Schematic & Wiring Diagram

This is a 2-station simple intercom circuit build based on transistors part and using common 8R mini speakers. The speaker works as microphone and generate sound, so there is no need microphone for this intercom. The "press-to-talk" switches should have a spring-return so the intercom can never be left ON, push on switch can be used for this kind of task.



The secret to preventing instability (motor-boating) with a high gain circuit like this is to power the speaker from a separate power supply! You can connect an extra station (or two extra stations) to this circuit design.

Build the circuit into two separated boxes (for 2 stations). On each box should be use 2 ports for external power supply port (if you use external power supply, not battery) and connection port to other stations. Create some holes for speaker push on switch.

Here the circuit diagram of Interior Light Fader for Automobile. The circuit is build using low power operational amplifier LM324 which only need around 3mA of current, so it won't bother the battery supply if left connected for extended periods. This circuit is similar to the fading eyes circuit above and is used to slowly brighten and fade interior lights of older cars.
The top two op-amps circuit module (pins 1,2,3 and 5,6,7) form a triangle wave oscillator running at about 700Hz while the lower op-amp (pins 8,9,10) produces a linear, 5 second ramp, that moves up or down depending on the position of the door switch. The two transistors and associated resistors serve to limit the ramp voltage to slightly more and less than the upper and lower limits of the triangle waveform. These two signals (700 hZ. triangle wave and 5 second ramp) are applied to the inputs of the 4th op-amp (pins 12,13,14) that serves as a voltage comparator and produce a varying duty cycle square wave that controls the IRFZ44 MOSFET and lamp brightness. The 5 second fade time can be adjusted with the 75K resistor connected to the door switch. A larger value will increase the time and a smaller value will speed it up.

When the door switch is closed (car door open) the voltage on pin 8 slowly increase above the negative peaks of the triangle wave producing a short duty cycle output and a dim light. As the ramp moves farther positive, a greater percentage of the triangle wave will be lower than the ramp voltage producing a wider pulse and brighter light. This process continues until the ramp is 100% above the positive peaks of the triangle wave and the output is maximum. When the door switch is open, the reverse action takes place and the lamps slowly fade out.

The MOSFET IRFZ44 shouldn't need a heatsink if the total load is 50 watts or less but the temperature of the MOSFET should be monitored to make sure it doesn't overheat. The on-state resistance is only 0.028 ohms so that 4A of current (48 watts) is only around 100mW. For larger loads, a compact heatsink can be added to keep the MOSFET cool. - Interior Light Fader for Automobile circuit diagram