large piezo speaker with Axon II

[BANE]

Hello all,
I have this piezo speaker from radio shack http://www.radioshack.com/product/index.jsp?productId=2062399 and i want to use it with the axon II.  I know how to make smaller piezo speakers around 5v work but this thing is rated up to 28v!  Can someone help me make a driver circuit for this device?

Can i use a npn mosfet transistor by any chance? 

any help would be greatly appreciated,
thnx

[Admin]

Can i use a npn mosfet transistor by any chance?

Yeap, thats exactly how you'd do it. Show us your schematic and we'll verify it.

Make sure you put a capacitor of small value in series to stop any DC current to the piezo, as DC current will otherwise kill it.

btw . . . 28V piezo . . . should be loud enough to clear the entire neighborhood of cats!

[BANE]

not sure where to place the cap or how many farads it should have.  Could you draw on it with ms paint?

update:  how does that look?   what value should the cap be?

[Admin]

Near perfect.

Add a flyback protection diode between ground and the - of the speaker, with the diode arrow pointing upwards.

As for the cap, I'm not sure. The speaker datasheet might have something, and I'd recommend googling around about it. If I had to guess, I'd say 0.1uF or less.

(btw, good call on the pull-down resister, I totally forgot about that!)

[Billy]

update:  how does that look?

What is voltage on the output of the Axon?

The transistor can take up to 4V to turn on. If at 3.3V or if you're PWM'ing at 5V you may find it doesn't turn on all the way. It's pretty typical to add a buffer to drive MOSFETs gates.  Good luck and remember to have fun.

[Admin]

What is voltage on the output of the Axon?

The transistor can take up to 4V to turn on. If at 3.3V or if you're PWM'ing at 5V you may find it doesn't turn on all the way. It's pretty typical to add a buffer to drive MOSFETs gates.

The output is near 5V, and ideally higher the gait voltage the better. Make sure it's a PWM optimized MOSFET, btw.

[waltr]

The IRF510 starts to turn on with a Gate to Source voltage (Vgs) of 4V. The Vgs is 10Volts to fully turn on that MOSFET.
http://www.irf.com/product-info/datasheets/data/irf510.pdf

A better choice is one of the Logic level MOSFETs that fully turn on with a Vgs of less than 5V.
http://www.irf.com/whats-new/nr060209.html
This one starts to turn on with Vgs of 2.7V.

Or use a bi-polar transistor to turn the MOSFET's gate on since you do have 24Volts avaiable.

[Soeren]

Hi,

not sure where to place the cap or how many farads it should have.  Could you draw on it with ms paint?

update:  how does that look?   what value should the cap be?

You shouldn't use a cap at all. It's not a speaker, but a buzzer, so have the oscillator circuitry built-in and just needs a DC voltage. A cap where you drew it would block the DC needed.

I don't quite get the obsession with MOSFET's - sure they have their places, but so does BjT's and this can be made with a BC547/2N9304.
And BjT's aren't very static sensitive... MOSFET's OTOH are, and ESD damage doesn't necessarily show right away. Further, you'd need a (true) logic level device and drive it correctly to get it working as good as the BjT.
Only use MOSFET's when they are the smarter choice.

For your circuit, a BC547 with a base resistor, a diode reverse biased over the buzzer and one over the c-e stretch of the transistor should have your ears "pierced" in a short time.

Had you used a plain piezo, the cap would have to be at least 10 times the cap of the piezo, but the larger the better (within reason), as you are creating a capacitive voltage divider (just like with resistors, but AC only of course) and if the DC blocking cap is 10 times, the piezo, you loose around 9% of the available voltage.
Plain piezo disks can be anywhere between a few nF to 100..200nF, depending on diameter and construction. Sometimes a smaller disk can have a higher capacitive value than a larger one, so either you measure or you play it safe with something around 1..2µF (non-polarized preferably).

In case it's a bit too loud (eg, when testing), put cell tape over the hole and make a small prick in that with something pointy.

