I want to make a loud siren, similar to a smoke alarm, using a small (~1"diameter) 8-ohm speaker. input voltage is limited to 3 volts@ 1.3amps. most of the piezos I tried simply aren't loud enough at that voltage. shooting for 90-100dB+. I am unfamiliar with audio circuits/signals in general so I am not really sure where to start. oscillator circuit? amplifier chip? any other ideas.
too bad its not a 12 volt system, a DEI513T is like 100db of peizo pain for ~10 bucks
The noise that the speaker produces is roughly proportional to the power that the speaker consumes.
Applying 5V to an 8 ohm speaker calculates out as:
P = E2 / r
P = 25 / 8 = 3.125W
That's the maximum available with with 5V and an 8 ohm speaker.
To get more noise, you need more power. Thus, you need to use a larger +V to get the needed sound level.
Using 12V, would produce 18W of power -- if the speaker can handle it.
Controlling a voltage greater than the Arduino's 5V has dangers.
I would use an opto-isolator (like the 4n35) to isolate the 12V from the Arduino. I am attaching a schematic that uses a 4n35 to control a higher voltage motor. You can see that it is relatively simple.
thanks for the input. unfortunately I am limited to a 3v battery, I can't use an external voltage for this application. I have seen some circuits that use oscillating transistors connected to transformer to boost the voltage, but I don't know which transformer I would need and it would have to fit in a 2"x.75"x.5" space.
Wow. Then you need to look for extra high efficiency. My best try would be here: Challenge Electronics
Good luck!
water resistance is also a major consideration. The 8ohm speaker I have sourced for the project utilizes a plastic diaphragm. anybody know how effective piezos are in damp/wet conditions?
3 volts@ 1.3amps.
Have you tried 3V into an 8-Ohm speaker (with a MOSFET to boost the current)? At a high-pitch, it might be loud enough.
BTW - You won't get 1.3A through an 8 or 4 Ohm speaker at 3V. ([u]Ohm's Law[/u]). And, you won't get 375mA out of the Arduino (rated 40mA max), so you need a MOSFET (or something) to boost the current capability.
With an [u]H-bridge circuit[/u] you can get double the voltage-swing +3V to -3V across the speaker, which will result in twice the current and 4 times the power, and 6dB more loudness. And, going from 8 to 4 Ohms will double the current for twice the power or another 3dB. So, with those two "tricks" you can get an additional 9dB.
Have you tried 3V into an 8-Ohm speaker
no not yet. a quick look at the math (as many have stated) hints at the outcome being unimpressive. I want more than just a pleasant tone; I am going for downright painful. ]
but alas I may not have many other options.
The h-bridge is a clever idea I will definitely keep that in mind.