High-sensitivity sound detector designed with four op amps LM324 LM324 is a quad op amp integrated circuit, it uses 14-pin dual in-line plastic package, the shape is shown in the figure. It contains four groups of operational amplifiers in the same form. In addition to the common power supply, the four groups of operational amplifiers are independent of each other. LM324 pdf
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Each group of operational amplifiers can be represented by the symbol shown in Figure 1. It has 5 pinouts, where "+" and "-" are two signal input terminals, and "V +" and "V-" are positive and negative power supplies. Terminal, "Vo" is the output terminal. Of 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 in-phase input terminal, indicating the signal of the op amp output terminal Vo Same phase as this input. The pin arrangement of LM324 is shown in the figure below. 
Here is an example of LM324 application circuit: high-sensitivity detector (in fact, it is the same as hearing aids)
With this device, you can hear very weak sounds in the distance, its strong directivity and extremely high sensitivity, it can fully collect the low-pitched whispers of athletes and coaches on the sports ground, and it is very interesting to use. 
The working principle circuit is shown in the figure above. The microphone installed in the special tube 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 one of the LM324's four op amps, with a gain of 110 times the gain, and the second stage consists of another op amp with a gain of 500 times. Such a high amplification capacity is enough to amplify a very weak sound signal, which is output by the headphones. It can be used to hear the faint sound that human ear can't hear directly from far away.
Precautions
1. Four operational amplifiers are integrated in the LM324, only A and D are used here, the wiring method can refer to the above figure
2. R1 = R2, the value range is between 10K and -100K
3. Power supply + 6V-9V, two (or three) battery clips can be used in series,
4. The sensitivity of this machine is extremely high, do not speak near the MIC during the test!