# help in a project

Discussion in 'Electronics Homework Help' started by Ghost69, Jun 25, 2013.

1. ### Ghost69

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Jun 25, 2013
i am trying to make a delay circuit which gives a delay of 4 minutes,during which the
supply is disconnected from the output.The delay must start once power is cut off from the circuit,and even if power is back before 4 minutes the power must be cut off from the output till the 4 minutes are over.all the components must be powered by the input source.please do help..

2. ### john monks

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Mar 9, 2012
Did you consider a battery backed-up count-down counter with a relay?

3. ### Ghost69

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Jun 25, 2013
ya i already solved it using a 555 timer powered by a battery and a relay to the output.but the project coordinator has now specified that a battery cannot be used for powering the delay circuit

4. ### john monks

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Mar 9, 2012
How about a large capacitor? What is fair game anyway? Something has to power a timer circuit.

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
How about a mains operated electromagnet is used to hold an anvil.

When the power goes off the anvil is released causing the anvil to fall onto a see-saw.

At the other end of the see-saw is a rock. The rock is thrown into the air striking the cage of a large parrot.

The parrot will emit a loud squawk which will awaken a sleeping cat.

the cat will leap up and run off. The string tied to its tail will pull over a piece of wood which is preventing a metal ball from running down a track.

The metal ball runs down the track, finally coming to rest against a pair of electrodes. The ball completes the circuit and the device is again powered.

6. ### Ghost69

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Jun 25, 2013

i was thinkin the same thing,but is there no other compact way to deal with the problem?

7. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
It makes one ponder and muse. Has there ever been college credits for the most creative Rube Goldberg?

Seriously though, 4 minutes is a long period for a 555 and that's without any power source concerns. In fact it's not easy to do with any RC based timing scheme. Any noise that infiltrates the ramp will make timing repeatability erratic.

How about a super cap and a micro placed in Sleep mode?

Chris

8. ### BobK

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1,688
Jan 5, 2010
Simply have a capacitor that is charged when power is on and discharges through a resistor when power is off. If, when power is supplied, the capacitor is above a threshold, the device is held in the off until the cap discharges.

Bob

9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
I've moved this to homework help for the obvious reason that it is homework.

I am now also wondering how a 555 would behave if I used a 1F supercap and reasonable value resistors... (I realise it's not a solution to this problem)

10. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Rereading this it occurred to me that we don't know what your power source is and we don't know what your load is. Saying "when power is cut from the circuit" could mean many things. Are we talking Mains power?

Chris

11. ### Ghost69

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Jun 25, 2013
yes mains power is concerned here.i would also like too know how to trigger the 555 IC only once when power is cut off.

180
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Feb 23, 2013
You will need some sort of power source during that 4min period. I guess you can use a phototransisotor to send a signal to the 555 so it will "know" when there is power and when there isn't.

Last edited: Jul 1, 2013
13. ### CDRIVEHauling 10' pipe on a Trek Shift3

4,960
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May 8, 2012
Has your instructor locked you into a 555 or are you free to choose your components, less a battery source? I also have a few more questions,

What is supposed to happen if 4 minutes expire but power has not been restored?
What is supposed to happen if power flickers on - off during the 4 minutes? Is it supposed to reset to the start of the 4 minute count?

Chris

180
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Feb 23, 2013
so you are going towards a picaxe with this right?

15. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Actually no. Not because I don't love them but because it's doubtful that his instructor will let him use one. I asked because I liked Bob's suggestion but I need to know the answers to my questions to see how I can implement it, or even if it can be implemented. Personally, I was surprised that no one responded to his suggestion, .. including me. I have an excuse though. It's too much multitasking. I get involved in more topics simultaneously than my aging brain can handle. At any time of the day or day of the week I have three to four Sim's up on my screen, all from different topics.

Chris

16. ### Ghost69

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Jun 25, 2013
nope is not necessary to use a 555 IC.
when the 4 minutes expire the mains should be connected again to the output.
even if power is back before 4 min the count is not supposed to be reset.

17. ### Ghost69

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Jun 25, 2013
and does anybody know anythin about a discharging type monostable circuit?