All Torque No Action?

...or maybe when too little torque is, well, not enough, perhaps? No, not really.

I'm trying to control 2 points with one servo and it just proved too hard.

I did an experiment today inspired by a friend of mine - Mr Merrigan. I have a number of points for switching lines which really can only move together (unless you enjoy derailment).

Chris suggested trying to make thes emove usnig a single servo. Well, I had a red hot go at this on Saturday and a good part of Sunday morning.

The idea it to mount the motor roughly in the mid-point of the meeting switches and have a beam going out across both switches with rods hooking into the cross-bars of the switches. One motor, two moving switches in synchronicity.

You can see the servo above and the spar above that with a nice mount point for the motor.

Just to make this a bit more obvious I have mounted the spar on top of the tracks so you can see how it would work. I've also put a videoon YouTube where I show the movement by hand.

Now this looks like a great idea but in fact it does not work well for a lot of reasons:

• You need very careful alignment to make this work and that is not insurmountable but is hard.

• The motion of the corss bar is straight across but this is radial and so puts more strain on the motor at the extremes of the switch positions. That's not a problem - just use a big motor. But that means a $20 motor instead of 2 x $4 = $8 :-(
  
• The long central spar (15cm) means that even a small turn of the motor (1 deg)  results in a big movement, so your points no longer switch nice and smoothly.

I tried and tried but no luck. When I thought through the logic I realise it was a dead loss really.

For all of these reasons, I'm moving back to one cheap servo per point. Sorry Chirs, nice idea though.

I've got my servo mounting system down to about 10 minutes per point.

Of course, there are other ways to do the mechanism so 1 servo will drive 2 points but it then takes more than 10 minutes per point to mount!

Shift Registers

I'm trying to get as much I/O out of a simple Arduino board as possible and I have previously written about shift registers and their use here.


I thin thought about designing my own PCB to use this and discovered this little beauty on the littlebirdelectronics web site.


I've bought a few and am using them to drive LEDs for signals, relays and some ULN2803s. You can get some nice right angle headers to make it easy to daisy chain these bad boys. Some photos below.

Daisy chained and plugged into a breadboard using the long headers made popular for Arduino shields.

All driven by 3 pins on the Arduino.