On the October 14, 2024, a design idea (DI) by Nick Cornford entitled “To press ON or hold OFF? This does both” appeared. It is a very interesting DI for DC voltages, but what about AC voltages?
Wow the engineering world with your unique design: Design Ideas Submission Guide
After reading this DI, I decided to design a circuit with similar operation for the much-needed AC voltages since many of our gadgets are connected to 110V/230V AC mains. In Figure 1’s circuit, if the single push button SW1 is pressed momentarily once, the mains AC voltage is extended to the output where a gadget is connected. If push button SW1 is pressed for a long time—4 to 5 seconds—power is disconnected. In my opinion, a shiny modern push button looks more attractive and elegant than a toggle switch.
Figure 1 If you press SW1 once, the AC output terminal J2 gets AC supply. If you hold SW1 for a long time, i.e., 4 to 5 seconds, the path from the power supply to terminal J2 gets disconnected. One single pushbutton provides both ON and OFF functions for AC voltage.
In this circuit, mains AC is fed to output terminal through the U5 triac. It should be selected according to voltage and current requirement. When you press SW1 once momentarily, it triggers a monostable U2A. Its raising edge pulse output sets the flip-flop U4A. Q2 becomes ON and current flows through the photodiode of U1. Optotriac U1 conducts and hence triac U5 also conducts. Thus, mains voltage is extended to output terminal.
If you press SW1 for a long time, i.e., 4 to 5 seconds (this time can be adjusted by changing R4, R5), capacitor C1 charges. When its voltage reaches the reference voltage set by the R4, R5 division, the comparator U3A output goes HIGH which resets flip-flop U4A. Thus, the flip-flop output goes LOW, switching Q2 OFF. At this point, no current flows through photodiode of U1, hence U1 and U5 are switched OFF. This way, the mains voltage to output is disconnected.
When you press SW1, C1 is charged. When SW1 is open, there must be a path to discharge C1 for proper operation of the next cycle. This is done by Q1. When SW1 is open, current flows from C1 through the emitter-base of Q1 and R1. Hence Q1 gets saturated and discharges C1. When SW1 is pressed, voltage is applied to base of Q1 via R7 and hence Q1 becomes open and allows C1 to charge. Being CMOS IC-based, the entire circuit draws very little current.
VDD here is 5 VDC. The VDD and VSS pins of U2, U3, and U4 are not shown in the circuit. They must be wired to VDD and VSS inputs shown. If you want a more simple circuit, the U1, U5 circuit can be replaced with a simple relay.
Jayapal Ramalingam has over three decades of experience in designing electronics systems for power & process industries and is presently a freelance automation consultant.
Related Content
- To press on or hold off? This does both.
- Smart TV power-ON aid
- Latching power switch uses momentary pushbutton
- A new and improved latching power switch
- Latching power switch uses momentary-action pushbutton
The post To press ON or hold OFF? This does both for AC voltages appeared first on EDN.