home-built calculator

[john]

Hi

My son (13 years old) would like to build a calculator, basic four function,
for a science fair project for next year.
He wanted to build it out of discrete components, however he has now,
(thankfully?) decided maybe some VLS chips would be better and alot easier.
So does anyone know of a set of plans, suggestions or other information that
might help with this project?
I am trained as a tech, but have not worked in the field in a decade.

Thanks

 

[John Miller]

My son (13 years old) would like to build a calculator, basic four function,
for a science fair project for next year.
He wanted to build it out of discrete components, however he has now,
(thankfully?) decided maybe some VLS chips would be better and alot easier.
So does anyone know of a set of plans, suggestions or other information that
might help with this project?
I am trained as a tech, but have not worked in the field in a decade.

Alas, I can't fetch the magazine out of deep storage right now, but
Popular Electronics had exactly such a project, fall of '68, if memory
serves.

--
John Miller, N4VU
email domain: n4vu.com; username: jsm(@)
Surplus (For sale or trade):
Tektronix 465B oscilloscope
New Fellowes leather brief/notebook case

 

[Mark Haase]

Hi

My son (13 years old) would like to build a calculator, basic four function,
for a science fair project for next year.
He wanted to build it out of discrete components, however he has now,
(thankfully?) decided maybe some VLS chips would be better and alot easier.
So does anyone know of a set of plans, suggestions or other information that
might help with this project?
I am trained as a tech, but have not worked in the field in a decade.

Thanks

You could do this with a PIC or HC11 (I recommend the Adapt11C24DX) and
some 7-segment LED displays, but its harder than you might think.
Scanning a keypad and decoding input requires writing a fast interrupt
service routine...Also, most multi-digit 7-seg displays can only display
one digit at a time, so to display multiple digits you have to employ
persistence of vision: displaying each digit in rapid succession.

 

[Bernd Felsche]

My son (13 years old) would like to build a calculator, basic four
function, for a science fair project for next year.
He wanted to build it out of discrete components, however he has now,
(thankfully?) decided maybe some VLS chips would be better and alot easier.

Easier doesn't demonstrate the understanding of how the machines
actually work; which may be a big ask for a 13-yo anyway! A
microcontroller such as an AVR could be used but it would be much
more of a software project than a hardware one.

To get a feel for the complexity required of an ALU, see
http://www.eecs.umich.edu/~jhayes/iscas/c3540/c3540.html

So does anyone know of a set of plans, suggestions or other
information that might help with this project?
I am trained as a tech, but have not worked in the field in a decade.

He should probably use BCD and "dedicated" ALU. BCD allows each
digit to have separate store and display... you need at least 2
"registers"; one for each value on which you're working. One of the
values can be common with the display register.

Division will be the hardest to handle. It's *very* ambitious to do
it at this level of integration.

Addition and subtraction are not out of the realm of possibility.

There used to be a bunch of MSI chips available for that
sort of thing 10 years ago. See
http://www.cs.uiowa.edu/~jones/logicsim/man/node5.html
for an overview of some TTL. Not a comprehensive list by any means.

 

[Michael]

Alas, I can't fetch the magazine out of deep storage right now, but
Popular Electronics had exactly such a project, fall of '68, if memory
serves.

--
John Miller, N4VU
email domain: n4vu.com; username: jsm(@)
Surplus (For sale or trade):
Tektronix 465B oscilloscope
New Fellowes leather brief/notebook case

Pop 'Tronics had a construction article on desktop 4-function electronic
calculator in late '71 or early '72. ISTR reading it when I was fresh out of
the Air Force, so it would have been then.

 

[Don Taylor]

If ...HE... is really going to build it, rather than this being an
ummmm... parent augmented project, perhaps a binary calculator
with bit serial operations and strings of leds to show the state
of the bits, perhaps handling + and - and entering a number, might
be a more feasible task to begin with.

But maybe 13 year olds are just getting a lot better than I remember being.

 

[Bernd Felsche]

Bernd Felsche <[email protected]> writes:

You lost an attribution in there!

 

[Zak]

He should probably use BCD and "dedicated" ALU. BCD allows each
digit to have separate store and display... you need at least 2
"registers"; one for each value on which you're working. One of the
values can be common with the display register.

For a 'simple' project I'd go for a naive implementation of an adding
only machine - which has the advantage of being nicely modular.

First make the keypad and the input registers with associated decoding
and displays.

Then add a bank of adders and more registers.

Use either 3 state buffers and connect everything to a 'display bus'
(enable keypad drivers when in 'key press' mode, or the adder output
after the 'add' key was pressed).

I can see this in a modular build as well. First start with the keypad
(diode encoder?) and a single input register and display, and add a
second one.

Then start working on the 'adding' register - with 3 digits width one
already had a nice demo.


Thomas

 

[Bernd Felsche]

Division is done by repeated subtraction - see 'restoring division'
for the algorithm. Yes, it will be slow, but once add and subtract
work, division is not such a big step !

Erm yes... counting the number of subtractions before the remainder
is less than the divisor. "Fractional" parts can be done by repeated
shifting of the decimal point until you run out of digits after the
decimal point. But it adds significant complexity in the form of a
counter and a comparison circuit.

In the days of mechanical calculators you could watch as the machine
actually did division by this process.

You could on early electronic calculators as well!

 

[Bob]

John,

Your son might enjoy _Code_ by Charles Petzold. It starts with using
telegraph relays, batteries, and light bulbs to allow two boys to
communicate between their homes and goes through (theoretically)
building your own computer. I suppose you could (theoretically) build
one from relays. His book can give your son an understanding of how
computers (and calculators) know that 1 + 1 = 2.

Bob