Homebrew PCBs

Some items of possible interest to PCB and CNC homebrewers:

12 inch paper shear with 6mm thick blade, is sturdy enough to cut 1/16
thick FR4. On sale in my local store for $12.99. On line catalog shows
$19.99. Item number 09805.

Several digital calipers are on sale. Item 47257 is a 6 inch digital
caliper, .0005 inch resolution, .001 inch accuracy, $15.99. Item 47261
is a 12 inch version, on sale for $34.99 in the on line catalog.

Item 93941 is an 8 inch caliper with "extra large LCD readout". Also
has "Built-in computer interface port". On sale in the on line catalog
for $29.95. In my local store the 6 inch version was on sale for
$19.99. There is also a 12 inch version, not on sale, for $64.99.

Any of these calipers should be good for direct readouts of XY tables
on drilling and milling machines, etc. I didn't figure out while I was
in the store where the computer interface was on the calipers that
have them. It's not shown in the instructions, and the calipers don't
come with an interface cable.

However, I got the Item 47257 (which is not advertised as having a
computer interface), and discovered that there is a small slide-off
cover which reveals 4 gold-plated edge-connector contacts at the edge
of the circuit board. I don't recognise the type of connector this
would work with. The contact pitch is about 60 mils or 1.5 mm. It
should be pretty easy and safe to solder wires to the contacts.

Oh, are you wondering whether that interface actually works? :-) Well,
it appears that the functions of the pins, from left to right, are:

* Ground

* A digital pulse stream, about 3 times per second (which is the
update rate of the display). Bit times are about 30 us.  The "message"
length is about 800 us. Maybe this is BCD. The "high" level is about
1.5V, the battery voltage. (Sorry, times and voltages are very rough -
I'm using a clunky old scope.)

* A digital pulse stream, about 3 times/second. Bit times are about 5
us. Message length is about 800 us. Maybe this is ASCII or something.

* 1.55V battery voltage.

Note, the pulses continue even when the device turns itself off or is
turned off with the on/off button. Also, the caliper doesn't lose it's
zero position even after being "off" for many minutes.

Perhaps the calipers that are advertised as having a computer
interface update at a faster rate, who knows.

Cheers,
- Jan