servos for turnouts

[gregh]

Back in GSM days there was a topic about using model aircraft servo motors to operate turnouts.
I posted my experiences at the time. I have recently installed a servo on a 2nd turnout, so thought I’d sort of repeat the previous.

I built a small styrene box to cover the servo. The usual circular actuator was used on the servo and a ‘handle’ added to allow local manual operation. A small micro switch was placed in another small styrene box to provide indication of the direction the turnout is set.

This pic shows the covers removed looking down.

To generate the pulses needed to operate the servo, I used a Picaxe 08 programmable controller. This has a command called SERVO which makes it very easy to use. The program I wrote just generates about ½ sec of pulses to move the servo to one end of its travel, on one output; and then ½ sec of pulses to move it to the other end, on the 2nd output.
A DPDT, spring-loaded, centre-off switch is used to:
1. Switch the 6V positive to the servo and
2. switch the required pulse train.
The servo therefore normally has no power applied so can’t chatter.
This diagram may help to explain it.

The control switch and electronics are mounted in a 15 cm square, styrene “gangers’ hut”. The roof flips up to allow access to the switches.

I use red LEDs to indicate direction.

 

[Bram]

Ooh, what a clever bottom, I am useless at the electronic side of things but I love to see other peoples solutions for various situations. Well done you

 

[Wobbleboxer]

Being a cheapskate I have been wondering whether there was a way of motorising points cheaper than the LGB point motor and supplementary switch method. And then I saw this thread. Great alternative, using servos, but I was hoping to do something smaller. BTW, are the servos weatherproof in their little huts?

The problems is I can do diodes and switches, beyond that I get all confused.

I understand the LGB switch uses a half turn motor. Are these motors or similar available to buy? How is the power turned off when the motor hits the end of the turn?

Sorry to hijack the thread.

 

[minimans]

LGB point motors arn't really motors at all they are more of a solonoid operated with DC current, switching the polarity to change direction of throw, LGB used half wave A/C current to power through there own switch box's which provided the diodes nessasary to give the half wave current.

 

[Wobbleboxer]

LGB point motors arn't really motors at all they are more of a solonoid operated with DC current, switching the polarity to change direction of throw...

Which probably means that something similar is not going to be generally available to the public.


Back to the drawing board.

 

[400Parker]

Colin,
Wouldn't a Peco "00" point motor (i.e. solenoid) do the job just as well? (And cheaper too)
Steve

 

[ntpntpntp]

I understand the LGB switch uses a half turn motor. Are these motors or similar available to buy?

There are other brands of point motor that use a motor drive rather than a solenoid. The cheapest around in the UK at the moment is probably the point motor by Conrad http://www.conrad-uk.com/ < Link To www.conrad-uk.com, search for item number 219998 - 89. Conrad are like a German version of Maplin. I see they're currently selling these at only £3.95 each: I paid around £5 each for a bunch of them a year or more ago, which I plan to use indoors with Peco G45 points. They use a bog-standard small motor with a rack & pinion and they work well, though not particularly weather-proof and the inbuilt polarity switch is implemented as thin wiper contacts that probably won't stand a lot of abuse.

I've seen a very similar design of motor marketed by Hoffman for 3 times the price (my club used some and were surprised when I showed them the Conrad item). Other motor-drive brands that I know of include Fulgurex and Lemarco (both expensive). Then you also have the stall-motor types such as the Tortoise.

To be honest, for outdoor use I reckon the LGB or Piko motors are a safer bet (and I think I've seen another brand of G scale weatherproof motor from one of the european track suppliers, but I forget who?)

 

[Wobbleboxer]

I don't think the Peco ones are weatherproof. I'll have a look at the Conrad site in the meantime.

I just fancy trying to do it differently and if I can save a few bob, all the better.

Sorry again Greg for hijacking your post. Great idea btw.

 

[Woderwick]

Well my LGB drives have lil electric motors in them driving a cog which drives a rack. Only 2 inputs whch have to be reversed.

 

[Gizzy]

I understand the LGB switch uses a half turn motor. Are these motors or similar available to buy? How is the power turned off when the motor hits the end of the turn?

Because you use a switch with a spring loaded action which you flick and release, or a push to make and break switch, this is enough to turn the motor just until it hits the end stop.

If you held the switch on, this would keep the motor moving and could therefore potentially damage it.


The LGB (or Piko) box and switches are quite expensive, so I personally use these panel switches, which come with the diodes and a wiring diagram;

http://www.gaugemaster.com/item_details.asp?code=GM511&style=main&strType=g

For comparison, this is the Piko switch box;

http://www.gaugemaster.com/item_details.asp?code=PK35260&style=main&strType=g&Mcode=Piko 35260

And here is the LGB switch box;

http://www.gaugemaster.com/item_details.asp?code=LGB51750&style=main&strType=g&Mcode=LGB 51750

 

[Gizzy]

LGB point motors arn't really motors at all they are more of a solonoid operated with DC current, switching the polarity to change direction of throw, LGB used half wave A/C current to power through there own switch box's which provided the diodes nessasary to give the half wave current.

I have to correct you there Minimate?

