Dual Tone Sounder Circuit

An outside horn-type speaker works best with the circuit. However, such devices require a great deal of power, so this sounder should only be used in alarm circuits where at least a 6-A SCR is used as the sounder driver. A single CMOS 4001 quad 2-input NOR gate, two 2N3904 general- purpose npri transistors, and a single MJE3055 power transistor combine to generate a two-tone output. Gates Ul-a and Ul-b are configured as a simple feedback oscillator with R2 and C2 setting the oscillator`s frequency. With the values shown, the circuit oscillates at about 500 Hz. Gates Ul-c and Ul-d are connected in a similar oscillator circuit, but they operate at a much lower frequency.
Dual Tone Sounder Circuit - schematic

The oscillator frequencies (and thus the tones that they produce) can be altered by increasing or decreasing, the values of Rl and C7] for the low-frequency oscillator and R2 and C2 for the high-frequency oscillator. Decreasing the values of those components will increase the frequency; increasing their values will decrease the frequency. The two oscillator outputs are connected to separate amplifiers (configured as emitter followers), whose outputs are used to drive a single power transistor (Q3, an MJE3055). 10-, 5-W resistor, R5, is used to limit the current through the speaker and Q3 to a safe level. To boost the sound level, R5 can be replaced with another speaker.

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