Two way intercom circuit construction
Two way intercom circuit construction
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.
If the pads of the push-on switch are shorted together and the switch removed then one unit can act as a remote listening microphone or 'bug'.
You can remove the speaker so that there is no possibility that noise interference will 'click' the speaker and give away its position.
For long term use in this mode a four wire flat cable can be used so that 2 of the wires can be used to supply the battery power to the unit from the listening location.
You may download the data sheet for the LM386 from the website of National Semiconductor at www.nsc.com
ASSEMBLY INSTRUCTIONS
Assembly is straight forward. Make sure you get the integrated circuit, microphone and the capacitors the correct way around.
The electrolytic and capacitors are
polarized; they have a + or - marked on them and they must be inserted correctly into the PCB. The overlay on the PCB shows the correct way to insert them.
The push-on switch is meant to point out from the PCB so that the unit can be mounted inside a box with just the cap of the switch showing.
Only three of the six terminals on the switch are connected in the circuit. The 8 ohm speaker can be connected either way.
Eachstation is identical. To connect the two wires connect one wire from point A to point A on the other board, and the other wire from point B to point B on the other board.
The electret microphone can be placed some distance from the
PCB.
The circuit diagram
CIRCUIT DESCRIPTION
both units are identical.
The detailed circuit diagram for each unit is shown on the next page. An LM386 audio power amplifier is used.
It is widely available, used a minimum of external parts and is well suited to battery operation. It will give just over 1W into the 8 ohm speaker.
The gain can be varied by the external components attached to pins 1 and 8. Gain can range from 20 (open circuit - pins 1 & 8 not connected) to 200 (pins 1 & 8 short circuited.) Stability at high frequencies is set by R3, C2 & C4. C4 decouples the power supply to the intercom for low frequency stability. C5 couples the output of the unit to the speaker.
COMPONENTS REQUIRED
Resistors 1/4W. 5%:
100R (brown black brown) 2
10R (brown black black) 2
10K (brown black orange) 2
Capacitors:
10uF electrolytic 2
100uF electrolytic 4
47nF mylar 2
100nF mylar 4
LM386N-1 IC 2
8 pin IC socket 2
Electret microphone 2
8 ohm speakers 2
Push-on non-lock, non-shorting switch 2
Switch cap 2
Two strand wire 10m
9V battery snap 2
Kit 21 PCB 2
FOLLOWING IS THE block for connecting the 2 modules with A to the A point and B with the B point of the other.
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