It's easy to make an electronic siren. We’ll need a single integrated circuit, a transistor, a speaker and a few other electronic components.
Our electronic siren consists of three circuit blocks. Two of the blocks are oscillators and the third block is a current amplifier implemented with a special transistor. An oscillator is based on gates IC1C and IC1D and a second one is based on IC1A and IC1B. The amplifier stage is implemented with the TIP110 containing a Darlington pair of transistors in a single case.
Both oscillators are astable multivibrators based on inverting gates. The 7400 IC contains 4 NAND gates. By short-circuiting the inputs in each gate, we actually make 4 NOT (inverting) gates.
The fundamental sound of the siren is produced by the oscillator which is based on the IC1C and IC1D gates. The values of C4, C5 and resistors R3, R7 and the potentiometer R4 determine the base operating frequency. This frequency can range from about 1200 to 3000Hz and can be adjusted by the R4 potentiometer.
The second oscillator, based on gates IC1A and IC1B, operates at a frequency of about 0.3Hz and is used to modulate the first oscillator. The modulation type is frequency modulation and the actual frequency deviation range is determined by the R4. Due to the frequency modulation, the sound produced at the output of the basic oscillator is of a varying frequency and thus the sound of a siren is being emulated.
The R6, C3 are used to smooth the relative sharp output of the low-frequency oscillator, in order to a produce a slow-varying modulation signal to simulate the familiar varying sound pitch of a typical siren. The sound is amplified by the TIP110 and drives the speaker. The Darlington pair inside the TIP110 acts as a high gain current amplifier. Thus, the loudspeaker is driven by a strong current in order to produce an adequate sound level.
Because the TIP110 operates at a relatively high current, it is essential to use a large heat sink in it. The amplifier operates at a higher voltage than the oscillators. The oscillators operate at approximately 5V, resulting from the main power supply voltage using a zener diode which acts as a stabilizer. Zener stabilization is practically feasible because the 7400 operates with very low power consumption.