Sun Jul 17 21:19:51 PDT 2016
1.5 Volt Transistor Radio
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Another way to use up batteries...This is a 1.5 volt AAA batter radio that drives a pair of earbuds. It works surprisingly well - it has few controls - in a minimalist quasi high quality high-fidelity Scandinavian audio buff sort of a way (not really). It is quite loud too. It uses about 3ma - so a fresh AAA battery lasts something in the region of 10 days (that is an estimate - I have not checked).
I made this a while ago - inspired by various pictures of matchbox size radios on the web. I made it using a standard tuner circuit and adding a couple of stages of amplification - initially it was a little unstable and prone to oscillate - which often happens with simple radios - but this was cured using a filtering capacitor which removes much of the RF before it gets to the audio amplifying transistors (I learned about this by looking at the pages here - which is also where the basic circuit came from). Thank you Andy Collinson!
The circuit is a little vulnerable to volume fluctuations if you have it in a pocket as you move around (changing the orientation of the direction ferrite antenna). In practice this is not too problematic. I have considered putting in an automatic gain control (AGC) of the types described here and here. Perhaps one day...
One interesting observation about making something like a little am radio is that (of course) I could buy such an item for a small sum of money and likely have a better radio. So I have been puzzling as to why I want to make one. One of the attractions of making something yourself - with relatively vague instructions and objectives - is that you need to solve various problems and make various compromises as you go. So there is an element of enjoying the journey as well as the destination. Well, that is my current excuse...
I will post the circuit soon - it has a few variations relative to the design published by Andy Collinson.