Électronique / Electronic
Here's a. different sort of amplifier: you listen through the Earphone to sounds being input through the Speaker.
After you finish wiring this Project, connect the Earphone. Turn the power ON and try rubbing your fingers across the front of the Speaker. Now try tapping the front of the Speaker. What do you hear?
Normally a speaker doesn't make a very good microphone. But in this circuit we have enough amplification to make up for this. Notice that we use two transistors in this Project for increased amplification.
We know how a Speaker works, but did you ever figure out how the Earphone works like we asked you back in Project A Push-Pull Amplifier in Amplifier Section?
Here's the story: we said it was a crystal earphone, It really has a piece of crystal in it. When electric current passes through the crystal, it vibrates and causes sound waves you can hear. But we've used the Earphone as a microphone in some Projects, This is because the crystal will give off weak electric currents whenever sound waves strike it. This relation between an electric current and a crystal is called piezoelectricity (it's easier to understand than it is to say).