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Servomotors...

A project log for MO5 interface boards

I'm a Thomson MO5 fanatic and despite its bad rep, it's a very nice machine that is easy to extend.

Yann Guidon / YGDESYann Guidon / YGDES 04/10/2018 at 01:240 Comments

Something strikes me now and it sounds almost ridiculous but as always, it's a matter of context.

One of the applications of this project is to control a model/miniature railroad circuit. Sensors are classic (contact, ILS, optical...) but actuators need more consideration. The most important part is to move the switches, it must be electromechanical. HO trains can do it with an electromagnet but mine don't have enough force so a RC servo is the way to go.

So now the challenge is to send the right pulses to the right circuit. Moreover the servos have significant idle power requirements and they tend to oscillate around the requested position so their power supply must be turned on only when a change is requested.

Fortunately, I expect that only one switch moves at a time and the control circuit is significantly simplified.

A 74CHT138 selects the power supply to turn on : the negative-active signal can drive a P-MOSFET to power one servo out of 8 (or 16).

A 74HCT238 can then send a sw-controlled pulse to this servo. A simple loop in BASIC will be enough...

16 individual outputs require 4 bits of address. The power and pulse signals require 2 more bits. Let's say that address 0 is a null/void address, so 6 output bits are sufficient. This could be implemented with a 74HCT273 or a MC6821 with room to spare.

For example, extra bits can drive a 74HC259 or two (to control lights or other signals)


So I'm trying to do something more or less like that :

However the LEGO implementation is expensive... I don't have such motor, and i'd need many, and I'm far from rich. However I have some cheap servos. I just need to find how to adapt the lever to the servo and I'm mostly done.

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