LM555 for Air Cannon

[4pyros]

Hi; I am new here but hope to get some help. I need a timer for an air cannon used for a haunted house to control valve timing when triggered by a presser mat. The mat is NO and I should have a relay output for the valve. I would like to pulse the relay when someonr steps on the mat for .1 to .5 seconds and then wait for 1 to 5 seconds befor it can be trigered again. I was hoping to use a 555 timer and tryed to get a MK111 adjustable interval timer kit to work but cant get it to hold for a few seconds befor it can be triggered again.

Can this be done with one timmer or will I need two?

Can someone help with a circuit that I can build with 555 timers?

Thanks Andy

 

[Reloadron]

Hi Andy & welcome to the forums.

OK attached is an image from an old, old booklet that I have. Credit for the circuit goes to Forrest M. Mims III. I believe you will need two timers as you mention. This can be done using a LM556 or two individual 555 timers.

The attached circuit has as can be seen two outputs. The second output goes high (active) following the active high period of the first circuit. That gives us two re triggerable one shots. Your problem is that following your trigger you want a delay before the timer can be re triggered. You want any triggers to your first timer inhibited for a period of time. Before I forget, output #2 is pin 9 of the chip.

Pin 4 of the chip is the reset for the first timer. For the timer to function (timer 1) pin 4 must be high and thus it is tied to V+. However, if pin 4 is held low the first timer will not trigger. You would set the period of the output for timer 1 for your desired time and use it to drive your air cannon device. That assumes the device doesn't overload the chips output. Additionally the output of the first timer starts the second timer.

Possibly the output of the second timer could drive a small transistor like a 2N2222 with the collector attached to pin 4 of the chip (the first timers reset) and to V+ through a 2.2 K resistor. Normally pin 4 would be held high by the 2.2 K resistor but when the second timer went active high it would turn on the 2N2222 and place pin 4 at a low inhibiting the first timer from doing anything.

The attached circuit shows pots for R1 & R4 and as to C1 and C6 between 1 and 100 uF caps. Those values could be changed to fixed value components to meet your timing needs.

I have not tried this so will not say for sure it will work but it is a thought. Possibly another member could simulate it. It is a thought and a rough thought with little more than a circuit to modify. I have not really thought it through but figured I would toss it out there.

Also the 2N2222 should have about a 1 K Ohm base resistor between the timer 2 out and the base.

Ron

 

[4pyros]

Ron; Mite work I will have to bread board it. I was thinking I would have to trigger both timers at once but triggering the 2nd one from the first makes more sense. Thanks Andy

 

[Reloadron]

Let me know if it works out for you.

Ron

 

[Boncuk]

Hi 4pyros,

this circuit will do.

The idea is to have two monostables triggering each other.

Timer1 (IC1) is triggered by a switch (S1) to ground. It will start and activate the relay via T1. When timed out its output will go negative triggering the second timer (IC2) which is a non-retriggerable monostable. With its output going high T2 will conduct and pull the collector towards ground, thus disabling IC1 by resetting it until timer2 times out.

The circuit might be triggered upon power up, causing it to reset IC1 until IC2 has timed out.

If this is an issue you might use a power up reset circuit for IC2.

If 10nF for C5 won't suffice use 100nF for coupling.

The PCB layout measures 2.3375X2.550".

Regards

Boncuk

 

[Reloadron]

Real nice job there Hans. That is one for the archives as there are countless request for circuits like this. Right down to the PCB too. Real nice!

Ron

 

[bryan1]

Hi Boncuk,
As per Ron's suggestion your post has been published as an article in the projects section under Timers. Please feel free to go edit the post as I did a quickie so it could be published.

Regards Bryan

 

[4pyros]

Boncuk vary nice, I have some questoins if you dont mind.

Has this been tested or waitting for me?
Should the trigger input to the 1st timer have a pull up?
Can I use a 2N2222 or 2N3904? Thats what I have on hand.

Thanks for all your work, Andy

 

[4pyros]

Bryan: How do I find the article? Andy

 

[Reloadron]

Hi Andy

The article can be found here, however, Boncuk will eventually work it up a little. Your initial question got me to thinking about the old circuit I posted and modifying it. Boncuk ran with it and did the modifications, pretty cool of him. Boncuk used a pair of 555 timers and put in the transistors. Also, yes, to answer your question you can use a 2N2222 or 2N3904 or just about any common NPN switching transistor. Start with the base transistor Boncuk used.

Ron

 

[4pyros]

Hi Ron; thanks for pointing me to the "Circuits" tab, some good stuff in thare. You said there are countless request for circuits like this, can you point me to any of them. Searching for air cannon timmer did not come up with much. Just woundering if anyone took a differnt approach to the problem and if thare mite be a simpler solution. In the minetime I will try and breadboard Boncuk's circuit with 2N3904 transistors because thats what I have on hand. Andy

 

[Reloadron]

Hi Andy

Maybe a poor choice of words on my part. There are countless post for timer circuits and many wish to do exactly what you wanted to do. Frequently people want a circuit where when triggered it begins a time interval before they want something to happen.

In your case you wanted to trigger an event for a short duration of time, OK that would be a "one shot" from a mono stable (one stable state) multivibrator. However, the circuit is re triggerable so as soon as the On time (out pulse duration) completes it will trigger again. The re trigger in your case was undesirable so the problem becomes how to disable it for a given period of time.

