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[tobyker]

I want to use a servo (I've got some spare) for remote actuation of a model railway signal. I have a servo tester, but I don't need proportional control - just bang to the left, centre, bang to the right etc. Is there a simple wired way of doing this?

[Ian Campbell]

Hello

RC servos require a repeating electrical pulse of variable duration for control so they require some sort of control unit. Such as Arduino or an animatronics board to operate without RC equipment.

However, if you were prepared to rip the electronics out of the servo then maybe some combination of limit switches and control switches may be able to provide 2 position control. Not as such simple though. I think it would require 2 SPST micro switches for limits and a DPDT toggle or slide switch for control

Regards
Ian

[cos918]

have a look on MERG (model electroincs railway group) website . I think they may have some thing to control servos.

john

[essex2visuvesi]

We had some of these on my sons old model railway
http://www.heathcote-electronics.co.uk/servo.html

[malcolmfrary]

There are lots of circuits on th'tinterweb using a 555 timer or similar to act as a servo tester. 
It needs two of such, one to set the servo to "this way" and the other set for "that way".  Connect the servo signal lead to them via a really cheap slide switch so that it gets one signal or the other.  Or use two testers and switches.  I can't really think of a simpler way of doing it, or, given the price of point motors and their switchgear, a simpler way.  Two circuits for the entire layout, one cheap slide switch per point.

[Subculture]

Sounds to me like you have the majority of items necessary already.

The servo tester will normally have a potentiometer inside, which can be replaced with a simple on-off-on switch. You may also want to wire in a pair 2.5k resistors either side to replicate the pots resistance when the switch is centralized. If you want to limit the movement of the servo you can bias the resitance to one side or the other by using a pair of trim pots in place of the fixed resitors.
 

[malcolmfrary]

Sounds to me like you have the majority of items necessary already.

The servo tester will normally have a potentiometer inside, which can be replaced with a simple on-off-on switch. You may also want to wire in a pair 2.5k resistors either side to replicate the pots resistance when the switch is centralized. If you want to limit the movement of the servo you can bias the resitance to one side or the other by using a pair of trim pots in place of the fixed resitors.

That would be an awful lot of circuits for an "average" layout.  Just one pair of circuits to supply the two pulse widths wanted, one cheap switch per point to select which pulse supply.

[inertia]

Reading your original post, you are operating just the one signal. Do you really need three positions (i.e. including a centre point) or just up and down? Please forgive me but I know nothing about railway signals!
A simple 555-based circuit such as the ACTion P22, without the pot but with a couple of fixed resistors and a switch, would do the job. I built one once for someone and it wouldn't take the work of years to find out what values the resistors were (they weren't the same, for sure). Having the servo to hand - or at least knowing the type and model - would be handy.
Dave M

[tobyker]

malcolmfrary has rumbled me - I have several surplus servos and am looking to use them as point motors! The signals bit was a red herring - just in case we had a point motor agent on here. I might even see if the old Acoms servos still work, if I can find them.

[Subculture]

Well to be fair I looked at the application in the original post- one signal. If you want to actuate lots of items then a different approach is desirable.

Personally for lots of points I'd use a Picaxe chip or equivalent. That would enable you to do other things too