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Hands Free Operation Two Wire Intercom Circuit

2017-08-16 07:31  
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This Kit is a two station intercom. It uses two wires to run between each intercom unit. Each is self contained with its own battery, speaker, microphone and amplifier circuit based on the  LM386 . To use it you just push the switch and talk. Releasing the switch automatically turns the intercom off and leaves it in a state ready to receive input from the other station. Each intercom station is identical.The design consists of an amplifier, a double-pole changeover switch and two loudspeakers: one for the master station and one for the slave. More than one slave unit can be used, but each requires an additional changeover switch. The power amplifier is a Type LM384 , which can provide almost 2 W output at a supply voltage of 15 V. Pins 3, 4, 5, 10, 11, and 12 are connected to ground and at the same time afford some cooling of the device. Because of that, the IC should not be fitted in a socket, but be soldered direct to the circuit board. The LM384 processes signals with respect to earth so that an asymmetric supply is sufficient. The amplification has been set internally to 50 (34 dB). The IC`s supply line is decoupled by C9. To ensure adequate input sensitivity, a preamplifier, IC1, is provided, which has an amplification of 11 (21 dB). Because this stage is intended for speech only, its bandwidth is limited to 160 Hz to 10 kHz. Divider R2/R3 at the input of the op amp is decoupled by C3. Special loudspeakers that can also serve as microphones are readily available: in the prototype, MS-55 units from Monacor were used, but a number of other makes will do just as well. The bandwidth of the MS-55 (used as loudspeaker) extends from 150 Hz to 20 kHz and (used as a microphone) from 20 Hz to 20 kHz. The MS-55 can handle up to 5-W output, lb ensure satisfactory operation, particularly as a microphone, the loudspeaker must be fitted in a closed box. Although it is advantageous for the "microphone" to have a low internal resistance, it is necessary for a transformer to be used at the input of the circuit. This has, however, the advantage that long cables can be used. An intercom system for a door and inside stations has a speech function for two-way communication with hands-free operation. The intercom system includes a two-wire connecting lead between the door station and each inside station to transmit a conversation current and an operating current.

Two-Wire Intercom