eli5: On old radios, how does turning the dial tune into a different station? what is happening inside the radio to achieve this?
In: 70
On the realy old designs, you change the value of a variable capacitor. They typically look like [https://en.wikipedia.org/wiki/Variable_capacitor#/media/File:Variable_Capacitor.jpg](https://en.wikipedia.org/wiki/Variable_capacitor#/media/File:Variable_Capacitor.jpg)
The simple capacitor is two parallel metal plates that have an air gap in between. Their capacitance depends on the area and the distance between the plates. If you one sideways the area where they overlap is reduced and the capacitance is reduced. That is exactly what happens in the variable capacitor in the image above. There are multiple layers of plates not just one to make it smaller.
A capacitor and an inductor can form an LC-oscillator. L is used for the value if inductance, I was already taking for current, and C stand for the capacitance.
It is a bit simplified way to produce a signal at a specific frequency. The frequency depends on the inductance of the inductor and the capacitance of the capacitor. The inductor will be fixed and the capacitor variable so you can change the frequency to one of the radio stations.
You can compare it to a spring with a weight attached, If you stretch out the spring it will start to oscillate up and down and the frequency depends on how strong the spring is and how large the weight is. You can change the frequency by just changing the weight, which is equivalent to changing the capacitor in an LC-oscillator.
The signals are then used to filter out the radio station you are listed to. The low-frequency sound can then be extracted from the high-frequency radio waves. In a simple AM receiver, the capacitor and inductor are used to filter the signal directly.
Regardless of how the LC-oscillator is used, you change the station by adjusting the frequency it oscillates at.
In newer designs, you might change the value of a resistor by mechanically moving a contact. That change will change the frequency of another type of oscillator. Today it is likly input to a microchip that in some way will control an oscillator that is used to pick out the signal you are interested in.
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