Michael said:
Thanks for the suggestions about using telephone cable.
Another quick question. In the schematic, the 100k pot's 3
leads connect one of them to resistor R1, another to capacitor
C2 and another to pins 3, 5 and 6 of the CMOS NAND Gate.
The 100k linear pot I bought at Radio Shack (still in bubble pack)
has no markings on the 3 leads. Which lead goes to which connection?
Pots almost always have the leads coming out in some 1 fold symmetry,
with two of the terminals symmetrical about a fold line passing
through the third. The two symmetrical terminals are the ends of the
resistor (100k, in this case) while the one on the line of symmetry is
the sliding contact that can be made to measure from 0 to 100k to
either of the others, by turning the pot. If in doubt, check the
resistance with a meter. This circuit has the wiper connected to one
end of the element to make a variable resistor in stead of a variable
voltage divider.
(Please refer to
http://charlesumlauf.com/PWMproject.htm)
Also what kind of socket is needed for the CMOS NAND Gate (mentioned
in the parts list)? I assume that it, like the MOSFET with its
heatsink, needs to dissipate heat.
CMOS chips draw very little power so the proto board is all the socket
you will need. You may have to lay the chip on its side on the table
and form the leads a bit to make them fit the board spacing and plug
in easily. They are made with the lead angled just a bit wide, so
that they can be squeezed by a gripping head on automatic insertion
equipment, to give a precise size, when held for insertion. The
outward spring helps to hold the chip in the board till soldered. But
this feature may make it hard to get the chip into the proto board
without an adjustment.
Again thanks to all of you who have been helping me with this project.
It brings back memories.
I don't know if anybody has warned you about static. Both the CMOS
logic chips and MOSFET power transistors have inputs that are
capacitors formed with a very thin coating of silicon oxide. Normal
body static can easily apply enough voltage to this layer to blast a
hole in it, destroying the part. This is especially problematical in
winter, when the humidity is low. The manufacturers recommend that
you work on a static dissapative table and ground yourself with a
ground lead and lots of other static precautions when handling these
devices. But you should have little trouble if you just keep in mind
how static might be built up and discharges through one of these
parts.
A wooden table or a table covered with a piece or cardboard is more
static dissapative than a formica surface or a metal surface (the
metal surface allows very large static currents to flow, while the
plastic surface can hold stored charge for a long time. Do not walk
across a room and reach out and touch one of these devices before you
touch a grounded surface, first. Do not carry one of these devices
and make contact with a grounded surface with the device. Enclose it
in your hand, and make contact with ground with your knuckle before
opening your hand. Never walk up to a circuit and touch one of the
pins on a chip first. Always touch something grounded and then the
battery. Well, you get the idea.
Oh, one other gotcha. Scotch tape produces prodigious static when it
is peeled. Be careful after having opened boxes that you had to peel
tape or labels off of.
I have never used a body grounding strap, but with such simple
precautions, I have never had a device fail because of handling. Just
get in the habit of thinking that there may be a static spark each
time you reach for something.
But it doesn't hurt to have a few spare parts for any project in case
you happen to zap something or connect the power backwards. We have
all done that.