I was not sure whether to post this as a topic or a blog. As it's probably going to be a bit "long winded", I thought a blog might be best - so here goes!

Like many others I have been messing around with servos to control the points on my OO layout. In the process I've discovered a few things that might be helpful to others who plan to do something similar. There are some advantages associated with using servos, but there are also some challenges that must be addressed to obtain the most reliable operation.

Probably the greatest advantage is the cost of the inexpensive servos available. For that reason, all my experience is based on the least expensive servo that seems to be available, the SG90.

The Advantages:

• Low cost
  
• Slow/smooth operation (no seismic events)
  
• Can produce large amounts of torque (turning force) relative to their size
  
• Control by simple single-pole changeover switch
  
• Switch position indicates point setting (no additional indicators required)
  
• Not difficult to interface with automation and interlock circuits
  
• Can be connected in parallel for crossovers, slips etc.
  
• One inexpensive driver circuit can drive almost any number of servos (this might take a bit of explaining)
  
• Low power consumption (when working properly)
  

The Challenges:

• Can produce intermittent or even continuous "buzzing" sounds
  
• Electrical interference (particularly with long connecting wire runs) can temporarily drive servos "bananas"
  
• Can be a bit tricky to make a reliable connection between servo and the point tie-bar
  

While there are not that many "challenges", they can present some serious problems that might discourage modelers from taking advantage of these inexpensive devices. Bear in mind that we are trying to use these devices in a manner that is quite a bit different from their intended application in radio-controlled aircraft, cars, etc. Both the application and the environments are quite different from a typical model railway, so it should not be a surprise that we have to account for these differences.

There is not a great deal of information available on the internals of these servos, and it's probably best to try to explain what I've been able to find and figure out for myself. The better we understand the beast, the more likely we will be able to tame it.

The next post will go into the internal operation in some detail.