Command-Operated Acoustic Tone

The circuit was designed to create an acoustic tone that can be controlled by giving a command.

• 7400 – a quadruple 2-input NAND gate employing Transistor-Transistor Logic (TTL) to achieve high speed at moderate power dissipation and provide the basic functions used in the implementation of digital integrated circuit systems due to its low output impedance, minimal variation in switching times with temperature, high noise immunity, and good capacitive drive capability
• 74121 – a monostable multivibrator featuring both positive and negative edge triggering with complementary outputs while having an internal 2K ohm timing resistor for design convenience to minimize component count and layout problems with features such as input clamp diodes, TTL & DTL compatible, Excellent noise immunity, variable & stable pulse width, compensated for Vcc & temperature variations, jitter free Schmitt trigger input, and triggered from active-high transition or active-low transition inputs
• BC550 – an NPN general purpose transistor with low current and low voltage used for low noise stages in audio frequency equipment

In the operation of the circuit, a short vibration in its input will produce an indication of an acoustic signal with a frequency of 2 KHz. The circuit contains a monostable multivibrator with an input from a Schmitt trigger. In a monostable circuit, one of the states will be stable and for a predetermined period, the circuit will flip into the unstable state but will eventually return to the stable state. The Schmitt trigger incorporate positive and provides greater stability and noise immunity for the single input threshold circuit.

The Transistor Transistor Logic (TTL) oscillator is oscillating in the frequency of 2 KHz within the duration of 100 ms. This TTL oscillator generates a stable square wave rich in harmonics. During this oscillation, a piezoelectric element or a loudspeaker produces a sound to represent the frequency of oscillation. The multivibrator is triggered by the square wave vibration that is applied in the input stage. This will drive the oscillator and the loudspeaker attached.

The use of switch S1 is to select the positive or negative polarity of vibration in the input. By changing the values of resistor R2 or capacitor C4, the frequency of oscillation proportionally changes. This change of frequency is what we are hearing in the loudspeaker with the change of sound. The intensity of the tone heard from the loudspeaker is adjusted using the trimmer TR1. The whole circuit is being powered by a 5 V supply.

The concept used by the acoustic tone circuit is being applied in computer editing & sequencing, in keyboards, in wind MIDI controller, or guitar MIDI converter, to produce high-quality monophonic voices. It can also be used in simulation of complex vibrations, reflections, resonances, and other acoustic phenomena.

Source:users.otenet.gr/~athsam/acoustic_tone.htm