How to connect Buzzer to PIC?

[Michael Wakileh]

Hi! I got a chance to look at your singing ballons on youtube... nice...
I'd actually be quite content if I reach that volume... You wanted them even louder?

[mtripoli]

To answer the last question first: I'm not sure what video you saw of the "Singing Balloon". The sound level output of the balloon is satisfactory; I wanted the higher voltage to get more headroom. In order to get the volume you hear I had to severely clip the (original) waveform. With a higher voltage (more headroom) I could have clipped the sound less and it would have sounded much (much) better. As with any disposable product, sub-fractions of a penny add up fast.

I didn't listen to the videos you attached but I think I get it; the card is programmed with DTMF tones (http://en.wikipedia.org/wiki/Dual-tone_multi-frequency) that are recorded (or generated via code) and reproduced into the mouthpiece of the phone. Phone systems recognize this and dial the number. Lot's of products have been on the shelves over the years that do this. The business card is cute but I'm not so sure of the practicality (my opinion - but we're not talking about that). You didn't say if this is exactly what you want to do. If it is, I'd offer the following...

The links I posted show how to connect the inductor for operation. I don't know that this would help anyway. When using an inductor in an oscillator you are tuning the components for the target frequency. With DTMF you are generating a couple of different frequencies so (if) you get it to oscillate it will not be very efficient and the voltage generated will be all over the place. Frankly, I'd abandon that approach.

You *could* however design a small high voltage step-up circuit. I'm going straight from memory but I think both Maxim and Linear Technology make IC's to do this. I have no idea of the Vmin values... You could then use this with a transistor to power just the piezo. I think these devices are current hungry and you won't get far with a lithium coin cell (they probably have an enable of some sort to turn them on and off, but...). And then there is the noise on the supply rail to deal with...

However.

You don't need high volumes to make this work, the XOR'd output from two pins driving the piezo should be fine; with 3V you get 6Vp-p like this (5 gets you 10). You're not going for resonant frequency so you can pretty much ignore anything to do with that other than holding the piezo solidly.

Keep in mind that waveform is not going to be a perfect square-wave when driving the piezo, nor will you get all the way to the rails.

I'd look for the piezo devices that have a crystal element on both sides of the brass coin. These are more efficient than than the single sided and only cost a few pennies more.

The thing you need to keep in mind doing this is the stability and accuracy of the frequencies being generated and that the tones are sine wave, not square. As I recall from doing a DTMF decoder years ago was that you have a very small window of deviation from the center tones to make it work.

PBP has a DTMFout routine that needs a 20Mhz to get it right. It's PWM from one pin and needs a filter to get it "sine-wavy". You'll loose voltage through the filter so don't think it'll be anywhere near the rail (its meant for going to into an amp). I've seen assembly code years ago to generate DTMF and used a few but that was a long time ago.

Good luck.

P.S. When preparing a lengthy post, before posting it copy it to the clipboard. If the session has timed out you can just paste it back in.

[Michael Wakileh]

Hi Mike,

Sorry for the late reply, and thank you for all the great information and the great links...

I'm sure I've had several past projects with badly mounted piezos, and
thanks to this thread I've finally taken the time to look at boost converters. I actually do a lot of my prototyping in smd and I've scrapped the flyback idea for now. (Common mode chokes are quite expensive and have lead times of >2months with farnell.at)

However, I've skimmed the Microchip technical briefs (TB053, AN258, ccp tips and tricks) on the regular boost converters in discontinuous mode, and I will be adding some chip inductors to my next order. Once I get my parts, I will test a small high voltage stepup (Ip = 10mA) and transistor method.

I'm actually not working on an embedded project at the time...
Just trying to learn a bit for future endeavors. The business card link was just an example of a battery driven, tiny gadget (the type of device I like to work on). Also, I was interested in sensing of Voltage spikes, and ways to protect the io pins incase of shock/trauma.


best regards,
Michael

[Telemachus]

I have taken apart a kitchen timer, and it has a similar circuit to the piezo buzzer mentioned earlier in this thread. There is a large inductor coil attached where the buzzer is.

In this picture:

http://www.picbasic.co.uk/forum/atta...9&d=1264966885

Can someone tell me what is the purpose of Q1? Looks to be a surface mount NPN transistor.

One of the leads to the piezo is always high, and the other one seems to be attached to the collector (not emitter!) of this transistor with the emitter going toward the IC through R1, although I could have my pinouts wrong.

How does the IC control the PWM going to the piezo by means of the transistor?

Thanks!

[mark_s]

This is a circuit I use. When the NPN is turned on the inductor is energized, turn off the transistor and a large counter inductive voltage spike is generated.
As stated in earlier posts its best to drive the piezos at the resonance frequency. Your picture maybe different

Your question about the IC control. The transistor acts as a switch, controlled by the IC. More important it protects the IC as a buffer with it's
higher current handling ability

[mtripoli]

I have taken apart a kitchen timer, and it has a similar circuit to the piezo buzzer mentioned earlier in this thread. There is a large inductor coil attached where the buzzer is.

In this picture:

http://www.picbasic.co.uk/forum/atta...9&d=1264966885

Can someone tell me what is the purpose of Q1? Looks to be a surface mount NPN transistor.

One of the leads to the piezo is always high, and the other one seems to be attached to the collector (not emitter!) of this transistor with the emitter going toward the IC through R1, although I could have my pinouts wrong.

How does the IC control the PWM going to the piezo by means of the transistor?

Thanks!

You've got the pinout wrong... the 22K resistor is on the base, the emitter looks to be tied to ground (check it with a meter). It sounds like the piezo is in the collector making it an open collector. You can tie the coil in parallel with the piezo to get the same flyback effect that Mark_s was talking about. Again, it's resonant so you have to play with the frequency to get it to "go"...

[Telemachus]

You've got the pinout wrong... the 22K resistor is on the base, the emitter looks to be tied to ground (check it with a meter). It sounds like the piezo is in the collector making it an open collector. You can tie the coil in parallel with the piezo to get the same flyback effect that Mark_s was talking about. Again, it's resonant so you have to play with the frequency to get it to "go"...

Thanks, after some more research, I realized the true pinout.

Actually, what I am trying to do is to remove the coil and piezo entirely, and have the transistor act as a switch to turn on and off a higher voltage (3.3volts) that will be fed to the input pin of a PIC. Therefore, when the kitchen timer goes off, the PIC will read the PWM output as a digital signal to run a routine prior to resetting the kitchen timer to start counting again.

What I have come up with so far is to disconect the coil from the transistor's collector, mount instead a 3k3 resistor between the collector to PIC VDD (3.3V) and connect the PIC input pin on transistor's colector. With the emitter of the transistor connected to GND (I will ground the PIC to the CMOS circuit). That way, when the timer goes off, it should trigger a "buzzer" that in non audible, but compatible with the VDD voltage that my PIC can read digitally.

This is a timer circuit to snap the shutter on a Cannon SLR camera, and I have found that using the kitchen timer is cheaper than it would be to buy the LCD and buttons needed build the circuit completely based on a PIC.

[mtripoli]

A digital Canon SLR? If I recall, there is a group of pads inside the the battery door. Bridging these pads results in focus adjust and shutter release. I used a 1K resistor (or something) to play around with this. I was going to make an RF remote control for my camera but got side-tracked with something else (about 5 years ago). There's a bunch of info on the net about this stuff. I think you can use just a relay to do this...

I'm missing the point of using the kitchen timer. You can do everything you want with just the PIC...