Low Level Sounder Circuit

Posted on Sep 28, 2012

This is a simple low-level noise maker that`s ideally suited to certain alarm applications. When the sounder is located in another part of the building, the sound level is loud enough to be heard, but is not loud enough to warn off an intruder. A single 2N3904 npn transistor is connected in a Hartley audio oscillator, with a 1 kO to 8- transistor-output transformer doing double duty. The circuit produces a single- frequency tone that can be varied in frequency by changing the value of either or both Ri and C1.

Click here to download the full size of the above Circuit.

Increasing the value of either component will lower the output frequency and decreasing their values will raise the frequency. Don`t go below 4.7 kQ for Rl because you could easily destroy Ql.

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