Posted on Dec 21, 2012

This circuit does not require an output transformer or an output coupling capacitor; the annunciator can easily be turned on or off by a control input voltage driving a 10Mohm input resistor, R3. For a smaller acoustic output, replace output transistor Ql, with a 100ohm resistor,

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

while also raising the voice coil impedance to 100ohm, to prevent loading of the IC.

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