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Old 9th Jun 2007, 21:30   #1
Gambler
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5 Second Timer

Hey all!

I'm a complete electronics newbie, but I still try!

Anyway, I need a little help. I want to put together a timer that works like this:

1. I close a switch
2. A timer delays 5 seconds
3. A second circuit closes after the 5 seconds.

I looked at the schmitt trigger, but I cant figure out how to put everything together.

Thanks!
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Old 9th Jun 2007, 22:02   #2
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So just to clarify, you close the switch on the timer, wait five seconds, then the other circuit switches on?
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Old 10th Jun 2007, 00:10   #3
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Yep, the main goal is to put lag in the button. Push...wait...ah... Releasing the switch can (and would be best) turn everything off.
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Last edited by Gambler; 10th Jun 2007 at 00:19.
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Old 10th Jun 2007, 00:54   #4
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http://555-timer.clarkson-uk.com/operation/delay.html
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Old 10th Jun 2007, 02:45   #5
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I warned you that I was an electronics newbie. I know that pin 3 is often used on a 555 for the oscillation, so I am assuming that is is the "Output" that is referred to. But...is the output negative or positive?

What is a common capacitor-resistor combination that I could most likely steal off some old projects?

http://555-timer.clarkson-uk.com/operation/mono2.gif

Thanks!

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Last edited by cpemma; 10th Jun 2007 at 12:21.
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Old 10th Jun 2007, 11:45   #6
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Quote:
Originally Posted by Gambler
But...is the output negative or positive?

What is a common capacitor-resistor combination that I could most likely steal off some old projects?
The output will be low until the 5 seconds has passed then the output will switch high. Try look for 220 uF 20.66 K or 470 uF 9.67 K they seem to be fairly common.
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Old 10th Jun 2007, 12:44   #7
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Quote:
Originally Posted by Gambler
What is a common capacitor-resistor combination that I could most likely steal off some old projects?

http://555-timer.clarkson-uk.com/operation/mono2.gif

Thanks!

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There's a calculator on the page, input '5 secs' and he gives you a list. You can round the resistors off to the nearest standard value, so 220uF & 20k is one combination, 100uF and 47k is another. There's no point in being too exact on the resistor when a capacitor tolerance is +/-20%; if you want 5.00 seconds use a variable resistor and tweak.

For what you say you want,
Quote:
1. I close a switch
2. A timer delays 5 seconds
3. A second circuit closes after the 5 seconds.
4. Releasing the switch can (and would be best) turn everything off.
the 555 output from pin#3 can be used to operate a relay to give power to the second circuit.



If the relay isn't a low-power-coiled one you'll need a transistor switch on the 555, there are thousands of examples out there.
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Last edited by cpemma; 10th Jun 2007 at 13:27.
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Old 10th Jun 2007, 14:43   #8
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Isn't this just a schmitt trigger though? If I am going that way could I use the 74HC14 and accomplish the same thing? I've looked at using the 74HC14, but I figured using a 555 would be a lot simpler, I guess not...

Thanks
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Old 10th Jun 2007, 18:37   #9
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You could make a crude delay with a Schmitt inverter, but you'll still need R1, C1, D2 and C3, plus you'll definitely need a transistor switch and its base resistor for current boost, so no simpler, less accurate, bigger footprint, higher cost.

If crude will do, just delay a transistor switch-on.

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Old 10th Jun 2007, 19:05   #10
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The circuits shown here are both simple and reliable, so my post is just another way of doing it.
You could also use a minimal microcontroller circuit(there are microcontrollers the size of a 555).
This suggestion is more like an overkill for a simple timer circuit, but it could be a nice and simple start to get into microcontrollers(if you ever planned on doing that).
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Old 10th Jun 2007, 23:53   #11
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The only reason I ask about doing it with a schmitt, is that you can use smaller capcitors. I dont have anything about about 100uF, nor do I have (or can get within a week, im impatient) 10,000 Ohm resistors.

There is no small way of doing this? A 470uF with 10,000 Ohms is going to be a fair sized capcitor. My 10uF are about as big as I can accommodate (size wise).
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Old 19th Jun 2007, 00:42   #12
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elctroJunky > Have a look at the ST7FLITEUS series, much, much, much smaller than a 555!
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Old 23rd Jul 2007, 11:04   #13
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Sorry to piggy back onto this discussion but the new site layout is slowing my search down It's been a while since I last visited

I want to create a similar timer that will turn off after 2 hours or so. I would like to use a switch to start the timer (then it would be inactive until it is re-pushed). I would prefer to have no power consumption until the button is pushed again.

This timer would be used to turn power on to a charger and then cut the power after the alloted time.

Hope this makes sense.

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Old 24th Jul 2007, 16:02   #14
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I would prefer to have no power consumption until the button is pushed again.
"no power consumption" means build it round an hour-glass or two , but you'll get very low power consumption from a CMOS 555 monostable plus MOSFET switcher, and I think it will handle a few hours delay with a low-leakage timing capacitor.

Another way to get long delays is with a CMOS4060 chip, built-in oscillator and divide its frequency by 16384 to create a very slow astable; you'll need a system to stop it coming on again after 4 hours.
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