At the next level of servo driver complexity, programmable controllers store a single servo action (defined by a start position, end position, and speed), and then trigger that action automatically. A switch on the board lets you set the trigger mode to respond to either momentary or continuous-on input.
With momentary, which is more frequently used with pushbutton or sensor input, the servo automatically returns to its start position after moving. A magnetic reed relay sensor detecting a train car passing by is a typical example, or input from an IR detector or inductive current sensor. With continuous triggering, like from a flipped toggle or slider switch, the servo remains in its end position until the input turns off.
To program the servo’s start, end, and speed settings, you follow a programming sequence using small buttons onboard the driver. The specific sequence you need to follow depends on the driver board you’re programming, but it’s generally a simple process.
The Singlet controller from Tam Valley Depot (above) is a basic programmable single-driver controller with a small footprint (1-1/4″×1-1/4″) that makes it easy to hide. The schematic shows how to configure it to support an automatic culvert unloader triggered by a long-lever micro switch mounted sticking up alongside a model train track. When a train car backs up to the ramp, its body closes the switch. This initiates the servo to tilt the ramp upward and unload the cargo into the car (below).
For more complex projects, we move up to a programmable 4-channel controller like the Quad-Pic from Tam Valley Depot (below). Measuring 3″×2.75″, this unit provides 4 servomotor drivers that you can program independently, and by configuring them to trigger each other, you can make them perform coordinated routines.
The schematic above shows a circuit that uses this controller for a model-loading platform in which a series of servo-operated paddles push barrels up a ramp. The ends of the servo horns opposite the paddles click 3 micro switches (SW1, SW2, SW3) in sequence, in each case actuating the next servo in line until all 4 servos have moved to their maximum clockwise position. The fourth servo then closes the last micro switch, sending power to the auto-reset relay. This causes the cycle to repeat, after a short delay determined by the value of the supercapacitor. Thus, a full load of barrels is conveyed one-by-one up the ramp (below).