Using Transistors?

[masterchief1517]

I was wondering if anyone knew of a site that had a simple guide on transistors, like which one to use and how to wire them. I wanted to run a little piezo buzzers with my picaxe, but it's not very loud running off the chip's voltage. What I think I do know is that a transitor would receive the tiny signal from my picaxe, then it would "open" the larger voltage source (around, 5 volts maybe?) and would turn on and off, making the piezo function like normal, except louder. I don't know if that is right, and I don't know which transistor to use or how to wire it, so any help would be greatly appreaciated!

 

 

[eclectic]

Chief.

Here's a very good starter.

http://www.kpsec.freeuk.com/components/tran.htm

e.

 

[Tom2000]

A Google search on 'transistor theory' will return many sites that have just what you need. Here's an example: <A href='http://www.tpub.com/neets/book7/25a.htm' Target=_Blank>External Web Link</a>

For your piezo sounder drive circuit, you can use any general-purpose NPN transistor, such as the 2N3904, 2N2222, etc. (In Europe, one commonly available is the BC458, I think. Ask around.)

The little sketch below shows how to hook it up.

Have fun!

Tom


<code><pre><font size=2 face='Courier'>



+6 to +9 volts
(refer to your piezo datasheet
for maximum voltage)
|
|
+-----+
| | Piezo transducer
+-----+
From |
Picaxe | collector
+-------+ base |/
O-----| 1 k |----------|
+-------+ |\
| emitter
|
ground



</font></pre></code>

 

[manuka]

What sort of voume &amp; fidelity are you after? I'm at an age when hearing issues arise but usually have no problems with Picaxe driven piezos! Small piezos make great simple/cheap/low profile sound output devices with basic Picaxe circuits, but you're probably not going to get much of an output boost even amplified. Aside from size issues their performance tends to peak around 4kHz as well, so forget any orchestral outputs! How large is yours? Tried various piezos? Is yours just a disk or mounted? Acoustic resonator maybe? Consider a small e'mag speaker if room filling sound is needed?

Guess it's worth trying however- just a simple NPN common emitter (CE) amp with a BC547 etc normally does OK. Although these classic amps have biasing &amp; voltage divider resistors,the setup above (thanks Tom2000 - saved me!) works a treat for simple Picaxe needs. Stan



Edited by - manuka on 05/08/2007 12:24:42

 

[xstamp]

If you need to increase the sound level above what can be achieved with a single transistor, try driving the piezo with push-pull outputs using three CMOS inverters. The pwmout output would be a good source of 40KHz drive for the inverters.

If you want an even higher level, use an RS-232 buffer chip to generate a +/-12 volt push-pull output (it will help to describe such techniques when we can post schematics on the improved forum).

 

[Michael 2727]

There are 3 ways to make a piezo really loud.
1: A fine tuned cavity, e.g. a custom made piezo disk, and to operate at its (optimum) resonant frequency then placed in a tuned cavity that reinforces the frequency, Cicadas have been doing it for years.

2: Drive the piezo with high voltage, 20V to 50V.
You will need a piezo element that is designed/capable of handling high voltages, many will take 20V to 30V, the smaller ones usually don't.

3: Use an Inductor (coil) in parallel with the piezo element. When an inductors magnetic field collapses it can produce up to 10 X the input voltage, but in reverse.
This high voltage is pushed back into the piezo (which is a capacitor) the piezo then discharges back into the inductor and so on.
This is called a Tank Circuit, the back and forth voltage dies away within a few cycles due to electrical losses. If pulsed from an open collector transistor the output from the piezo can be several times the output from a transistor only.

Many small beeps from electronic devices e.g. watches, calculators, toys are in fact a single pulse, the piezo/inductor do the rest.
Finding an inductor that is exactly tuned to the piezo element is the hard part.
You really need a good LCR Meter and calculator if you want to make your own.
Usually involves a couple of hunderd turns of very fine wire on a ferrite bobbin.

 

[masterchief1517]

Thanks a whole bunch, that article at kpsec really helped a bunch. And I bought my piezo from radioshack. I was there and I wanted something I could mess around and experiment with on my picaxe. It is a &quot;Piezo Mini Buzzer&quot;(part #273-074), it takes 3-12VDC, 70DB min. at 12VDC, and its resonant frequency is 4100 +or- 500Hz. Well I think I can safely order my transistors now, and I'm also buying a couple of solar panels to operate my picaxe, no more batteries for me! I'm also stuck buying another resonator cause I was assembling one of those LCD serial interfaces and I soldered the resonator in the wrong spot, and when I was desoldering it I ripped one of the leads out, joy. One last thing, how well would a general purpose amplifying transitor work as a switch?

Edited by - masterchief1517 on 05/08/2007 12:29:39

 

[manuka]

Whoa -<A href='http://www.radioshack.com/sm-pc-board-12vdc-70db-piezo-buzzer--pi-2062403.html ' Target=_Blank>External Web Link</a>seems to be a <b>buzzer! </b> Hence it's no doubt got inbuilt electronics already. Does it have Read/Black colour coded leads &amp; only work one way round on a 12V battery? For Picaxe use just a bare piezo disk <b>sounder </b> is what's normally used. These work on very low voltages (~3V) either way round, with the output sounds frequency determined by driving signals from the Picaxe itself. Putting one across 12V will just give clicks. See a typical one =&gt; <A href='http://www.maplin.co.uk/Search.aspx?criteria=KU57M&amp;DOY=5m8{/a] ' Target=_Blank>External Web Link</a>

 

[masterchief1517]

It is polarized (making the judgement by the big '+' on the side next to one lead), but I've already tested this piezo buzzer, it works with the picaxe and scales up and down some harmonics, but it is a really small range so if it is playing a sound that is too low or too high it does what it says in the name, it buzzes. And it does play at a very audible level without any amplification, but I want it to be louder so I could make some really annoying things if I really wanted to. And just for the record, it says right on the box that it is rated to run as low as 3V.

Edited by - masterchief1517 on 05/08/2007 13:02:14

 

[Tom2000]

OK, I see what's going on now.

Yes, you have a buzzer. That device operates on 12 volts DC, and generates its own tone.

I thought you had a piezo sounder, which is a passive device. You need to feed it with a high frequency square wave signal to make it beep at the frequency you're sending it.

You can still use the circuit I sketched. Just feed it with 12 volts, and drive the transistor with a steady HIGH (tone on) or LOW (tone off) from the Picaxe.

Tom

 

[masterchief1517]

Again, it has successfully made sounds already, which I programmed it to do with my picaxe 08M. I honestly have no idea why the call it a buzzer because it makes plenty of harmonic sounds with the sound command in the picaxe editor, and it has the about midi-level sound quality. *EDIT* I guess it actually is a buzzer, because I got clicks from it when just attaching it to a battery, but it still does produce harmonics if you make a program.

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Edited by - masterchief1517 on 05/08/2007 13:30:35

 

[boriz]

Tom2000.

Your circuit is missing a resistor. A piezo transducer is electrically very much like a small capacitor. You need to place a resistor in parallel with the piezo element, around 1k or 2k should suffice, to discharge it and pull the collector voltage back up to +V.

 

[Tom2000]

Why, thank you, boriz! I wasn't aware of that. Good to know (particularly since I've included a piezo sounder in my recent project. I'll amend the schematic asap, lest anyone gig me on a low-volume beeper.)

Tom

 

[boriz]

Very welcome.

BTW. The easiest way to make a passive piezo transducer sound louder is to glue it to a sounding board, usually the project case. This will more than double the volume when done carefully.