Peak Indicator for Audio Signal

Peak Indicator for Audio Signal


The circuit is designed to produce an indicator that will show the peak level of audio signal as it exceeds the prescribed limit.

Terminology 74C14 – a monolithic complementary metal oxide semiconductor (CMOS) integrated circuit with N- and P-channel enhancement transistors and functions as a hex Schmitt trigger with features low power, high noise immunity, wide supply voltage, and hysteresis voltage1N4148 – a small-signal silicon epitaxial planar diode used for fast switching applications with a reverse voltage of 100 V and forward current of 150 mA Circuit Explanation

The design of the circuit is based on the operation of a 74C14 which is a type of CMOS inverting Schmitt trigger with high input impedance on the peak indicator. The trimmer TR1 regulates the value of up to what extent the circuit will detect the level. During the operation of the circuit, D1 half-wave rectifies the input. The output of the second Schmitt trigger produces a logic HIGH when the upper trigger threshold is exceeded by the voltage across R1. The supply determines the trigger threshold voltage where a trigger level of 3.6 V corresponds to a 5 V supply.

The feedback to the input through C2 is the positive pulse at the output of the second trigger. During the time of charging C2, the logic HIGH level will be maintained if the input signal is very short, which is determined by the values of R1 and C2. The LED D4 will light up when the output of the third trigger is logic LOW during the time of C2 charging.  The lighting of D4 indicates a very high level of the input signal. The peak indicator will begin to function after the trimmer is adjusted to the maximum desired level. It can provide a reliable means of detection by identifying the very short voltage over swings.

Part List

TR1=1Mohms trimmer
C1=100nF 100V MKT
C2=150nF 100V MKT


The application of this circuit involves the measurement of real time audio especially in a loudspeaker system, tape recorders, and mixing consoles, where the driving limits are controlled. One of the functions implied by the circuit is the signal analysis where it is applied in machine performance analysis, product testing, audiometer analysis and calibration, maintenance troubleshooting and inspections, and engineering control studies. When the circuit functions as noise dose measurement, it can be applied in occupational and environmental noise compliance and measurement, noise ordinance enforcement, community noise assessment, and environmental impact studies. For functioning as frequency and time weighting, it can be used in frequency calibration and analysis.

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