Enter a two stage timer. The output from the first stage enables an output pulse but also when that output pulse goes negative or low it starts a second timer which we use to inhibit any triggers of the first stage (it turns it off for all practical purposes) for the duration of its output.

So don't think air cannon but rather what the timers are doing as this scenario could apply to other applications. Thus a Forum or Google search of two stage timers or timers in general would yield better results.

We can employ the use of a basic circuit for literally dozens of applications. Sometimes we take a basic circuit and modify it to fit our applications. Simple off topic case in point. If I Google "unanimous vote detector" I will get hits for what amounts to a giant AND Gate where if all the inputs are true the output will be true. However, if a single input is false the output will be false. What if I have a pump and I want a series of conditions to be met before the pump is allowed to run? I want line pressure greater than 30 psi and I want a specific valve open and I want a fluid temperature greater than 200 degrees F? Get the idea?

Ron

 

[4pyros]

OK I breadboarded Boncuk's circuit and finaly after some changes got it to work.
First off I added a .01uf cap to the trigger input to the 1st timer to make it a true oneshot, in series with pin 2 with 10k pull up resisters on either side of the cap. I did not use a transister on the output of the 1st timer because the relay draws less than 200ma. I did not use the diode to keep it simple. I used a 2N3904 with 10K resisisters on the output of the 2nd timer but I should be able to tie the output of the 2nd timmer directly to the reset of the 1st timer? to elimante the 2nd transister.
More later. Andy

 

[Boncuk]

OK I breadboarded Boncuk's circuit and finaly after some changes got it to work.
First off I added a .01uf cap to the trigger input to the 1st timer to make it a true oneshot, in series with pin 2 with 10k pull up resisters on either side of the cap.

I assumed nobody will be standing on the mat for hours to retrigger the circuit.

did not use a transister on the output of the 1st timer because the relay draws less than 200ma.

I always use a transistor to switch a relay. It's of course up to you to switch it directly from the timer output at low current.

I did not use the diode to keep it simple.

It's just a matter of arithmetic using or omitting a cheap diode to suppress back EMF of the relay coil. A new timer IC will definitely be more expensive than the diode.

I used a 2N3904 with 10K resisisters on the output of the 2nd timer but I should be able to tie the output of the 2nd timmer directly to the reset of the 1st timer? to elimante the 2nd transister.
More later. Andy

IC1 is being reset by IC2 until that one times out. IC2 goes active high when triggered. That means you will not be able to keep IC1 at reset condition to avoid early triggering. Observe the dot at pin4 (indicating an inverted input) of the timer ICs. They must be kept low to reset.

Reset must be inverted and a simple NPN-transistor is an easy way to invert levels.

Boncuk

 

[Boncuk]

Boncuk vary nice, I have some questoins if you dont mind.

Has this been tested or waitting for me?
Should the trigger input to the 1st timer have a pull up?
Can I use a 2N2222 or 2N3904? Thats what I have on hand.



Thanks for all your work, Andy

Hi Andy,

the circuit has not been taken off the shelf. I designed it just for you and it hasn't been tested, but should work the way I designed it.

Boncuk

 

[Boncuk]

If I Google "unanimous vote detector" I will get hits for what amounts to a giant AND Gate where if all the inputs are true the output will be true. However, if a single input is false the output will be false.
Ron

Hi Ron,

the "unanimous vote detector" was a 100% miss if you were talking about Forrest M. Mims II and "Engineer's Notebook II"

The output goes low at the end.

Regards

Hans

 

[Reloadron]

Hi Ron,

the "unanimous vote detector" was a 100% miss if you were talking about Forrest M. Mims II and "Engineer's Notebook II"

The output goes low at the end.

Regards

Hans

Damn, I guess I should have looked at it closer for my little example.

Ron

 

[4pyros]

Thanks again Boncuk;

I assumed nobody will be standing on the mat for hours to retrigger the circuit.

Sorry I did not make it clear that I only what it triggered once per group of peaple.

It's just a matter of arithmetic using or omitting a cheap diode to suppress back EMF of the relay coil. A new timer IC will definitely be more expensive than the diode.

It was not the relay diode I left out it was D2 what was that for?

IC1 is being reset by IC2 until that one times out. IC2 goes active high when triggered. That means you will not be able to keep IC1 at reset condition to avoid early triggering. Observe the dot at pin4 (indicating an inverted input) of the timer ICs. They must be kept low to reset.

Reset must be inverted and a simple NPN-transistor is an easy way to invert levels.

Yes after thinking about for awile last night I saw the need for the invershoin so T2 stays. Off hand can you give me the proper resister values for using a 2N3904 thare?

the circuit has not been taken off the shelf. I designed it just for you and it hasn't been tested, but should work the way I designed it.

yes it worked as desined just changed the trigger input to detter sute my needs.

Thanks Andy

 

[4pyros]

Ron; A Google search of two stage timers gets close. I was just woundering if thare is an easyer way. Thanks Andy

 

[Reloadron]

Ron; A Google search of two stage timers gets close. I was just woundering if thare is an easyer way. Thanks Andy

Hi Andy

No easier way that I am aware of. The idea behind "getting close" is to find something then modify it for a particular application. I believe there are commercially available timers out there that would do it through configuration from companies like ATC (Automatic Timing Controls) and Macromatic to name a few but they carry a price tag. The two stage that Boncuk put together is the best, simple and economic solution for your application that I can see.

Ron