You know the saying: "for the man who's only got a hammer, everything looks like a nail".
Stay tooled up

[BANE]

Make sure it's a PWM optimized MOSFET, btw.

it is; it says "high switching speeds....for pulse circuits"

Or use a bi-polar transistor to turn the MOSFET's gate on since you do have 24Volts avaiable.

i dont doubt there are many other transistors that would work better, but i already have a couple IRF510 on hand.  I will expand my transistor collection next time i order

You shouldn't use a cap at all. It's not a speaker,

So sorry for the confusion from the misleading schematics, but i couldnt find a piezo speaker (i was in kind of a hurry) in my EAGLE library and that symbol was the closest thing   But it is in fact a piezo speaker.

I don't quite get the obsession with MOSFET's

For some reason i dont get much luck with transistors other than mosfet (probably because i've never had a formal class that included them) unless im following someone else's schematics.  For most of my circuits i  want to simply turn on/off some high voltage with my 5v micro controller, so for now it gets the job done .  However, i hope to change this someday 

Ill update my schematic and reupload in a sec

[BANE]

If I had to guess, I'd say 0.1uF or less.

I've googled around and that seems to be the most common value. Any objections?  I just purchased a roll of .1uf so its fine with me

Also, the diode i picked at random looked correct.  I have a bunch of these http://www.radioshack.com/product/index.jsp?productId=2062576 Will these work?

[Admin]

You shouldn't use a cap at all. It's not a speaker, but a buzzer, so have the oscillator circuitry built-in and just needs a DC voltage. A cap where you drew it would block the DC needed.

hmmmm didn't occur to me that the cap is already built into the piezo . . . I'd check the datasheet just in case, but I think Soeren is right - no need for a cap.

I don't quite get the obsession with MOSFET's - sure they have their places, but so does BjT's and this can be made with a BC547/2N9304.

You and I seem to be at odds on this all the time

MOSFETS are more expensive, but for a one-off project thats only an extra 50 cents or so. They can also handle higher currents, and don't require resistors, making it easier for a beginner that wouldn't know how to calculate optimal values. The PWM optimized types are perfect for driving motors and stuff . . .

Plus, normal transistors are old technology for old people

(here is where you counter my argument lol)

[Soeren]

Hi,

You shouldn't use a cap at all. It's not a speaker,

So sorry for the confusion from the misleading schematics, but i couldnt find a piezo speaker (i was in kind of a hurry) in my EAGLE library and that symbol was the closest thing   But it is in fact a piezo speaker.

Not according to the page you linked to. They actually term it, not a buzzer, nor a speaker but...wait for it... a "Pulse"   
The page says
"1 x 90dB Piezo Pulse (Operates on 3-28VDC, carrying 12mA at 12V. Buzzer tone rating 2,800Hz)"
And that sort of tells the story.
Even without the word "buzzer", the single frequency and the current demands are certain giveaways.

For some reason i dont get much luck with transistors other than mosfet (probably because i've never had a formal class that included them) unless im following someone else's schematics.  For most of my circuits i  want to simply turn on/off some high voltage with my 5v micro controller, so for now it gets the job done .  However, i hope to change this someday 

Using a BjT as a switch is very very very simple and I can teach you that in say 10 minutes if you care?

IRF510 is not a particular good MOSFET anyway, but a TO-220, 100V, 5.6A device (with a high R_ds_on of 0.540 Ohm). A bit overkill (in size as well as power handling) for controlling 12V @ 12mA if you ask me.


And just to repeat myself. If it were a speaker and it had a capacity of 100nF (0.1µF) and you put 100nF in series with that, you will effectively be wasting half your voltage and that's a lot of SPL to neglect.

[BANE]

hmmmm didn't occur to me that the cap is already built into the piezo . . . I'd check the datasheet just in case, but I think Soeren is right - no need for a cap.

i've googled "radio shack 273-060a" and cant seem to find a relavent data sheet, but im known to overlook things .  Ill take your word; so no cap

Using a BjT as a switch is very very very simple and I can teach you that in say 10 minutes if you care?

I'm all ears   Might you include what type and best place to buy?  I doubt i have any laying round and i need to order from digikey soon

[Soeren]

Hi,

You and I seem to be at odds on this all the time

MOSFETS are more expensive, but for a one-off project thats only an extra 50 cents or so. They can also handle higher currents, and don't require resistors, making it easier for a beginner that wouldn't know how to calculate optimal values. The PWM optimized types are perfect for driving motors and stuff . . .

