Teaching kids about computers...

[Tony Duell]

> You're right about the PSUs though - plus I think that for small SMPSUs (i.e. 
> without lots of protection circuitry on the 'hot' side) there's some trick to 
> making them work from 110V anyway (i.e. the '110V operation' stuff can be 
> added after the fact if needed).

Most small SPMSUs start out by rectifying and smoothing the mains to give 
about 340V DC (in Eurrope). If you have such a supply, you can replace 
the input stage with a voltage doubler circuit and run it off 115V -- the 
output of that will be about 340V DC, so the rest of the supply will 
neither know nor care.

You run into problems if there's other stuff run directly off the mains 
input -- like a small linear supply to provide the startup voltage, or a 
cooling fan, or...). You'll need to modify that too.
> > Tandy CoCo 1 or 2? It's essentially the Motorola application circuit for the 
> > 6883 SAM, so it's straightforward. OK, the CPU is a 6809, which is not as 
> > common as the others, but it _is_ a nice, easy, CPU to learn. 
> 
> Yep, I saw that other post before I saw yours. That's definitely looking like 
> a good candidate, assuming the hardware doesn't prove to be too rare. I've 

I see them quite offten on E-bay. I don;t think they're rare _at all_.

> never prodded a 6809 myself (just 6502 and Z80), but I guess I could stand to 
> learn too :-)

You'll love it. It's waht the 6502 should ahve been. 16 bit index 
registers, 16 bit stack pointer -- no make that 2 16 bit stack pointers, 
progam counter relative addressign modes and long branches for 
position-independant code, 'zero page' in any 256 byte page of memory, 2 
accumulators that can be combindd to make a 16 bit one, the most 
orthogonal instruction set of any 8-bitter, etc, etc, etc.

I always wish the Beeb had used that as its CPU. Acorn clearly knew about 
it (they'd made a 6809-bsed System CPU board), but for some odd reason 
they didn't use it in the Beeb.

-tony