The Frigoboat Merlin Smart Speed Control is a plug-on circuit board that can be installed on any system using the Danfoss BD35 or BD50 compressor.
The Merlin preforms three functions:
Soft-starts the compressor for protection of the electronics.
Automatically controls compressor speed for highest possible efficiency.
Gives visual indication of compressor speed.
Please see the links below for full details.
Speed Controller Guide