Solenoids are AC devices, as they use 2 coils to move a former to and fro. This former is attached to the point tiebar. You only sustain a magnetic field in a coil by using a changing current, hence the use of AC voltage.

The LGB point motor does use an electric motor turning 180 degrees or a half turn. The motor is fitted with a pinion which meshes with a rack that attaches to a tie bar. The motor needs to change direction to change the point and therefore has to be a DC motor. The power supply is AC, but the current is rectified to DC via diodes. Thus by using a 2 way centre off spring loaded switch, you can supply either positive or negative DC current to your motor which changes the direction accordingly....

 

[Wobbleboxer]

I use a TE controller so was planning to use the points control accessory with it. More expense, hence trying to keep the cost of the motors down. I also need to switch track current so would need to use the LGB accessory switch on top of the motor making each point about a £40 outlay.

The Conrad motors look excellent and only £3.95 each. Shame they are indoors only. Do you think they'd be OK if I built trackside huts like gregh has for his servos?

 

[adeshers]

The Conrad motors look excellent and only £3.95 each. Shame they are indoors only. Do you think they'd be OK if I built trackside huts like gregh has for his servos?

If it was well ventilated, as condensation can be a killer.

 

[ntpntpntp]

Solenoids are AC devices, as they use 2 coils to move a former to and fro. This former is attached to the point tiebar. You only sustain a magnetic field in a coil by using a changing current, hence the use of AC voltage.

er... how come then that many railway modellers (myself included) can quite happily operate our "solenoid" Peco point motors using a CDU (Capacitor Discharge Unit), which is most definitely a DC output? I've built well over 50 CDU's ('cos I wanted individual CDU's for each point on my N gauge layout in case I got into computer control long before I got into DCC). What about DC powered relays, and DC electromagnets etc? Same principle.

My vague memory of schoolboy physics says that you need a changing magnetic field to generate a current in a coil (which is how a generator or a transformer works), but not so sure about the need for a changing current to sustain a magnetic field? A simple DC electromagnet (a coil and a battery) would seem to indicate that a steady current is fine. As long as there's current flow in the wire you get a magnetic field.

Severe thread drift - sorry! Love the way those servos have been cased to look like "real" point machines.

 

[Gizzy]

Solenoids are AC devices, as they use 2 coils to move a former to and fro. This former is attached to the point tiebar. You only sustain a magnetic field in a coil by using a changing current, hence the use of AC voltage.

er... how come then that many railway modellers (myself included) can quite happily operate our "solenoid" Peco point motors using a CDU (Capacitor Discharge Unit), which is most definitely a DC output? I've built well over 50 CDU's ('cos I wanted individual CDU's for each point on my N gauge layout in case I got into computer control long before I got into DCC). What about DC powered relays, and DC electromagnets etc? Same principle.

My vague memory of schoolboy physics says that you need a changing magnetic field to generate a current in a coil (which is how a generator or a transformer works), but not so sure about the need for a changing current to sustain a magnetic field? A simple DC electromagnet (a coil and a battery) would seem to indicate that a steady current is fine. As long as there's current flow in the wire you get a magnetic field.

Nick, you are of course correct and I am wrong.

What I should have said was that you cannot 'sustain' a magnetic field with DC as it would only be momentary. This would be enough to move a solenoid for a point tie bar. I was getting a little confused for a moment between solenoids and transformers. Tranformers do need a changing current, i.e. AC, to work.

To clarify then, yes solenoids will work with DC, although AC is better. In a model railway application to move a point, a DC solenoid will do the job, with a momentary switch action. However, an AC power supply is used as it can be rectified and the direction of the DC current can be changed to move the point tie bar.

LGB point motors use a half turn motor, which is DC current.

Again, an AC power unit is required and the current rectified to DC via diodes, in order to move the point in the direction required.

Sorry for the confusion peeps!

 

[Neil Robinson]

Sorry for the confusion peeps!

I'll add to the potential confusion.
Early (pre "EPL") LGB point motors had three terminals, so I assume they were twin solenoid types.
LGB EPL point motors don't necessarily need an AC power unit as they may be operated by reversible polarity DC as well as half wave rectified AC.

 

[simon@mgr]

I beg to differ on the classification of a solenoid.
A solenoid is a coil with a core, moveable or fixed, these can be DC or AC.
Most relays are DC operated and will hold continuous application of voltage, the magnetic field exists for as long as power is applied.
In fact when power is removed the is a reverse voltage generated in the order of several hundred volts in the coil, which is why snubbing diodes are fitted across the coil.

 

[corgi]

Railcon in Australia make and market a servo driver for model railways. They come in two versions, analogue can be operated by a switch and DCC operated from an address.

 

[Steveford666]

I don't know if they still make them, but Maplin used to sell a passing contact switch. With the aid of a vairable potentiometer and a transistor you had the pefect system for poiint operations.

 

[Glengrant]

I read all the foregoing with interest but, at the end of the day, whatever you use has to be protected from the ravages of the weather, and I'm afraid that the only one I can see that will do that is the LGB, and even then it needs attention. I was really keen on the Lemarco, so prototypical in its operation, but it didn't last one winter. If anyone disagrees with me then let's have your views, I'd be very interested