This is a test of the automatic speed regulation circuit for the Locodoco project (das-labor.org/wiki/Locodoco).

An AVR Atmega8 connected to an L6201 motor controller controls the current on the track with a PWM (pulse width modultaion). It constantly measures the voltage that the motor of the loco induces back to the rails in cycles where the power is turned off. While the motor controller isn't feeding any current, the so called "generator voltage" can be measured across the motor (and rails). The generator voltage is relative to the speed of the turning motor and gives a good indication how fast the loco is running.
When measuring voltages below the desired treshold, the Microcontroller generates longer pulses to make the motor turn faster.
The microcontroller in this video is programmed to keep this voltage relatively low, running the loco very slowly. Despite the different slope, the controller keeps the motor speed well adjusted.

The loco is a simple analogue driven model, having no DCC controller or similar device on board. The motor of the loco is directly connected to the rails.

More about this project: das-labor.org/wiki/Locodoco

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