Acting on a capacitor charging, one oscillator (pins 1,2 and 3 of lC1) can modulate another oscillator in frequency, as described in this project. The produced tones thus run from high to low and vice-versa at a rate determined by the first oscillator frequency and also by P1 and P2 adjustments.
The circuit can be used as a siren, part of alarms, games and many other applications. With a 12Y power supply the output power is up to a wattage representing an excellent audio level in a loudspeaker.
The basic circuit has tone, modulation rate and depth adjusted by trimmer potentiometer. There are also some components that can be changed to alter the performance of the siren. G1, for instance, determines modulation rate and can range from 1 to 47 uF. C2 determines modulation depth and rate and can also be altered within a large range of values. C3 determine tone frequency, ranging from 0.01 to 0.1uF.
Power supply can range from 6 to 12V. With a 12V power supply current drain is as high as 2 or 3 amps. Fl is important prevent problems with shorts. Ql is a power FET and equivalents can be used. You also can replace this transistor with a Darlington NPN power transistor such as TlP110, but you must also add a series resistor with base terminal. Use a 10,000 ohm resistor tor this task.
The schematic for the Frequency-Modulation Siren is shown in circuit . Observe that two gates are used as oscillators and the other two of a 40938lC are used as buffer-inverters.
Components placement on a homemade printed-circuit board is shown in circuit. For experimental purposes the circuit can also be mounted on a solderless board, and it is important to place the transistor on a large heatsink.
Position of the polarized components, such as electrolytic capacitors, should be observed.
The speaker should be a high-power type for better performance. Installing it in an enclosure will provide better sound level.
Parts List - Frequency-Modulation Siren
lC1 - 40938 CMOS Integrated Circuit
Q1 - lRF630 or equivalent Power FET
SPKR - 4 or 8 ohms, 4- to 8-inch speaker - see text
F1 - 5A fuse
R1 - 100,000 ohm, 114W,5% resistor
R2 - 4,700 ohm, 114W,5% resistor
R3,4 - 10,000 ohm, 114W,5% resistors
P1 - 2,2OO,OO0 ohm trimmer potentiometer
P2 - 47,000 ohm trimmer potentiometer
P3 - 100,000 ohm trimmer potentiometer
C1 - 4.7uF,16 WVDC electrolytic capacitor
C2 - 1,000uF, 16 WVDC electrolytic capacitor
C3 - 22uF, 16 WVDC electrolytic capacitor
C4 - 0.022uF ceramic or metal film capacitor
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