Plus, normal transistors are old technology for old people

(here is where you counter my argument lol)

OK, since I have to

It's not the prize of a MOSFET that I'm on about, although I remember as a kid, I would stretch my budget as long as possible by annoying the salesmen for next to peanuts

It's simply that they have different applications and you'll always get the best result by using the component most optimum for a given task.

IRF510 (TO-220) over a BC547 (TO-92) for a 12mA load... You be the judge.


The "higher current" depends on what device you pick and I seriously don't see many circuits being discussed here that need all that substantial powers, so in general, this is not a valid argument at all - more an apples vs. pears.
For instance, we can compare something like a 2N7000 (the MOSFET equivalent of a "TUN") to a BC546, then you have two valid candidates for a serious discussion.
They are very alike in the "usual parameters" (max. current, max voltage etc).
2N700 has a V_ds_on of 0.4V @ V_gs = 4.5V and I_d of 75mA - not particular impressive.
If pulsed, it can handle up to 0.5A, at which the V_ds_on goes up to 2.5V and needs a V_gs of 10V - not impressive either.
What else... Switching speed 2N7000 = 10ns i.e. 100MHz (300MHz for the BC546).
Nope, I'm really not impressed.
So, the critical voices would ask, where would the 2N7000 be the best choice...
As an example, in eg. a 12..20V circuit which needed very narrow pulses of 1A, say for IR comms, it would certainly be better than the BC546 (which would die screaming).


You should always use a gate resistor (or a tiny inductance like a ferrite bead), or you risk oscillations, or driver latch-up if driven from eg. CMOS.

There's a common misconception that you don't need current to drive a MOSFET gate, but that doesn't hold true, as the gate capacity needs current, more so if you want clean switching.

When currents get above 10A or so, MOSFETs will usually be the best choice.
When eg. there's little drive voltage BjT's will usually be the best choice.
In 3.3V circuits, you need quite exotic MOSFETs to do what just about any BjT can do.

The knowledge to calculate a switching transistor can be acquired in 10 minutes flat and for most projects like switching a buzzer (or other stuff), anything between 470 Ohm and 10k will do as a base resistor, so it really aren't rocket science (and what's the fun if you don't have to learn little bits here and there ).

 
"PWM optimized" is a marketing term used (by Vishay AFAIK) and just covers a fast switching MOSFET. It's allways best to get the fastest switching device, when you operate with large currents at high frequencies (hundreds of kHz and up) - to minimize switching losses.
Fast switching is best for any high current, high frequency switching application, whether PWM or not.
MOSFETs will improve in switching speed over time. Every one and his uncle is working on getting the gate capacitances and propagation times down and they're still in their (2nd) infancy when it comes to potential I think.


But, I don't see the 'bots build here as totally cutting edge, so chasing tiny bits of efficiency is completely wasted, if you throw your circuits in a chassis welded from water pipes or similar.
For a 5kHz PWM control driving the motor of a 5kg robot, I seriously doubt that you will see any difference ("PWM optimized" contra regular fast types) worth mentioning.


Yes, BjT's are for oooold knowledgeable people... But we don't mind sharing our wisdom with kids like you, so that you can all reap the benefits of our ancients potions and spells     


(If anything sounds messy or doesn't make sense, it's probably because my GF keeps bugging me that dinner is getting cold, so gotta post NOW  )

[BANE]

thanks soeren,
now back to business; i just breadboarded the whole thing and the only issue i had was the diode is facing the wrong direction.  Other than that it works great.  Currently im only using 12v and my ears are already bleeding

[Soeren]

Hi,

Currently im only using 12v and my ears are already bleeding

Great... Or rather... Great that it works, but careful now, you don't want a hearing loss like mine (from 4khz to 6kHz it drops 45dB and from 6kHz to 8kHz it drops a further 20dB) in a decade or two - I annoy the living daylight out of most people, when my wrist watch alarm has accidentally been set to ON - I don't hear it, while people 3..5m away are going bananas and asking me to stop the noise... "What noise" I have to ask.

The limit is 85dB (based on an 8 hours workday), but in DK it's 80dB and if it goes to 85dB, the employer must assure that everybody has got proper hearing protection and that it's being used.
You can get a permanent hearing damage as good as instantly if subjected to around 130dB or more.

Loss of hearing in not always just a loss of hearing. Often you get extremely noise sensitive (sounds odd but is very real) and that's usually accompanied by tinnitus, which isn't all that entertaining either.