A few weeks ago, my uncle asked me if I could design an AC ON delay generator for him. As always I thought up something, a monostable using NE555. But there was a condition he put forward that he wanted an AC appliance to be turned ON when he presses a switch and should be able to turn it OFF whenever he wants, even before the delay timeout with that same switch. So a monostable with push-button trigger input won't work because we are talking about SPST switch here. Somehow I have to control the power to the circuit or the relay if I'm going to use it (sure, I'm going to use a relay to switch an AC load). Following is a circuit with such feature.
We could design a similar circuit using only transistors, but with 555 the circuit would be more versatile. The above is an adjustable monostable multivibrator. The capacitor C3 and trimmer R2 determines the time delay. A capacitor C4 is connected between Vcc and the active LOW input of the IC ie. pin 2. R3 is used to pull pin 2 to Vcc. Usually when the circuit is first powered up, the output will be LOW at pin 3. But here, the capacitor C4 triggers the pin 2 and initiates a timing cycle at the same time we power up the circuit through SW1. This causes the initial output to be HIGH which keeps the transistor Q2 used to drive the output relay, OFF.
After completing the first cycle, the output at pin 3 becomes LOW. This turns ON the transistor Q2 and it drives the relay causing it to switch from NC to NO. The circuit stays in this mode as long as there power. When the power is cut, the relay turns OFF.
D1 is the flyback diode. The transistor Q1 is used to discharge the capacitor. You can adjust the time delay using the trimmer R2 or changing the capacitor C3. The time delay can be calculated using the following formula.
But I didn't built this for him, instead I overthought things and designed something else in the first place. I thought I'd need a monostable to control the time delay and a latch for switching. So I designed the following circuit using an NE556 (dual 555 timer) and assembled the circuit in a perforated PCB ! The NE556 circuit works exactly as the NE555 circuit and is no better than the latter. Then why didn't I choose the simpler one ? The answer is, my bad! He was in a hurry too
The circuit is self explanatory. One internal 555 is used as a monostable and the other as a bistable latch. T2 is used to discharge C1 of any residue charge and C6 is used to suppress contact noise.
All images were taken with Moto G2.