transmitter/encoder unfolds across two PCBs (printed circuit
boards). One has the CPU, power regulator, NTSC resistor
network, and crystal for the processor. The other is just
switches, LEDs, and resistors. I did it this way so that I could
stack the switch/LED piece on top of the CPU element, so
the whole package will be small enough to fit into a box on
top of the transmitter case. The two PCBs are joined by DIP
headers on either end. These provide electrical connections
and physical support. The switches actually attach the
circuit board to the chassis face.
The circuit itself is very straightforward but there are a
couple of things that need to be pointed out. The two data
outputs back to the transmitter are current-limited with 10K
resistors to protect everybody. You’ll also notice that the
switch LEDs are not hooked up sequentially on the ports.
This is because the ports on any side of the Propeller have a
maximum current which is less than the sum of the
individual ports. Putting all of the LEDs on one side would
overload the Prop if they were all turned on simultaneously.
Having software control of them makes it possible to do all
sorts of start-up dances. And, since there’s a lot of unused
processing power, I tossed in a red-green LED driver cog.
We haven’t talked about the receiver yet, but the two
transmitter PCBs and the receiver PCB all fit on a single
ExpressPCB mini-board. All the ExpressPCB files for this are
on the SERVO website ( www.servomagazine.com). The
good news is you get three PCBs on one order. The bad
news is it must be cut into three individual boards when
you get it. I’ve found the easiest and cleanest way to cut
PCBs is with a band saw. If you don’t have access to a band
saw, score them on both sides several times with a box
cutter and metal ruler, then bend to break. A dremel tool
with a wheel cutter works well, too. Make sure you don’t
cut too close to the traces on the adjacent boards. If you
don’t want to order these PCBs from ExpressPCB, send me
an email ( firstname.lastname@example.org). I have extras available,
so I’ll cut them and put them up on eBay.
For real estate reasons, there are components on both
sides of the PCBs. There are also three regulators using the
surface for heat dissipation on the receiver/decoder so
remember this when mounting. It will need a stand-off of
at least 1/4” inch for electrical isolation and cooling. The
PHOTO 3 and PHOTO 4. Transmitter/Encoder case.
tag for a component on the underside of the board (green)
shows up on the top (red) side of the board, so you have to
be careful. The bottom side components are bypass caps
and half of the voltage divider resistors for the ADC.
The chassis for the switch can be pretty much any size
PHOTO 6. The rev 4 receiver/decoder board ready to go.
SERVO 04.2010 59