A 3 Servo DCC decoder based on an Arduino.
This decoder can be used to control 3 servos, the choice of servo is determined by the state of the function buttons, F0, F1 and F2. If the function is off then the servo will not move, if the function is turned on then the corresponding servo can be control by the speed knob of the controller.
Each servo has a set of CV values that are used to define how the servo behaves, a lower and upper limit on the travel of the servo and the maximum speed the servo will travel between the lower and upper limits.
Two modes of operation are supported, relative or absolute mode. In relative mode the speed knob and direct control the direction of movement of the servo and the percentage of the maximum speed at which the servo moves. In absolute mode the servo ignores the speed setting in the CVs and simply moves to a position that matches the current speed knob setting. E.g. if the speed know is set to 40% then the servo will move to a position 40% of the way between the lower and upper limits set in the CVs for that servo.
The DCC input signal should be connected to pin 2 of the Arduino and the acknowledge signal to pin 3 of the Arduino.
The servos should have the control input of the servo connected to pins 9, 10 and 11 respectively.
Pin 4 of the Arduino may be used for a "soft start" approach. If this is used to control the supply of power to the servos then it will stop servo jitter on startup. Pin 4 will be high on startup and taken low after the decoder has receved power for 5 seconds. This output can be used to control the voltage supplied to the servo power, but must not be used as the direct power to the servo as it is not capable of supplying the required current.
The onboard LED (pin 13) will be lit when pin 4 is set low.
The decoder supports the following CV values:
| CV | Description | Default |
|---|---|---|
| 1 | Short address of the decoder | 3 |
| 7 | Decoder version (read-only) | 2 |
| 8 | Manufacturer ID | DIY |
| 17 | Extended address uppper 6 bits | 0 |
| 18 | Extended address lower 8 bits | 3 |
| 29 | Configuration options | 0 |
| 30 | First servo lower limit angle (degrees) | 0 |
| 31 | First servo upper limit angle (degrees) | 180 |
| 32 | First servo travel time (seconds). The time to move better lower and upper limit at 100% throttle. Ignored in absolute mode | 5 |
| 33 | First servo configuration flags (see below) | 3 |
| 35 | First servo lower limit angle (degrees) | 0 |
| 36 | First servo upper limit angle (degrees) | 180 |
| 37 | First servo travel time (seconds). The time to move better lower and upper limit at 100% throttle. Ignored in absolute mode | 5 |
| 38 | First servo configuration flags (see below) | 3 |
| 40 | First servo lower limit angle (degrees) | 0 |
| 41 | First servo upper limit angle (degrees) | 180 |
| 42 | First servo travel time (seconds). The time to move better lower and upper limit at 100% throttle. Ignored in absolute mode | 5 |
| 43 | First servo configuration flags (see below) | 3 |
The servo configuration values (CV33, CV38 and CV 40) are defined as follows
| 7 | 6 | 5 | 4 | 3 | 2 | 1 & 0 |
|---|---|---|---|---|---|---|
| Unused | Unused | Unused | Unused | Unused | Absolute Mode | Initial Position |
Absolute mode controls the effect the speed knob has on the servo position. A value of 0 in this bit field results in the speed setting controlling the speed of the servo movement, relative the minimum travel time set in the relavant CV for the servo. A value of 1 means the speed knob position sets the absolute position of the servo. In absolute mode the end positions are still taken into account, so a 10% speed setting means 10% of travel from lower limit to upper limit.
Initial position bits control the movement of the servo at startup.
| Value | Description |
|---|---|
| 0 | No movement of the servo at startup |
| 1 | The servo moves to the lower limit position at startup |
| 2 | The servo moves to the upper limit position at startup |
| 3 | The servo moves to the mid point between the lower and upper limit position at startup |
Changes to the servo mode will only take effect after the decoder has been power cycled.
This project is based on my DCCServo library and the NMRA DCC library.