The TTL circuit we present is for recreational purposes, but that does not mean that it cannot serve as a basis for work on digital logic. It is a two-tone siren, still based on the 7400 integrated circuit. The circuit is shown in the figure. In this circuit, two ports form an audio oscillator controlled by the other two ports, which form a slow modulation oscillator. In the first oscillator, the audio tone is given by the 220 nF capacitor. The reader will be able to experience 100 to 470 nF. The modulation is given by the capacitor from 10 to 47 uF. The intermittent rectangular signal generated by the circuit is amplified by a BD135, or equivalent like the TIP31, and reproduced by a loudspeaker. For greater audio power, only the transistor can be powered with a higher voltage, keeping the 5 V of the integrated.
