Broadcast A/V alarm. Help with a 555 timing circuit.

Thread Starter

Quantum Flux

Joined Feb 19, 2010
18
This is my project to upgrade an A/V alarm at my workplace. My knowledge of electronics is still full of holes so your input is much appreciated.

The design presently in use is simply a mutable piezo alarm. The new design will incorporate some features which have often been talked about as desirable. Below are the schematics and illustrations. I will update this first post as I progress and will add photos once i start building.

Explanation of intended operation:
The unit is essentially a video black/ audio silent warning. There are two independent alarms which are each triggered by a DB9 pin 1 going high. Two ring illuminated push on/ push off's serve as mute buttons for each alarm. When a port triggers its alarm, the ring will flash. Upon muting, the ring goes solid red and a duration selectable interval timer generates a chirp every X seconds as a reminder that the alarm is still muted. A volume control provides a small amount of comfort adjustment if the buzzer is too loud. A panel brightness selector is useful for the night shift, for which the house lights are down and a full brightness panel would be irritating.

RENDERED ILLUSTRATION OF THE 1RU PANEL MOUNT:


THE 556 BASED INTERVAL TIMER (a reminder chirp when the alarm is left muted):
Much credit to Bill_Marsden for the design.


THE 555 OSCILLATOR CIRCUITS (Providing a selectable tone to differentiate between other alarms in the same room):



HELP CURRENTLY NEEDED ON:

* To conserve components and PCB space, is there a way to incorporate both 555 tone generators into a single cct using only one 555 chip?
* Even if I have to order overseas, I am looking for a black metal push on/ push off red ring illuminated button with a life of around 5000 to 20,000 cycles.
 
Last edited:

Wendy

Joined Mar 24, 2008
22,145
Also the first timing period is a little longer than the rest (which are constant). I want every timing period to be exactly 1 minute from the start.

CAN ANYONE PROVIDE A 555 TIMER CIRCUIT SCHEMATIC TO THIS EFFECT?
This is actually a good question, since the problem is the capacitor is starting from 0V and charging from there. Once the oscillator has started it bounces between 1/3 and 2/3 of the Vcc, but starting condition is pretty constant.

People have shown schematics that do this on this site, but I don't have them at my fingertips. The other way is two 555's, or a dual 555 chip called a 556. The first method involves extra resistors to precharge the capacitor to 1/3 the Vcc, if you are turning this circuit on with the power supply it will be defeated. Pin 4 however, is the remote on and off for a 555.

So before we can make a circuit to do what you want, we'll need to know how your are turning it on and off, and if this is negotiable. The other thing is how stable your power supply voltage is, the more stable the better.

Something I'll mention, 555s are dandy chips, but they aren't the only way to do this sort of thing. Two transistors, two capacitors, and a handful of resistors would also work well.
 

Thread Starter

Quantum Flux

Joined Feb 19, 2010
18
So before we can make a circuit to do what you want, we'll need to know how your are turning it on and off, and if this is negotiable. The other thing is how stable your power supply voltage is, the more stable the better.

Something I'll mention, 555s are dandy chips, but they aren't the only way to do this sort of thing. Two transistors, two capacitors, and a handful of resistors would also work well.
It'll be turned on and off as power is provided to or cut off from it.

I was using a 9v DV plug pack. I do have a few LM7805 that put out a very stable 5.75V it think. But at that voltage the piezo isn't very loud.

EDIT:
On further thought, I suppose i could have the output drive a transistor and run the piezo off the 9v supply
 

Wendy

Joined Mar 24, 2008
22,145
The two 555 idea would do it. Another thought, use CMOS 555s, never turn the power off, and use pin 4.

I'll be back with the dual 555 (or 556) oscillator in a bit. Is it important the circuit start beeping immediately? Given your 1st post I would assume so.
 

Thread Starter

Quantum Flux

Joined Feb 19, 2010
18
The two 555 idea would do it. Another thought, use CMOS 555s, never turn the power off, and use pin 4.

I'll be back with the dual 555 (or 556) oscillator in a bit. Is it important the circuit start beeping immediately? Given your 1st post I would assume so.
Basically the alarm sounds constantly when the port activates it. When it's muted, there is a delay of 1 minute then a chirp that repeats every minute until the alarm is un-muted.
 
