Servo - April 2010

PHOTO 7 and PHOTO 8. Configuration screens.

transmitter. One 1/4” x 20 bolt 1” long attaches the encoder to the top of the square bar. Coming out of the side of the case, it provides a rotational adjustment to get it just right.

The Transmitter Software

The encoder software is incredibly simple (and well commented in the SPIN code). It counts the clock cycles. On every eighth pulse, it sends a framing marker and sets up the next data bit. Each data bit and framing marker is held in place for at least one complete cycle. Switch positions and LED status are updated at the beginning of each bit cycle. This all resides in one cog. The NTSC generation happens in another cog but unless a video display is added to the transmitter above the switches, there is not a lot of use for this beyond debugging. Since the encoder is so incredibly simple, there is not much to display. In the code, there are remarks to point the switches and LEDs to the ports you have chosen.

The red-green LED driver is fun too. It’s off if you send it a zero; a number between one and 255 will give you a combination of red and green — one being all green and 255 being all red. All of the code is available in the download package on the SERVO website.

limited with 10K resistors. The output level of the signals will vary with the power supply of the receiver, but they do go to zero when low. The resistors compensate for the upper voltage variance.

The audio and video switching section is facilitated by a SN74CB3Q3253 2 x 4 to one analog multiplexor. One of four separate A/V inputs can be selected and routed to a transmitter or the tap connector. Channel 0 is hard-wired to the audio and video outputs of the Propeller and is the default position of the selector. Channel selection is accomplished electronically by the bit configuration of Av0 and Av1.

The servo output section of the board is pretty cool. Two 74HC595 serial-in, parallel-out shift registers have been cascaded into 16 outputs. These outputs drive the signal pins on 16 servo connectors. (It takes five Propeller ports and a little PASM code to create 16 PWM output channels.) You’ll note that the power rail for the servo connections is routed from the receiver through a one ohm, five watt resistor, R45. This provides a voltage difference between the supply and the power rail which facilitates a current calculation for the servos. (The math is noted in the SPIN

The Receiver Circuit

The receiver side is more complicated than the transmitter side because it handles more processing and switching. It is divided into functional groups in the schematics but it all resides on one board. Functionally, there is the EEPROM and CPU power, the PWM receiver inputs section, the servo output section, the audio/video switching and 12V power, an NTSC signal generator DAC, an audio output, an eight-channel ADC, and finally, some indicator lights for power and activity. The block diagram rounds all of this up at a higher level. The EEPROM and crystal circuits are copied directly from the Propeller demo board.