Servo - April 2010

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 ( jim@cannibalrobotics.com). 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.