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Arduino, Hardware, Software

USB Configurable Arduino DDC Servo & Accessory Decoder: Add a Port to a Servo to Switch Electrofrog Polarity

When a port is configured as a servo, we have the option to also configure one or even two outputs to go along with it.

This can be used for many different purposes, but the main application of the extra outputs is to switch a relay to accommodate a so called ‘electrofrog’ turnout. These turnouts require the DCC signal on the frog and the frog point to change polarity with the state of the turnout: straight or bent.


When one extra output is configured, it switches halfway the rotation of the servo. This is OK for the turnout types where the frog point is isolated and can be switched any moment without creating an electrical short.

There are turnout types however (like Peco N scale) where a second relay is needed to switch the DCC off before the servo movement starts. The other relay then switches DCC polarity halfway, and when the movement has finished the second relay switches DCC back on .

The graphs shows the timing diagram for this setup.


The video shows how to setup a servo to use one or two additional switching ports.

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