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help with a 555? Answered

hi, to make a long story short i don't want to have to go into the cupboard and turn on my amp every time i plug it in, it has a button on it that needs to be pressed about 5 secs after power is turned on and then left like that, could i take a feed off the power supply, connect it to the 555 or another ic and make it activate a transistor 5 secs after power was applied? i only need it to do this once every startup, i don't want it pulsing on and off and it needs to be as simple as.
thanks, luke



8 years ago

Use an RC circuit with a 2N2222 or 2N2907 transistor depending on polarity. R*C is time constant of voltage rise at junction. Transistor switches at about 0.7 vdc. Collector of transistor provides output to control delayed signal.


8 years ago

555 can be configured in 3 (common) configurations.

1) Astable -- 'not' stable, it oscillates, back and forth forever.
2) Monostable -- 'one' stable, it oscillates once, then waits for input (trigger) and then repeats.
3) Bistable -- 'both' stable, also known as a flip-flop, it changes once with an input, and waits for another input.

Now, does that button have to be held in, or can it be hit once and it stays on?
You'll use monostable for this configuration, where circuit powers up, waits in the astable portion (about 5 seconds), then oscillates, and stays there -- the output (or inverted output) can be used to drive the button you want. You may need to use a relay as the 555 might not be able to source enough power for the button.

If the button has to wait 5 seconds, then just pulse once, you'll need 2 555's, or 1 556 timer (2 in one package).
In this case you use 2 monostable, where the first generates the 5 second delay, then when it triggers, it activates the second timer to do it's one pulse, and both return to stable wait mode.

http://www.bowdenshobbycircuits.info/ explains all.


Answer 8 years ago

no, it does not need to be held, you hit it once and it stays on, but if it is then hit again it will turn off, i am not very clever with these things but could put something together from a labeld schematic, if it isn't too mutch to ask, could you e-mail one to me at: ljthevj@googlemail.com ?
thank you - luke