Design of High Sensitivity Sound Detector Based on Four Op Amp LM324
The LM324 is a quad op amp integrated circuit packaged in a 14-pin dual in-line plastic package, as shown in the figure. It contains four sets of identical operational amplifiers. The four sets of op amps are independent of each other except for the power supply. LM324 pdf 
Each set of operational amplifiers can be represented by the symbol shown in Figure 1. It has 5 pinouts, where "+" and "-" are the two signal inputs, and "V+" and "V-" are positive and negative supplies. End, "Vo" is the output. In the two signal input terminals, Vi-(-) is the inverting input terminal, indicating that the signal of the op amp output terminal Vo is opposite to the phase of the input terminal; Vi+(+) is the non-inverting input terminal, indicating the signal of the op amp output terminal Vo. Same phase as this input. The pinout of the LM324 is shown in the figure below. 
The following describes an example of the LM324 application circuit: a high-sensitivity horn (in fact, a hearing aid)
With this device, you can hear very weak sounds in the distance. Its strong directivity and high sensitivity can make the whispers of athletes and coaches on the sports field very easy to use. It is very interesting to use. 
working principle
The circuit is shown in the figure above. The microphone installed in the special package receives the sound in a certain direction (the sound in other directions is suppressed) and sends it to the amplifier for amplification. The amplifier consists of two stages. The first stage consists of an op amp in the LM324 quad op amp with a gain of 110 times the gain, and the second stage consists of another op amp with a gain of 500 times the gain. This high amplification capability is sufficient to amplify the very weak sound signal and output it from the headphones. Use it to hear the faint sound that the human ear can't hear directly from far away.
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