Circuit and PCB design for the Futaba-Multiplex Trainer cord
After the description on the previous page the circuit is almost self-explanatory.
On the input side the signal from the Futaba transmitter is fed to the base of a transistor. On the collector side of this transistor the led from the optocoupler is biased by a 8K2 resistor. When the transistor is conducting the anode of the led is grounded. So the signal of the Futaba is put into the optocoupler on the output side of which this signal is retrieved and amplified by the first of three transistors. This first transistor is included in the optocoupler and the other two are discrete components. A nicely squared signal with a positive baseline and negative pulses can be found at the collector of the second 2N3903 transistor at the right (output) side of the circuit. Various capacitors are used to suppress unwanted signals from RF sources. The parts around the 78L05 are used to create the regulated 5 volts.
The PCB design follows the circuit as can be seen on this overlay where the components placement is also drawn in.
The copper traces on the PCB are shown in this drawing. The original Easypc file, used to produce this PCB can be downloaded from the page with the downloadable files. Look for the PUPTEACH.PCB file.
