This electronic siren circuit is designed using LM389 chip. With transistors T1 and T2 was built astabil multivibrator whose frequency can be adjusted with potentiometer P1 between 1 and 7 Hz. Multivibrator produces a square wave that is available to the collector of T2 (pin 11 of IC1) and the control transistor T3. This transistor is connected to the emitter or muting the amplifier input. Now muting input is connected on the frequency rate of multivibrator with supply voltage and thus attenuates at the same rate, the output signal of the amplifier.
This simple microphone preamplifier circuit is based on the LM318 op amp . The LM318 op amp is operated as a standard non-inverting amplifier. Resistor R1 provides an input path to ground for the bias current of the non-inverting input. The combination of R2 and C2 provides a frequency roll-off below 30 Hz. At 30 Hz and above the gain is relatively flat at about 50 dB, set by the ratio R3/R2. R3 furnishes negative feedback from the output to the inverting input of the op amp. C3 ac couples the preamp to the tone control section.