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I am designing a board which communicate wirelessly with a smart phone. When I send a specific command, I want it to sound an alarm to the user, making it able to be found when lost. My board runs on a coin cell 3V battery, which has a current drain which should not go above 15mA. The sound unit should not exceed 10mm in diameter. I struggle to find any speakers or buzzers which can operate within these conditions, so my question is:

What methods and/or components can be used with 3V, max 15mA circuit to notify a user using an audio signal around 70dB SPL?

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  • \$\begingroup\$ How many dB SPL do you need? \$\endgroup\$ – Andy aka Aug 13 '13 at 9:31
  • \$\begingroup\$ @Andyaka I guess about 70dB \$\endgroup\$ – chwi Aug 14 '13 at 11:57
  • \$\begingroup\$ An ordinary piezo disk should meet your electrical requirement while being fairly loud at suitable frequencies. Current demand will only be while voltage is changing, so be sure to have sufficient supply caps and to measure the time average. \$\endgroup\$ – Chris Stratton Aug 14 '13 at 12:42
  • \$\begingroup\$ @ChrisStratton I thought that these disc needed around 30V? Do you have any examples? \$\endgroup\$ – chwi Aug 14 '13 at 13:47
  • \$\begingroup\$ 70dB, at what distance is that? 10cm? 1m? (Without the distance the number doesn't mean anything) \$\endgroup\$ – Johan.A Aug 14 '13 at 13:53
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PUI has the AI-3035, a piezoelectric buzzer rated for 2-5 Volt operation, nominal 3 Volts, and with a maximum current requirement of 9 mA.

Buzzer

The dimensions are 30 mm diameter, 20.5 mm height excluding leads. The Digikey pricing is $3.55 each, but you can find them on eBay sometimes for as little as $1 including international shipping.

PUI also has a piezo buzzer rated for a nominal 1.5 Volts, ~5 mA, the AI-1201B, but no datasheet seems to be available for it, so it isn't definite whether it can withstand the 3 Volt supply mentioned in the question.

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The closest you can get to the solution are magnetic buzzers used in pagers and cell phones. They run from 1V up, but they use to need more than 20mA. Look the options in this page: http://goo.gl/aIh5cn Piezos won't work without a tranformer, they need high voltages. For the magnetic buzzer solution, shall you find one that is 30mA, for example, you may charge a capacitor to help it beeping. It will demand some tinkering, a transistor eventually.

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  • \$\begingroup\$ The claim that Piezo's wont work without a transformer is rather debatable - it really depends on the poster's precise power needs, and what frequency they are willing to use. One shouldn't assume a piezo won't work without actually trying it, and the electrical power required can be much less. \$\endgroup\$ – Chris Stratton Sep 20 '13 at 15:55

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