I've taken on the task of building a radio receiver to help further a interest in radio
and electronics. There are a number of quirks to the requirements:
1) Must have low current drain. Their electricity supply is sometimes uncertain,
so it needs to be able to run on batteries, or solar cells, or a wind generator, etc.
2) Has to be fairly simple to build and use. True, I have a very well stocked junkbox,
and access to additional parts as needed, but I want it to be understandable, and
easy to modify and/or duplicate depending on interest.
3) Should cover the AM BC band (MF), and some SW BC and/or ham bands as well.
My current plan is a regenerative set similar to this one:http://www.qsl.net/7n3wvm/regen.html
with separate regenerative stage and detector. I'd probably look for a dual-gang
variable capacitor, such as those used for AM/FM radios, with one large section
(for MF) and a smaller one (for SW). Probably at least two SW bands, covering
something like 5 - 10 and 11-16 MHz, depending on the bandspread available
from the capacitor. (Might just be multiple taps on the input coil, allowing a wide
range of frequencies to be covered.)
I'm looking at using a band switch rather than plug-in coils because then there
are no parts to loose, but it may require several sets of contacts, especially
to change between MF and SW.
Using a regenerative receiver avoids the requirement for a dual-gang variable
as would be required in a super-het. And adds to the "retro" feel - apparently
they like that sort of thing, so I might look for an old ARC-5 case or similar
to build it in, otherwise I'd use a piece of double sided circuit board for the
front panel with the circuit on the inside and the outside polished to look nice.
Best life from a 9V battery would be to design the circuit to operate from
+5V and use a low dropout regulator - that works down to about 5.5V.
I'd also provide for an external DC input (up to +28V?), and/or a larger holder
for AA, C or D cells for extended use. AC power would be via a wall wart,
and I might even put a full-wave bridge rectifier internal to the set so that
just about any type could be pressed into service - even if the output was
AC instead of DC, or if the polarity were reversed. Once I have a sense of
the current draw I can see if I can fit enough solar cells along the top of
the case to power it that way.
At 5V, I'm probably better off using bipolar transistors rather than FETs
(though I have some SMD units that would work well at that voltage.)
Audio output would need to be able to drive headphones or a small speaker.
An LM386 might be a good compromise here for low current draw, though
discrete transistors wouldn't be a bad choice if I can manage it with a
2N3904/2N3906 or similar pair to keep the quiescent current low. Output
should be able to drive 16 ohms.
So, while I have a number of sample circuits already, I though I would
see if anyone has any special favorite circuits to try, or other thoughts
on the design of such a receiver.
How would you go about designing a receiver to meet these needs?