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Wendy

Joined Mar 24, 2008
22,145
OK, so the beeping doesn't have to begin immediately, but after the 1 minute interval is OK. This simplified things a bit (one or two fewer parts).
 

Thread Starter

Quantum Flux

Joined Feb 19, 2010
18
I'd like to use a 556 to conserve space if possible. and im drawing up another 555 circuit to generate a tone for the piezo so the tone can be altered. This is because there are many alarms in the room and its very hard to hear which is coming from where because they all sound the same. The design of the room reflects the 3khz tone all over so you cant tell where it's coming from.

Would it be easier to have the circuit powered all the time and trigger it somehow?


Shortly, I'll post up my schematic as it is now. That will probably make it clearer what im trying to achieve.
 
Last edited:

Wendy

Joined Mar 24, 2008
22,145
I'd like to use a 556 to conserve space if possible. and im drawing up another 555 circuit to generate a tone for the piezo so the tone can be altered. This is because there are many alarms in the room and its very hard to hear which is coming from where because they all sound the same.

Would it be easier to have the circuit powered all the time and trigger it somehow?


Shortly, I'll post up my schematic as it is now. That will probably make it clearer what im trying to achieve.
Yes.

You can experiment with it, but I don't thing you will be able to change the tone of the piezo at all. You can modulate it, but the frequency is fixed (I think). However, experimenting is all part of the fun.

I think I have a simple fix for the duration issue. I'll post it shortly.
 

Wendy

Joined Mar 24, 2008
22,145
OK, my thought is the monostable will force C1 to discharge entirely, which will equalize the cycles as you want.



It is probably pretty obvious I haven't build this, so it is a try it and see proposition. The resistors will need tweaked I'm pretty sure.

My blog is working again, you might find some of this interesting...

Bill's Index

The 555 Projects

My Cookbook

LEDs, 555s, Flashers, and Light Chasers
 

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Thread Starter

Quantum Flux

Joined Feb 19, 2010
18
...but the frequency is fixed (I think).
After doing a bit of reading it looks like the frequency is fixed because the crystal has a single resonant frequency. Maybe I can find a micro speaker or something but i dont know if i could make it anywhere near as loud with low power.

EDIT:
I think i can still use a piezo but vary the sound without varying the frequency. I could build in 3 selectable options like so.
1. A sine wave varying the voltage up and down to created a pulsing kind of effect.
2. A square wave to create a fast chirping/clicking kind of sound
3. A constant tone which is just the piezo driven directly from power without the 555 circuit.
 
Last edited:

Wendy

Joined Mar 24, 2008
22,145
Opps, drafting error there.

Here is the revised drawing...



Sorry about that. I'm starting a similar build using LEDs to verify this design (the 556 version). When I have something I'll post it, protoboard drawing similar to my 555 projects.

Mine will use LEDs similar to the other projects.



Checkout of the protoboard layout pending.
 

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Wendy

Joined Mar 24, 2008
22,145
I'm not having a good day it seems. Rev C, I figured this one out from the protoboard.



It's been a long day, and all my 556 are CMOS (not a good thing for this design). I'll pick it up again tomorrow.



I verified this one, it works exactly as planned, the time immediately after power up is constant. You may need to lengthen out the beep duration, I'd double the resistor R2. Again, I used LEDs instead of the piezo. I tried a conventional 556 and a CMOS, it didn't make much difference.

This design has been one massive brain fart on my part.
 

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Wendy

Joined Mar 24, 2008
22,145
Check the transistor and pin 4. It is the equalizer section. It doesn't sound like pin 4 is ever going low (it goes low when the piezo is on).
 

Thread Starter

Quantum Flux

Joined Feb 19, 2010
18
All good now. One thing though. I reduced R2 to 270 ohm to get a very short beep which is what i'd prefer for this project. But it gets very hot. Should I move up to a 1/2 watt resistor?
 

Wendy

Joined Mar 24, 2008
22,145
Leave R2 where it was (2.2KΩ) and drop the capacitor to 10µF.

When the timer is idle that resistor is connected to ground through the chip.

If you want to reduce the current even more use a 1µF and 22KΩ. The math works the same for all values.

So, what did you find?
 
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