need help with 555 delay timer

gretsch27 ha scritto:

Post your schematic (imageshack.us)

You can see the schematic at

thanks

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

View in Courier:

+V>---> | | O | +-----+--------+ | | | [R] | | | | \\ | \\ +----| >O---O| >-->OUT | | / | / [C] | | | | | GND>----+-----+--------+

-- John Fields Professional Circuit Designer

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

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

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

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

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

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

--
Well, in order to give you advice successfully we need to know some
more stuff, like, what battery voltage are you planning on using,
how does the parachute get triggered (that is, what do you need the
timer to do when it times out? Operate a relay or something like
that?) and how do you know when it\'s time to trigger the chute?

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!

--
Almost! :-)

How do you plan to start the timer, and will that little battery
supply enough current for the ignitor to work and hold in a relay at
the same time?

I guess the question is really "How much current does it take to
make the ignitor work?"

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.

8000.000 hits for google delay timer

On 19 Apr 2006 14:08:36 -0700, "gretsch27" wrote:

--- 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 trigger the ignitor.

I went to a few of the model rocketry sites on the web and looked up ignitors, and it seems like they need 3 amps or so to fire, so the relay needs to be able to handle _at least_ that.

Here's a simple circuit that simulates nicely: . . +----+-----+---------+----------+--------+ . | | | | |K | COM O-->TO +6 . | +->[200K] | [1N4001] [COIL]- - -| . O | | C | | O--> | . |NO | 2N4403 B--[1K]--+--------+ |NO . O |S1 | E | +-------->TO IGNITOR . | | | | . |+ | [1K] | .[12V] | | C . | +---------+-----------------B 2N4401 . | | |+ E . | | O [1200µF] | . | NO| | | . | S2| O | | . | | | | . +----------+---------+-------------------+ .

It starts timing out when S2 is no longer held closed (by your magnetic latch?) and allows the 1200µF cap to charge up (through the

200 kohm pot) to the point where the 2N4401 starts to conduct. That will start to turn on the 2N4403 whichwill turn on the 2N4401 even harder, which... So you get a quick snap-in of the relay.

Here's the LTSPICE circuit file:

Version 4 SHEET 1 1268 852 WIRE -464 272 -464 64 WIRE -464 624 -464 352 WIRE -400 528 -400 496 WIRE -400 624 -464 624 WIRE -400 624 -400 608 WIRE -272 496 -400 496 WIRE -272 624 -400 624 WIRE -272 624 -272 544 WIRE -224 480 -224 432 WIRE -224 624 -272 624 WIRE -224 624 -224 560 WIRE -112 64 -464 64 WIRE -112 272 -112 64 WIRE -112 432 -224 432 WIRE -112 432 -112 352 WIRE -112 480 -112 432 WIRE -112 624 -224 624 WIRE -112 624 -112 544 WIRE 16 64 -112 64 WIRE 16 160 16 64 WIRE 16 288 16 256 WIRE 16 432 -112 432 WIRE 16 432 16 368 WIRE 112 208 80 208 WIRE 272 64 16 64 WIRE 272 112 272 64 WIRE 272 208 192 208 WIRE 272 208 272 176 WIRE 304 432 16 432 WIRE 368 64 272 64 WIRE 368 96 368 64 WIRE 368 208 272 208 WIRE 368 208 368 176 WIRE 368 384 368 208 WIRE 368 624 -112 624 WIRE 368 624 368 480 WIRE 368 672 368 624 FLAG 368 672 0 SYMBOL pnp 80 256 R180 SYMATTR InstName Q1 SYMATTR Value 2N4403 SYMBOL npn 304 384 R0 SYMATTR InstName Q2 SYMATTR Value 2N4401 SYMBOL res -128 256 R0 SYMATTR InstName R1 SYMATTR Value 200k SYMBOL cap -128 480 R0 SYMATTR InstName C1 SYMATTR Value 1200e-6 SYMBOL res 0 272 R0 SYMATTR InstName R2 SYMATTR Value 1k SYMBOL res 208 192 R90 WINDOW 0 62 58 VBottom 0 WINDOW 3 64 54 VTop 0 SYMATTR InstName R3 SYMATTR Value 1k SYMBOL voltage -464 256 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL voltage -400 512 R0 WINDOW 0 -53 5 Left 0 WINDOW 3 -242 110 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value PULSE(1 0 2 1E-6 1E-6) SYMBOL sw -224 576 M180 WINDOW 0 32 15 Left 0 WINDOW 3 32 44 Left 0 SYMATTR InstName S2 SYMBOL ind 352 80 R0 SYMATTR InstName L1 SYMATTR Value 100e-3 SYMATTR SpiceLine Rser=240 SYMBOL diode 288 176 R180 WINDOW 0 24 72 Left 0 WINDOW 3 43 36 Left 0 SYMATTR InstName D1 SYMATTR Value 1N4148 TEXT -488 688 Left 0 !.model SW SW(Ron=1 Roff=10Meg Vt=0.5Vh=0) TEXT -498 708 Left 0 !.tran 0 20 0

-- John Fields Professional Circuit Designer

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 
address)

Never take a laxative and a sleeping pill at the same time!!

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

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