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555 one shot question

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mainframe

New Member
Hey gang! I'm working on a prop for an airsoft scenario and I have a question.

A while back I purchased the timer kit from Denkimono. What I'm wanting to do is hook it up to a 10 second 555 one shot, which I've found schematics for and am confident I can make, to push a 120db siren so it can be heard from a distance. The problem, is when the alarm goes off it sends pulsed voltage to piezo buzzer and the voltage is incredibly low.

I did a little research and it looks like a monostable multivibrator will allow me to take the low, pulsing voltage and make it steady voltage and step it up enough to activate the one shot.

So I guess the question is; am I on the right path?

TIA
 

mainframe

New Member
Ok. I'm a bit confused. The more I read the more it looks like a 555 one shot is a monostable multivibrator. When I was researching before, the schematic had two 555s, an opamp and a bunch of other bits.

I'm starting to think I'll be better off with two 555 one shots.

The first one is triggered by the speaker pulse of the timer activating a relay. Once the normally open relay is closed it will trigger the second one shot and close a separate circuit bypassing the first one shot to keep itself powered preventing the second one shot from resetting.

That makes sense, right?
 

Boncuk

New Member
Hi mainframe,

you see me confused as well.

I don't understand what you are getting at.

Here is a classic monostable made with an NE555.

R1 is the timing resistor, C1 timing capacitor, R2 is a pullup resistor for the trigger input to have a defined high level if not triggered. C3 is a decoupling capacitor. BZ must be a selfcontained buzzer (with internal sound generator).

VDD might be between 6 and 15V.

Boncuk
 

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Wp100

Well-Known Member
That makes sense, right?
Kind of !!

First - what signal is going to activate the timer ?

A monostable 555 will produce a single high pulse for nn seconds -

An astable 555 with produce a continuous stream of high and low pulses.

Unsure what you created to produce a 10 second burst of pulses - was it 2 timers connected together ?

The 555s output cababilities are limited in terms of voltage and current - so we need to know the maximum current and voltage of the siren you intend to use - it may need an extra transistor or little relay to drive it.

Can you post a diagram of what you built so far..
 

mainframe

New Member
Sorry guys, and thanks for being patient with me. As if it wasn't obvious, I don't know enough about this to even call myself a noob :p

I'm not sure the current of the siren but it's rated for 6-14 volts and I'll be powering that portion with a 9v battery. I currently have that siren hooked up to a alarm system motion sensor with a 5v 1A relay and a 6v lantern battery and everything works fine. Here's a pic of that.

If you follow the link above to Denkimono's site they have a video of the timer in action. Initially I though I could just replace the buzzer with the siren I bought, but the voltage going to the buzzer is only .3V and it pulses so there is no way to push the siren. But, at the same time I don't want a 120db siren blaring until I can get to where it is to turn it off. So what I really need to have happen is is have that .3v pulse activate the alarm for 5 seconds (after thinking about it 120db for ten seconds is kind of overkill), and only do it once until the while shebang has been reset.

I guess the reason I thought I needed two was so I could get the relay from the first locked into a closed state by looping power to it when it closed (so it wouldn't reset and start the process over again each time the one shot got triggered) and allow the second one to run it's natural cycle. But then I found this one that says it's non-retrigerable.



I've built something similar to these years ago. It was a touch circuit using a 555 that I used to open a stealth drive bay on a gaming rig by touching a bolt on the faceplate. I think these all basically work the same as far as causing a voltage drop on pin 2 to trigger a response right?

If I were to use a transistor to act as the "push button" switch I could trigger it with the .3v couldn't I?
 

mainframe

New Member
So I built the one Boncuk showed with a couple exceptions. I changed R1 to 470KΩ resistor to shorten the time and the 555's I have on hand are TLC555. The pinout is arranged different but the pins are the same.

Here's what I'm getting. When I close the "button" side of the circuit and release it my siren clicks and in about 6 seconds it clicks again. So the timing is acceptable. But, if I keep the button closed I don't get the second click from the siren until after I release. I checked the voltage, and I'm getting just under 2v from pin 3 to ground so I can't try to work my 5v relay in there.
 
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Boncuk

New Member
So I built the one Boncuk showed with a couple exceptions. I changed R1 to 470KΩ resistor to shorten the time and the 555's I have on hand are TLC555. The pinout is arranged different but the pins are the same.

Here's what I'm getting. When I close the "button" side of the circuit and release it my siren clicks and in about 6 seconds it clicks again. So the timing is acceptable. But, if I keep the button closed I don't get the second click from the siren until after I release. I checked the voltage, and I'm getting just under 2v from pin 3 to ground so I can't try to work my 5v relay in there.
Hi mainframe,

you're obviously using a sound transducer instead of a selfcontained buzzer.

Just for your information: A selfcontained buzzer generates an audible signal when connected to a DC power source.

If you use a sound transducer you'll have to make the sound signal yourself, which you can do using a dual timer chip, one wired as monostable and the other as astable multivibrator.

The monostable will trigger the astable (set to the desired audio frequency) to have the piezo transducer sound at your desired frequency and time.

The monostable I suggested is a retriggerable one, meaning it will keep the output high as long as the button is depressed + time delay set by the timing RC combination.

Boncuk
 

mainframe

New Member
Hi mainframe,

you're obviously using a sound transducer instead of a selfcontained buzzer.

Just for your information: A selfcontained buzzer generates an audible signal when connected to a DC power source.

If you use a sound transducer you'll have to make the sound signal yourself, which you can do using a dual timer chip, one wired as monostable and the other as astable multivibrator.

The monostable will trigger the astable (set to the desired audio frequency) to have the piezo transducer sound at your desired frequency and time.

The monostable I suggested is a retriggerable one, meaning it will keep the output high as long as the button is depressed + time delay set by the timing RC combination.

Boncuk
The siren is self contained. It the picture in the forum I linked to the motion sensor trips the relay allowing the 6v from the lantern battery to flow freely to the siren. It screams like a banshee.

I think the problem is that the 2 volts coming from pin 3 isn't high enough to hit the low threshold on the siren which is rated for 6-14v.

If my understanding about transistors is correct, I can run undiluted voltage through it to the siren and use the 2v from the output pin to activate it. Am I right with that assumption? And if so once the voltage from the output drops out will current through the transistor stop? Would I use a NPN or PNP transistor for this effect?
 
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