This was an experiment to see if there was an easy way to avoid having to make a variable capacitor. The previous designs I made using a home-made variable capacitor, although cheap to make, still needed quite a bit of time and effort. This one was surprisingly easy and works really well. It’s a bit strange having to tune it by swinging the coil around, but one gets used to it.
This design used fixed capacitors in the tuning circuit. Tuning is accomplished by varying the inductance instead. This is accomplished by a variometer – a coil wound in two halves, one inside the other in such a way that the angle between them can be varied.
One of the things I hoped to achieve was to get full band coverage just with the coils, but unfortunately I wasn’t able to do it. I ended up using a switch and different capacitor values to select either the top or bottom half of the AM broadcast band.
Here is the circuit diagram:
The coils were wound with 26 swg copper wire on some loops of thick plastic sheet (about 0.9mm). Other materials could be used, such as thick paper or cardboard. The center coil is fixed to a length of plastic tube which rotates on a disposable chopstick, around 5.5 mm diameter. The chopstick is glued into a small block of wood which is glued to the base. The windings were painted with a clear varnish to make sure they don’t slip off.
Dimensions and details are here (click on images to enlarge):
The blue LED is there to provide a low voltage for the detector, around 2.2 volts. It also serves as a handy reminder that it’s still on.
Pins 1 & 8 of the IC are normally connected via a 10 uF electrolytic capacitor. I didn’t bother to put it in, since a direct connection seems to work fine in this configuration.
The feedback winding is wound on the outer coil. The inner coil is intended to be rotated and is connected via multi-strand wire which is looped around the shaft to give it some flexibility. Unless you’re moving it constantly, it’ll probably last for years before the wire breaks.
100K would also be fine for the regen pot.
Hand capacitance will influence the inner coil most. I found it best to achieve rough tuning with it, then fine tuning with the outer coil which is less influenced. I haven’t bothered to use stranded wire to connect the outer coil, so I try not to move it very far.
Here is a shot of the PCB I etched for it, for those who may want to copy the layout:
And this one shows a bit more construction detail: