Mike_in_SD said:
I have a GPS that uses a piezo for proximity alarms.
I HAVE to have it louder. With help of the boys over at
electronics.design I have found the piezo and unsoldered it.
Here is a pic of the board to show you why I had a problem
finding it.
www.mikeamick.com/all.jpg
the round thing that LOOKS like the piezo is made of ferrite and is
a compass module ( I belive ) .. the white chip below it .. is
the piezo.
ANYWAYS ....................................
I soldered a couple leads to where the piezo used to be .. and have
been buying every small piezo I can find and hooking it up to the leads
to find one that is louder ..
Of course I only use one's that are close to the voltage and current
parameters of the first one.
Its amazing the varience .. some a little louder and some you can
bareley hear.
question .. can I hook up two of them in parellel(sp?) will it hurt
anything ?
or is there something else I can hook up .. that is triggered by the
old piezo signal ... ??
thanks
mike
Hi, Mike. If you HAVE to get a bigger sound from that dinky piezo
beeper, it's probably going to be a little more complicated than just
buying another beeper. But the good news is you can probably cobble up
what you need from the Radio Shack in the Pierre mall.
First, a little background. Bare piezo beepers are driven by AC -- the
alternating voltage causes the piezo element to flex like a speaker.
If you flex the piezo element at its mechanically resonant frequency, a
lot of the electrical energy will be converted into sound.
Since 4KHz sine waves are kind of hard to generate, you can do
something like this with two microcontroller output pins (view in
fixerd font or M$ Notepad):
|
| Piezo
| Beeper
| |.-.|
| || ||
| |\ || || /|
| A o--| >O--|| ||--O< |--.
| |/ || || \| |
| || || |
| |'-'| |
| |
| A'o---------------------'
(created by AACircuit v1.28.6 beta 04/19/05
www.tech-chat.de)
If A' is the inverse of A, you will have one side at 5V (if that's the
operating voltage of the uC -- could be 3V or 3.3V) and the other side
of the piezo element at 0V. When A and A' switch, the second side has
5V and the 1st side has 0V. That's the same as applying a 10Vp.p.
square wave from a function generator to the piezo element, and
actually produces louder sound than a square wave, because the fast
transitions cause the piezo element to move faster.
Now you could just replace the piezo element with a bigger one, but
you're probably not going to get better results. That's because the
piezo element is seen by the two digital outputs as a capacitive load
(probably several thousand pF). The primary limitation will be how
hard the uC outputs can drive that capacitive load. Not only that, but
different piezo elements have different mechanical resonances. If it's
the wrong frequency, you'll waste a lot of the energy from the two
logic gates, which means it won't even be as loud as the original.
Now there is another way to do this. If you can pick off the logic
signal, you can use it with an external power supply and a transistor
to switch, to drive a separate self-driven piezo beeper. Here's the
circuit:
|
| A o-------o---------o---o-----------o------------.
| | |10K| | |
| 1000pF | .-. | | |
| --- | | | | |
| --- | | | | |
| | '-' |e .-----o-----. |
| | ___ | |< | + | |
| B o--|<---o--|___|--o-|2N3906 | Beeper | |
| 10K b|\ | with | +| 9V
| 1N4001 |c | Control | ---
| o-----oC Pin | -
| | | RadioShack| |
| .-. | 273-68 | |
| 10K| | | - | |
| | | '-----o-----' |
| '-' | |
| | | |
| '-----------o------------'
|
(created by AACircuit v1.28.6 beta 04/19/05
www.tech-chat.de)
Every time the piezo turns on, A' will be several volts lower than A
3000 times a second or so. That will result in current being pulled
out of the base of the PNP transistor, which will turn it on. A
positive voltage will appear at the control pin of the 80dB piezo pulse
buzzer, which will result in an annoyingly loud intermittent beep.
Trust me, this will work fine with a 9V battery, only uses 10mA when
it's on and essentially nothing when it's off, and will be pretty loud.
So go to the Shack, get a 9V battery with snap on connector with flying
leads, a small perfboard, some 10K resistors, a 2N3906 or other PNP
small signal transistor, and the specified beeper, and you're good to
go.
Cheers
Chris