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Control by Microprocessor

4QD controllers are quite suitable for use with a microprocessor e.g. Pi or Arduino etc, but may require some simple interfacing, depending on the output of the digital system used.

We have a schematic and some sample Python code for driving the DNO by a Pi here.


Most PLCs and micro controllers have a PWM output, this should be fed into the pot wiper connection of the input. Our controllers all have a ramping circuit which acts as a low pass filter. This will average out the PWM signal, accepting the mean d.c. voltage as the speed reference input so no extra filtering is normally required. To alter the speed, simply vary the mark/space ratio of the signal. You may need to adjust the gain of the controller to give full speed at the 100% mark.

Pot Fault Detection There is a caveat on the above – some controllers have pot fault detection circuitry fitted, on controllers that use resistance detection (4QD series, Pro-150, Pro-120, DNO, VTX), you will need to fit a resistor in place of the pot to override the pot fault detection circuit. A 10k resistor between pot max and pot zero will usually suffice.

The PFD circuit on the Uni works by measuring the voltage on the wiper of the pot. If this voltage instantaneously exceeds a threshold, then the PFD circuitry engages and turns off the controller. There is more detail on driving the Uni via PWM on this page.



The reverse line on all controllers are high impedance and can easily be operated digitally. Easiest method is a pull up resistor to positive line (10K will usually suffice) with a transistor to 0v. The method is shown in the diagram. Virtually any NPN transistor will do.



The ignition line is easiest operated by a PNP transistor pulling it to the positive line, since extra impedance here will increase the battery under-voltage protection level (assuming the feature is present). The ignition current is small, about 600Āµamps, so a 100K base resistor is fine. The method is shown in the second diagram.

Or you could use a relay, the coil operated by the digital system and the normally open contacts as the ignition switch.

Expansion connector

In the Pro-120, DNO and VTX it is possible to input direct to the modulator via the expansion connector. This is still decoupled and will still accept a PWM signal.