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K7: A switch-tuned radio

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In a cassette box

Fancy building your own AM. radio? Sfay tuned. This very compact radio haś switched tuning and runs from a single 1.5V battery celi. Ali you need for this “Hey, what have you got there!” project is an old cassette box and a handful of components.

by HENK MULDER

The K7 has switched tuning. The six preset channels are tuned with the six trim capacitors at the right.

this “reduction” race, a lot of manufac-turers tend to overlook the manageabil-ity of their products.

Take a modern TV remote control, for example. Not only do you need good eyes to read the text on the control, you are expected to have some sort of a Computer background to under-stand the operation of the little keyboard. On top of that, using a thumb of average size will often result in morę than one number keying at a time. In-dustry sometimes overshoots the mark.

You will encounter a similar problem trying to tune a modern pocket size radio receiver. The tuning dial is gen-erally so smali that tuning is like a jack-pot: if you are lucky you will tune in to a good station!

In our new “K7” design we have tried to overcome this problem by

I    Gazińg into those glossy catalogues,

j    kindly spread around by the hifi shops,

^!    you wonder How on earth anyone can

manufacture and sell a walkman-type AM/FM radio with headphones for less than 20 dollars. Looking at the price of components you would expect them to cost a fair bit morę than that. Some-times it seems that there is no point in constructing one’s own electronic equip-ment, as most of the stuff is commer-cially available at lower prices.

However, one thing that money can’t buy is the satisfaction you get out of building your own and being able to boast to your friends “Look, I built this myself!” You also learn a lot aboiit electronics, too . . .

The trend in electronics has always been towards miniaturisation of components and finał products. However, in

providing the radio with 6 presets (a little like a car radio). This restricts the nuisance of tuning to the construction phase of the receiver. Once you have allocated the six positions of the switch to your favourite radio stations, you will find it morę than easy to switch from | one to the other whenever you get bored with a record, a chat or an adver-tisement for washing powder.

The K7 radio is powered by a single AA size 1.5 volt battery. The power j consumption of the Circuit is very Iow, so the battery should last for quite a while.

Bored with the ubiquitous black “jiffy” project cases, we decided for this project to use the most handled but least applied case — a cassette box.

This neat little see-through box supplied us with the name for the project as well. K7, pronounced the French way, says: cassette!

AM?

If we are honest about it , then we , have to admit that the K7 is nothing but an old-time “crystal” receiver in up-dated modern disguise. The Circuit ćan easily be split into three parts: the tun- j ing Circuit, the detector and the audio amplifier. Have a look at the simplified diagram, Fig. 1..

Radio signals are picked up by the antenna, whose coil is part of the tuning Circuit. The tuning Circuit has a double function; it selects the radio signal for which it is tuned and it śuppresses all the other unwanted signals.

How does it work? The tuning circuit or resonant circuit, consisting of ą coil (inductor) and a capacitor, resonates at a certain frequency: the resonance fre-quency. If you inject a signal of this , resonant frequency into the circuit, then the energy in the circuit will build up.

In other words, the amplitudę of the signal in the tuning circuit will get larger and larger. In theory, if the tuning circuit were ideał, i.e., without lośses, then the amplitudę could become infi-nitely large . . . However, in practice there are losses: the capacitors leak, the coils have resistance and on top of that the tuning circuit loses energy through electromagnetic radiation. Still the tun-

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ELECTRONICS Australia. September 1987