Puggy Potty Alarm

[CRE]

Nope, no joke, one of my pugs is getting out of hand with his "marking" behavior..... It's driving us nuts. So, I plan on placing some alarm in every area he's "tagging". I've got bare copper tape (used in stained glass work) which I'll run in two horizontal lines with only about a millimeter gap.

The part I'm having trouble with is the audible alarm. I'd like to build a piezo driver which'll push about 80-90 dB. I'm going to use a 555 monostable to only allow the sirem to scream for a few second when triggered, then reset.

Anyone have a schematic for a good loud piezo driver? I've got a few different piezo transducers on hand but I'll probably use the larger ones I picked up at RatShack (273-073A).

Thanks!

Edit: One more thing, I do want this circuit to run at an audible frequency, so I know when something's been done. Also, the electric dog whistles I've found are too quiet.... any idea how make a louder version:  http://www.zen22142.zen.co.uk/Circuits/Misc/whistle.htm That's one I've tried.

[audioguru]

Hi CRE,
Sure it's a joke! 
If you drive the piezo transducer with a bridged audio amp like an MC34119, it will have 4 times the power. It will be so loud that your pug will stop what it is doing in mid-stream and be afraid to "mark" again.
I used an MC34119 in my son's alarm clock and it can be heard down the street.
You will also get a lot more performance from a battery powered circuit if you add a bypass cap across the battery connections on the pcb, maybe 100uF.

Those piezo thingy's are much louder when they are mounted in a small jar or something that resonates at their frequency. Many are made mounted in the back of a plastic housing that is their resonant cavity. Since the piezo and its cavity are resonant, you must tune the oscillator's frequency for the best, loudest match.

[Alun]

Try this circuit you could use a 7555 for low power of a 556 dual timer to save space/money or a 7556 to get the best of both.

To achive a very large amount of power use a bridge op-amp driving a 8ohm to 1K audio transformer with the input on the 8ohm and piezo on the 1K secondary.

But be warned these circuits can damage your hearing and the audio transformer idea may destroy a piezo element if the output voltage is too high.

[audioguru]

Hi Alun,
I don't think that your opamp will work, it will latch-up.

[Alun]

Silly me, I've got +/- the wrong way round, try this:

[audioguru]

Hi Alun,
He, he. Silly you again!
                                                         

[CRE]

Hey guys, thanks for all the responses.

For the time being I've set up a small analog buzzer from RatShack which made for some VERY amusing moments last night.... so funny, so sad.... poor startled little doggy..... so funny!I think I'll give Alun's circuit a shot, but I was wondering if I could substitute the CA3130 with another op amp such as a LM740 or a LM1458? These I have on hand. 

[audioguru]

Hi CRE,
Did your startled little pug stop mid-stream?
You can use any opamp for Alun's corrected circuit, but you didn't notice the elbow that I gave him. His latest circuit must be trying to save battery power because the opamp isn't the required inverter so the piezo gets nothing!

[Alun]

Yes silly me, I needed an inverter not a follower, I knew this but for some reason I got the circuits mixed up! 

You can use any op-amp but I recommended the CA130 because it boasts rail to rail swing and will give you the most volume, yes a LM741 will do but it won't be as loud. You could also use a hex invertor logic chip and paralell the inputs and outputs, you could also use a complementary pair of MOSFETs, or a bipolar transistors (with an inverter first) - they're many ways.

[audioguru]

Hey Alun,
Silly, silly you again and again!   
Your 'fixed" opamp still won't work with its non-inverting input grounded. It needs to be at mid-supply. 
With its gain of -1, your opamp would mirror the non-rail-to-rail output of the 555. It needs some gain to go full swing. 
Nobody would notice the loss of about only 1dB anyway when using an opamp. 
Where's your supply bypass cap? A 555 draws a supply current spike of up to 400mA when its heavy-duty totem-pole output switches, that would kill the battery's voltage for a moment without a cap holding the voltage up. 

[Alun]

Oh of course, the one I posted before would be ok for a duel supply but not for single supply operation.

I never bother drawing bypass capacitors on the schematic, I just add them to the circuit when I build it. I put a 100nf on the supply of each chip or near any switching transistors, and a 10 to 100uf near the battery. I've added a 100nf to this circuit but you may need to use a bigger capacitor as well.

[CRE]

LOL... 

Oh I've been there a few times.... then again I'm still a newbie.... but even I know that once I've "Mastered" something I'll always find more to learn.

Thanks for going to the trouble Alun, I'll probably have time to breadboard this tonight. I'll post my results... well, I'll post whether it works or not... as for actual results... depends on my doggy.

[CRE]

Okay, I've built the circuit and it works well, I'll need to modify the trigger a bit, either a one-shot circuit or a long delay before resetting as the contacts may take a while to dry and stop conducting (if I'm not home).

Also, am I correct in thinking that the gain is set by the 10K resistor between the op-amp's output and the inverting input? Not that I'm likely to get much more out of it while running on a battery.

I did replace the 22K resistor in the R/C with a 100K pot... makes it alot easier to find the piezo's "sweet spot".

Thanks a ton Alun and everyone.

Do you guys think adding another 555 timer running astable would work alright if set with a cycle of something along the lines of: 5 sec. high, 10 min. low? I don't think it would work well, but if it's only gonna loose some precision I might be able to deal with it. If not, can anyone offer another suggestion?

[audioguru]

Hi CRE,
Changing the gain of the opamp won't make much difference because it already does exactly what the output of its driving 555 is doing, but is inverted. Maybe your piezo is drawing more current than the CA3130 can deliver. Measure its AC output voltage, it should be the same as the 555. Try a 741 opamp if it is too low.

I don't think a 555 will give a duty-cycle as extreme as the 120 to 1 that you were planning.

Since you are using a battery, I recommend using Cmos 555's for low power drain.