Audio-Operated Room Monitor Circuit

Figure 1 

The circuit is aimed to create a monitoring and surveillance scheme for a remote location to function as a room monitor or baby alarm with the use of an audio input.

• LM380 – a power audio amplifier used for consumer applications with wide supply voltage range (10V-22V), low quiescent power drain (0.13 W), voltage gain fixed at 50, high peak current capability (1.3A), input referenced to GND, high input impedance (150K ohm), low distortion, and standard dual-in-line package
• Amplifier – an electronic device used for intensifying the amplitude or power of a transmitted signal by manipulating the output to counterpart the input signal but with larger amplitude
• 1N4001 – a 1.0 Amp Silicon rectifier with voltage range of 50 to 1000 Volts and possessing features such as guaranteed high temperature soldering, high current capability, diffused junction, low reverse leakage, utilizes void-free molded plastic technique for low cost construction, and carries Underwriters Laboratory Flammability Classification of 94V-0 by its plastic package
• Electret Microphone – a type of condenser of capacitor microphone that utilizes a permanently charged object to eliminate the use of a power supply

The circuit may be powered by mains power supply or a 12V battery source. The power mains consist of a transformer with the primary being connected to the local electricity supply while the secondary is rated at 1A with 12V output. Four 1N4001 make up the bridge rectifier to achieve a full-wave rectification or conversion of AC input into varying DC output. Compared to a conventional full-wave rectifier, a bridge rectifier is capable of producing a voltage output that is almost twice of conventional full-wave circuit. The rating of a bridge rectifier is based on the maximum voltage they can withstand and the maximum current they can pass. In this circuit, its rating was 1A at 50V peak voltage. Capacitor C4 removes any high frequency noise along with the supply while the large value electrolytic capacitor C3 performs the smoothing by acting as a reservoir as it connects across the DC supply. This tends C3 to supply current to the output in case the varying DC voltage is declining from the bridge rectifier. This capacitor will discharge upon supplying current to the output but will charge rapidly near the peak of the varying DC.

A single LM380 comprises the audio amplifier which in turn driver an 8 ohm speaker by delivering 2 watts power output. The IC has features such as standard dual-in-line package, low distortion, high input impedance, quiescent output voltage at half of the supply voltage, high peak current capability, ground referenced input, low quiescent power drain, wide supply voltage range and fixed voltage gain at 50. To prevent the audible oscillation of the mains alternating current, known as power hum, capacitor C1 is positioned to provide further supply decoupling from hums. Resistor forms the zobel network to make the impedance of the loudspeaker to appear as a steady resistance to the amplifier. Along with the zobel network is the series capacitor that will be used to prevent a wideband oscillation at 10 MHz. In practical operation, the absence of zobel network tends the LM380 to consume excessive current and to produce distortion on the audio during the occurrence of wideband oscillation.

From the remote or monitored room, the electret microphone and amplifier is placed with 3 wires in connection to the power supply and amplifier. The wire may be made of telephone cable or alarm cable. The electret microphone may come in the form of two terminals or three terminals for older types. The input signal from the microphone is amplified by a direct coupled amplifier. These two stages of amplification are made possible by an NPN general purpose transistor BC549C which contains low noise stages in audio frequency equipment.

The 10K ohm preset controls the gain of the circuit while the 10K ohm potentiometer adjusts the volume of the listening room location at the remote room. The RC circuit containing 1K ohm resistor and 200 uF capacitor performs the decoupling of the preamplifier power supply. To achieve a high signal to noise ratio, the first stage is made at a low collector current whereas, to achieve low output impedance for handling long cables, the second stage conducts at emitter follower mode. Because of this, a screened cable is not required since noise interference and hum will not be present under low output impedance.

Numerous applications can be attained from the room monitor such as remote observation of the audio from kids, nannies, spouses or other family members in the home, as well as in some businesses and offices with imposed security rules. One important use of this circuit is being a baby or infant monitor system which is an essential accessory to a household. This is very beneficial when giving the baby a nap while a parent finishes his house chores. It may also span into 2 rooms with an LED indicator to know which room is being monitored.

The LM380 2.5 W power audio amplifier can be used in phonograph amplifiers, intercoms, line drivers, teaching machine outputs, alarms, ultrasonic drivers, TV sound systems, AM-FM radio, small servo drivers, power conductors and others.

Source:www.zen22142.zen.co.uk/Circuits/Misc/roommon.htm