Saturday, January 08, 2011

Building a 40m DC receiver in an Altoids tin

I've made these before but they're fun. My objective is to eventually make a direct conversion receiver for 40m that is small enough to fit inside an Altoids tin, including the 9V battery. As you can see, I'm a long way from that objective:


This is the classic NE602 LM386 design from EMRFD except that I'm using a permeability tuned oscillator. I'm hunting for long brass machine screws (and the nuts to fit, probably about 3cm long and 3mm diameter. (If anyone can suggest a source I'd be most grateful). A steel screw works but quenches the oscillator as I screw it in. (Is that why brass is used?).

The other issue in miniaturisation is getting small components - some of my capacitors are huge, due to voltage rating, and I need to collect smaller ones.

1 comment:

Alan Yates said...

Looking good mate, I think you could almost squeeze a 9 volt in there now if you moved the audio amp over a little...

I think I got my brass threaded rod from Bunnings, but maybe it was Mitre 10. Worst case I have a foot or so left, but no matching brass nuts, only gal which can produce noise with the high gain of a DC RX.

Brass tunes up, iron down when advanced into the coil. As you say solid ferrous material is lossy and will kill the oscillator quickly, powdered iron works better. Ferrite can be used in a pinch, but it isn't very stable, still you don't need much. A bit of thick copper wire soldered to the end of a normal hardware would work too, just keep the iron away at max penetration. Got plenty of thick Cu magnet wire I can drop you off some at lunch if you'd like?