need help with 555 delay timer

What I need is a simple circuit for a 5-10 second delay. No voltage for 5-10 seconds, and then turn on and stay on. I've googled this, and simulated the circuits that people have designed for this purpose, and they don't seem to work. I know someone who tried actually making the circuit and it still didn't work, so I think my simulation is correct. Can anyone help?

Nick

Reply to
gretsch27
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gretsch27 ha scritto:

Post your schematic (imageshack.us)

Reply to
ZeroDue

You can see the schematic at

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thanks

Reply to
gretsch27

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Unless you\'ve got some requirements you didn\'t mention, all you
really need is an RC driving a couple of  Schmitt trigger inverters.
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Reply to
John Fields

John, what does letter "O" (which I marked with *

Reply to
Ignoramus16588

unfortunately, I am an Aerospace Engineering major, and I'm not very good with electronics. What is an RC or a Schmitt trigger? The application is for a rocket timer. i am building a rocket engine, and need to test it in a rocket. In order to recover it successfully, I need a delay to trigger the parachute.

Thanks for your helps guys!

Nick

Reply to
gretsch27

I have to wonder if a safer and more reliable way of triggering the parachute would be to sense that your rocket is in free fall (no gravity). That could be done with just a little weighted spring contact. Make sure to wire in a safety switch that would allow this circuit to work only if turned on, and turn it on before the launch.

Any thoughts?

i
Reply to
Ignoramus16588

Apparantly "you don't have to be a rocket scientist" to be a rocket scientist!

Luhan

Reply to
Luhan

I think the problem could be with the high value of the timing components, 100uF, 1Meg. ISTR having this problem years ago, there was enough current leakage into pins 6 and 7 that prevented the capacitor reaching the trigger voltage.

Try the CMOS version of the 555, or easier use John Fields approach

martin

Reply to
martin griffith

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No.  As a matter of fact, when power is removed the charged
capacitor will keep the circuit running for a while.
Reply to
John Fields

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Well, in order to give you advice successfully we need to know some
more stuff, like, what battery voltage are you planning on using,
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Reply to
John Fields

I'd like to use a timer because a simple acceleration switch would trigger when the acceleration becomes negative, which is right after the engine burns out. The rocket has to coast to get any altitude. The burntime of the engine is only ~1 second. Flight time is around 20 seconds, so a 10 second delay would deploy the parachute shortly after apogee. It seems easier to figure out the time to apogee. A magnetic apogee detector is expensive, and a flight computer is more expensive, and heavy. Timers are usually used as a backup system for rocket recovery.

The ignitor for the ejection charge runs on 4 AA batteries in series. To save weight, I'd like to use one of those little 12 volt batteries. It looks just like an n-size, but it's not. I figure I will use that

12V battery to run the timer which will operate a relay to complete the ignition circuit. Hopefully this is enough info.

Thanks for the help, and I apologize for being such a newb!

Reply to
gretsch27

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Almost! :-)

How do you plan to start the timer, and will that little battery
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Reply to
John Fields

I was going to use 4 AA batteries for the ignitor. I'm not sure exactly how much current the ignitor takes. I know that 2 AAs isn't enough, but 4 works. The 12V battery will just have to keep the relay closed so that the AAs trigger the ignitor. I'll probably start the timer using a magnetic switch that disconnects from the rocket when it takes off.

Reply to
gretsch27

8000.000 hits for google delay timer
Reply to
Sjouke Burry

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Great!  I misread it and thought you were going to use the little
12V battery (I think it\'s called an A23 or something like that) to
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Reply to
John Fields

Nick, according to the diagram you drew of -your- circuit, I see that the voltmeter is connected from the 555 output pin (3) and the Trigger pin (2). Try connecting it from pin 3 to circuit ground (Pin 1). Also, pin 2 is the trigger input. You will need to simulate a negative-going pulse on that pin to trigger the timer. A switch to ground on pin 2 should work. I didn't check your values, but they look to be in the ballpark. And make sure you're using the correct version of the 555 as specified by the author of your test circuit. The CMOS version is different from the bipolar version. Download the datasheets for each and use the one that's appropriate for your parts bin. The datasheets will give you more diagrams and explanations on how they work.

Cheers!!!!!

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
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Reply to
DaveM

wow, thanks! I'll see what I can do with this.

Reply to
gretsch27

formatting link

Use the monostable design (like the one you posted) and then use the Schmitt trigger (described at the end of the link) to invert the signal. For a relay I'd use a low voltage power mosfet, but it's all up to you.

Ben

Reply to
Ben Gustave

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