You can use the standby/on switch in Figure 1 for industrial or telecom applications in which the circuitry must somehow “remember” its state—standby or on—after a power failure that occurs when no operator is present. An alternative approach uses a battery or a supercapacitor and a flip-flop. This approach is less reliable, however, because the circuit can lose its state if leakage current drains the battery. Another alternative involves the use of a microcontroller and EEPROM, but that approach requires software plus a provision for start-up time. Also, a stand-alone EEPROM for this application has an awkward interface.
You can use an electronically programmable voltage reference, IC4, as a single-bit nonvolatile-memory cell. To remember the state of the standby/on switch, this circuit programs IC4’s output voltage high or low and can reprogram it at least 50,000 times. IC1 is a low-dropout linear regulator with reset output and a wide input-voltage range that extends to 72V. A microprocessor supervisor, IC2, debounces the standby/on pushbutton and supports the programming of IC4 by increasing the pause length between pulses. IC4’s output drives IC5, an inverter with Schmitt-trigger input, which in turn drives the gate of transistor Q2 to control the main power supply.
Flip-flop IC3 helps to change the standby/on state with each press of the control button. At the end of IC4’s programming cycle, a low-to-high edge at IC3’s clock input sets the flip-flop to its opposite state, thanks to the feedback from the inverter. IC2’s reset triggers this action at power-up to ensure that the switch is ready to change state. Transistor Q1B and IC1’s reset output prevent the programming of incorrect states by blocking IC4’s adjust input during start-up and power-fail conditions.
You must block the effect of IC2’s power-up or -down reset pulse on IC4’s adjust input; C2 therefore sets IC1’s reset time-out to be longer than IC2’s reset time-out. The threshold voltage of IC2, 2.9V, is also lower than that of IC1, 4.6V. The worst-case 1.32V input-threshold voltage of IC5 guarantees the standby position at first power-on because the factory-preset output for IC4 is only 1.2V.Click here for the illustrations:
Caption
Figure 1: The circuit remembers its standby or on state if power fails with no operator present.
