555 Timer Set-Reset Latch

This circuit uses the 555 Timer as a Set-Reset Latch with switches at the THRESHOLD and TRIGGER terminals. Sreset, when pressed connects THRESHOLD to Vcc through a series resistor. This resets the output to logic LOW. Sset, when pressed connects TRIGGER to GROUND through a series resistor. This sets the output to logic HIGH. Rtrgpu is a pull-up resistor for TRIGGER. Rthrpd is a pull-down resistor for THRESHOLD. When Scfg is open, RESET through THRESHOLD has precedence over SET. Thus, Sreset can override Sset. This behavior, however, can vary depending on the manufacturer of the specific 555 Timer. When Scfg is closed, THRESHOLD and TRIGGER are connected together, Rtrgpu and Rthrpd now biases these terminals to mid-supply. When Sreset and Sset are simultaneously pressed the output is preserved (No Change), the resistors in series with the switches to Vcc and Ground limit the current to the switches. Sclear serves as a master CLEAR. Qoc is an auxiliary open-collector output.