Classic Intercom

This is a two-way intercom system.

Figure 1: Device.

You can see my device working in this video:

Step 1: Design the Circuit

I have drawn the circuit on Easy Eda software (https://easyeda.com):

Figure 2: Circuit 1 Design.

The two potentiometers/variable resistors should be 1 kohm.

The way the circuit is designed is that two people cannot speak at the same time to prevent oscillations (signal from speaker entering the microphone).

Step 2: Make The Preamplifier

I made a preamplifier with three 470 uF capacitors for each transistor amplifier stage. Although in the circuit design there are only two 470 uF capacitors per amplifier stage.

Figure 3: Make the Preamplifier.

You can also see that the base resistors need to be adjusted to the different current gains of each transistor.

Step 3: Make The Power Amplifier

I made a power amplifier with a 1 kohm to 8 ohm impedance transformer.

Figure 4: Make the Power Amplifier.

You can also see that the base resistors need to be adjusted to the different current gains of each transistor.

Step 4: Encasement

I used two cardboard gift boxes for encasement (each circuit was made twice - receiver/transmitter). 

Figure 5: Encasement Photo 1.

Figure 6: Encasement Photo 2.

Step 5: Testing

Testing showed that the microphone is sensitive:

Additional talking:

Conclusion

I designed the circuit after reading the article about earphone intercom on this website:

https://analogue-electronics.blogspot.com

I simplified the circuit from the link on the website above.