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Remote Audio Detection And Security Monitoring

2015-01-24 19:13  
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Figure 1 

The circuit is designed to detect audio signals on a remote area for security purposes.

Preamplifier (preamp) – a device normally used together with the sound equipment to be of assistance in improving the overall quality of sound by detecting and strengthening weak signals for subsequent and more powerful amplification stages
LM386 – a low voltage audio power amplifier with features such as battery operated, minimum external parts, wide supply voltage range, low quiescent current drain, high voltage gain, ground referenced input, self centering output quiescent voltage, low distortion, and available in 8 pinMSOPpackage

The LM386 has been designed to work with low voltages with the current being consumed from a power source or a battery is quite minimal. A 12 V supply was used in this circuit which can be obtained from a car battery or a power adapter and it drives the remote microphone preamp circuit and the amplifier circuit on the output side.

The preamplifier on the remote side utilizes an electret condenser microphone (ECM) to identify the audio signals. TheECMcan be very durable, very sensitive, extremely compact in size and has low power requirements. To amplify the weak input signal from the microphone, a 2 stage amplifier is constructed around Q1 and Q2. These amplifiers TL071 are designed as low noise versions with low input bias and offset currents, low harmonic distortion and fast slew rate ideally suited for high fidelity and audio preamplifier applications. The capacitor C1 provides high frequency roll off while resistor R2 acts as a gain control to the overall audio response. For a high signal to noise ratio, the Q1 operates at low collector current. For a maximum dynamic range, the Q2 collector is biased around half the supply voltage.

To prevent undesirable transfer of energy, known as coupling, within the power supply, capacitor C4 acts as a decoupling capacitor by shunting the noise caused by other circuit element through the capacitor, reducing the effect they have on the rest of the circuit, as the power is being supplied on the resistor R6 and R10. While the capacitor C4 prevents AC voltages in the power lines, capacitor C6 prevents DC voltage of the power supply as it allows audio signals to pass, coming from the amplified audio output from Q2 collector.

On the other end of the circuit resides the loudspeaker for the amplified audio signal via capacitor C7. Due to the nature of the circuit having a long wire, capacitor C8 is necessary to provide high frequency response above 100 KHz after the signal amplification made by TL071 to avoid minimal radio interference from the wire. The LM386 will receive the amplified signal while the volume is adjusted by resistor R14. The loudspeaker is rated 8 ohms with 1 watt of audio power and minimal distortion percentage. Alternatively, headphones can also be used but considering a resistor in series with the same impedance as the headphone.

The LM386 power amplifier is designed for use in low voltage consumer applications such as AM-FM radio amplifiers, power converters, small servo drivers, ultrasonic drivers, line drivers, TV sound systems, intercoms, and portable tape player amplifiers.

The preamplifiers can be utilized with semiconductor charged particle detectors, proportional counters and ionization chambers, time and energy spectroscopy, first stage of an instrument amplifier, as masthead amplifier used with television receiver antenna or a satellite receiver dish, as part of a stand alone channel strip built into an audio mixing desk, and a stand alone unit for use in live music and recording studio applications.

The electret condenser microphones (ECM) can be used in several applications such as high quality recording, and lavaliere used to built-in microphones in small sound recording devices and telephones. It is also used for television, theatre, and public speaking applications, in order to allow hands-free operation known as lapel mic.


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