periodic pulse generator using a comparator or an operational amplifier. For single supply operation switches S1 and S2 should both be in UP position. For dual supply operation switches S1 and S2 should both be in DOWN position. With switch S3 open the timing capacitor Ct is charged and discharged from the op amp output. Diodes Dchg and Ddch separates the charge and discharge paths. Charging is through resistor Rthi, discharging is through Rthilo. With S3 closed: Rthi = timing resistor, along with Rthilo, for HIGH portion of the output signal. Rthilo = timing resistor both for HIGH and LOW portion of the output signal. When the output is HIGH the timing capacitor Ct is charged through diode Dchg. The charging voltage can be found by first subtracting the diode voltage drop from the output voltage. Then the Thevenin voltage is determined by voltage division through Rthi and Rthilo. The charging resistance is the Thevenin equivalent resistance (Rthi and Rthilo in parallel). When the output is LOW Dchg blocks the output signal. The timing capacitor Ct is then passively discharged to ground/Vee through Rthilo. If the desired mode of discharging is only passively to ground/Vee (S3 omitted and one end of Rthilo permanently connected to ground/Vee), diode Ddch can be removed. At lower frequencies polarized capacitor may be necessary for higher timing (Ct) capacitance. If dual power supply will be used S4 should be thrown right, this connects the negative terminal of the polarized capacitor to the negative power supply of the operational amplifier instead of to the ground.