im trying to figure out how to build a time delay circuit, new here.

[davy87nj]

Hi my name is David, i live in NJ and im new here. Im an automotive technician and have electronics training, but some of this stuff i searched blows me away, but i would like to learn whatever i can. So yeah, im interested in making a 3-5 second time delay circuit. I searched and im not really understanding the math maybe. Can anyone explain a good way to make one of these cheaply and maybe explain the math if there is any? I was not able to find a cheap time delay switch online but maybe im not looking in the right place. please forgive my ignorance, gotta start some where i guess. what i would like to do with this is flip a switch and have a light bulb filiment like device turn on and heat up enough to create ignition but i would like a around 3 second delay. I dont really know if this is possible but it made sense to me, thank you. And what a great site hopefully i can learn other things to widen my knowledge of electronics. thanks, David.

 

[Jaguarjoe]

Google around for LM555 or NE555 or just 555. This is a very inexpensive, very versatile timer chip. You will find numerous circuits for time delays and a dozen other things. You should also be able to find timing calculators specifically for the 555 that figure out T= 1.1RC. You will need to know the current that your filament draws to determine what will switch it on and off.

 

[Reloadron]

Per Jaguarjoe, a good choice would be to use a 555 timer chip. What you actually want to do is configure the 555 as a one shot monostable multivibrator. You flip a switch or push a button and the output would go high for a period of time then return to low. That is if I read you right as to what you want it to do as far as timing. Sometimes when we say time delay we mean an event happens and then after a prescribed time something happens. Really need to know exactly what you are after.

Ron

 

[KMoffett]

what i would like to do with this is flip a switch and have a light bulb filiment like device turn on and heat up enough to create ignition but i would like a around 3 second delay

Do you want the bulb to start heating 3 seconds after you through the switch?
Or, do you want the bulb to start heating as soon as you throw the switch, and shut off 3 seconds later.
And, what do you mean by: "heat up enough to create ignition"

Ken

 

[KeepItSimpleStupid]

I have to ask:
a) What voltage are you switching?
b) What current are you anticipating?
c) what voltage do you want the timer to operate from?

 

[davy87nj]

Thanks for the answers i really appreciate them, and im going to look up those timers, and the calculators, and when i figure out the bulb filiment amperage ill make sure to get the right one.
Sorry let me clear a few things up. Im looking to maybe use a 9 volt battery, but really i could settle for any source voltage as long as it would work, and be portable, i dont want to hook up a car battery to it. im trying to find a way to light(on fire) magnesium, which doesnt really need extream temp to light. Im trying to do a little project to make a completely homemade thermite grenade(incendiary granade). Purly for educational purposes of course no sarcasim intended. I just saw a few examples on youtube and it looks dangerous to use a torch or lighter. I could use a timed fuse but it would be cool to use a timed switch. So basically im looking to flip the switch and have about 3 to 5 seconds. i understand the filiment will take some time to work but i think it will be very small amount of time. I looked up a few things and i was a little confused because it said it would give a small amount of voltage then once the timer is up it will hit the intended max voltage, that wont work. i might be reading into it wrong, ill look somemore up. please excuse my ignorance again with the correct terms. Thank you.

 

[davy87nj]

i was just looking up the diagram and the explaination of how the 555 works. It looks like the monostable mode works oppisite of what i would like to do. i would like to see 0 output voltage for 3 seconds after i hit a switch, then after about 3 seconds i would like to see full Vcc voltage at the out put. I might be wrong, but it seems i might be able to wire it up to do this, im just not sure how. as it is described in monostable mode it is basically oppisite, turning the voltage off after 3 seconds or any giving time instead of on. Maybe someone can clear that up for me if they got the time? thanks

and also one other question, im looking at the diagram on wikipedia, i see the resistors, the transistor and the compacitor, but im wondering what the large triangles are. I assumeing a one way check vave, diode? Im not positive, been almost a year since i really looked to understand out a circuit worked. Thanks.

 

[Jaguarjoe]

I don't think this is a good plan. Any malfunction in the time delay circuit could possibly light up your filament and your bomb. This wouldn't be very cool if it happened at the wrong time. Heck, it might not even be too cool if it happened at the right time!
I have seen circuits for pyrotechnic igniters and most of them center around making the circuits fail safe. One circuit was triggered by a microprocessor which was required to make 3 high/low transistions in short order at its output in order to trigger the ignition. That way, if the uP failed high or failed low it wouldn't light off. All of the other components in the circuit (caps, diodes and resistors) were similarly failsafe.
I wonder how many back yard terrorists would be interested in this thread?

 

[KeepItSimpleStupid]

Some 30-40 years ago when I was I kid, I did ignite magnesium ribbon with a torch. It wasn't that easy, but it may have been the thickness of the ribbon. I also made a homeade ignitor,but not for the ribbon. I wrapped magnet wire around a match. The varnish insulation would burn too.

Model rocket ignitors may be what you need. 4'' Igniters D21-E30 (6) (aro89892) Aerotech Model Rocket Engines Igniters They require about 3 amps at 12 V. You won't get that out of a 9 V battery. Two P500 polopulse battery may work. **broken link removed**, but it looks like they are unavailable: P500 P500 Polaroid Polapulse for Sinclair FTV-1 6V Lithium. Powerdex 6A1400, Powerdex VP6-1400, Gould-Powerdex 6A1400, Polaroid P500, Polapulse P500 They were used in Poloroid film packs. Another possibility is Ni-chrome wire or just plain wire.

I've seen the thermite thing up close and personal. That stuff puts a hole in 1/2" steel plate like it's butter.

The 555 in my opinion is a very unstable monostable timer.

 

[Reloadron]

It looks like the monostable mode works oppisite of what i would like to do. i would like to see 0 output voltage for 3 seconds after i hit a switch, then after about 3 seconds i would like to see full Vcc voltage at the out put.

OK, I had a feeling about that. What is generally done in a case like this is you configure a dual timer. A popular chip is the 556 which is no more than a pair of 555s in a single package. That or just use a pair of discreet 555 chips.

Concerning your application, I see where Jaguarjoe has hit on safety. It takes a hot flame to get magnesium going and once going it has this reluctance to stop burning till it is depleated. Just be careful and that from someone who loves things that go bang! Pyrotechnics are our friend.

The idea behind a dual timer is that once initiated the first stage needs to run its time before the second stage goes active. Like a "delay on make". You can try a Google of two stage timers or delay on make timers. Maybe someone here can point you to a recent thread on the subject as there have been plenty. I am at work so my abilities in the forum are limited right now but this all should give you an idea.

Ron

 

[KeepItSimpleStupid]

So basically im looking to flip the switch and have about 3 to 5 seconds.

Explain please. Is that 3-5 sec to get away before it activates or 3-5 sec of activation?