How to connect Buzzer to PIC?

[Michael Wakileh]

Every so often I find myself buying products just to find out how they work... Recently I bought a cheap battery powered Piezo alarm...very small and quite loud...
(I believe it's sweeping btw. two frequencies to be even more
audible...so it's probably a piezo transducer.)

I'm looking for help in figuring out how they made it so loud.

(The audio jack is being used as a switch...alarm goes on when
pin is pulled out...pin is stopped from coming out via housing...nice hack)

I will draw up a schematic soon... here some pics

Unfortunately I've never seen the critical part in question (black with 3 pins).
I'm guess I'm looking at a transformer? inductor?

Can anyone give me hints towards reading material or electrical tests I can do to find out more...

[ScaleRobotics]

Looks like the large 3 pin part might have heat shrink around it. Is that the case? If so, can you cut it off, and see a part #?

[Michael Wakileh]

Unfortunately no part number... just a bunch or really fine coils... I tried to be careful but I cut through them.... Perhaps I'll buy another unit so I can find out how many coils etc... but I guess we have an answer. I am not familiar with transformers. Logic tells me this one has a comon ground...input is boosted drive the piezo at higher voltage... I did a very brief search of farnell but didn't find anything similar... Anyone know part numbers or supliers of small 3 pin tranformers?

[mtripoli]

It's a coil (inductor) to generate flyback voltage. There's a switching circuit that creates a high voltage to apply across the piezo element. Piezo elements are generally designed to be run at high voltage; 30-200V is not uncommon. In a really good design the piezo will also be in a Helmholtz enclosure as well to really get everything from the system as a whole. Years ago I designed these things for car alarms. 120dB was on the low end of what you could achieve...

Do some searching on piezo elements; off the top of my head I'd say have a look at Murata. I seem to recall that they have some pretty good doc's on these things.

Mike Tripoli

P.S. I found the doc I was talking about pretty quickly: http://www.murata.com/catalog/p15e6.pdf. It's a good starting point...

[Michael Wakileh]

Thanks! I've glanced through it and it's the best info I've seen sofar. I'll need some time but I'm going to work on a simple solution for pics (need to read up on inductors etc). Seems like this is something that gets asked about often on the forum.

regards,
mike

[Michael Wakileh]

After some thought, I think I will go with max232/3232 or equivalent. May get back to this if sound output is not satisfactory...

[mtripoli]

Will that help? I'm going off the top of my head but you'd have to use both outputs and invert the signal going into one of the inputs to get the voltage swing across the piezo (assuming using one output pin on the PIC)? You'd only get about 14VAC? What is the capactive loading spec of the 232 device; can it drive a cap load (piezo's are normally a "relatively speaking" large capacitive load).

Have a look at this website: http://www.discovercircuits.com/P/piezo.htm. There are a couple of circuits shown. All you really need is a small inductor, a transistor and the piezo. The PIC is your oscillator. I'd have to think this is cheaper and easier than a MAX232?

Keep in mind the resonant frequency of the device as well. Thinking back, one of the problems that the car alarms I worked on had when I was brought in was the guy before me was trying to brute force the piezo to get the output needed (+120db @ 3 feet). It was at around +97db. Tweaking the frequency made it jump to over +120db. The other thing that is more important than you'd think is in the way the piezo is held. I've done edge fixture mostly; the tighter this is clamped the better for output.

I invented the "Singing Balloon"; this uses a piezo attached to a "foam speaker". It is edge mounted as well and driven with PWM from the sound chip. The output is around 10VAC to the piezo (in this case it reproduces music). The piezo *really* needed about 30VAC but the product (disposable in every sense of the word) couldn't stand the cost of the added circuitry to make it happen. I think to date there's been about 30-35 million of them produced.

Mike Tripoli