cheap servo control gates or points

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adamc
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cheap servo control gates or points

Postby adamc » Fri Jul 25, 2014 2:55 am

in advance of some loco gates, I have been looking at using an arduino and motor driver to do some PWM loco driving and point operation. But for now at least, a simple circuit to move a servo from one position to another. It uses a 555 and a few components and a switch and think its about a 1/3rd of a price of a tortoise, maybe a lot less as the components are about £3 the biggest cost is the actual switch

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its basically a 555 with an upper and lower limit resistor to dial in the limits of travel. I also use the switch to turn on and off the power so the servo isn't running.


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Postby adamc » Fri Jul 25, 2014 3:08 am

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Postby Boghopper » Fri Jul 25, 2014 6:42 am

Ingenious! Wish I had you electronic savvy. :-(
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Postby scratchbuilt » Fri Jul 25, 2014 10:49 am

Is there a circuit diagram available please..?

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Postby adamc » Sat Jul 26, 2014 1:56 am

i will draw it up, its a basic 555 servo driver with two presets. the switch turns on the power and selects the resistor for each range

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Postby adamc » Sat Jul 26, 2014 2:05 am

i am also working on an arduino controller speed and points unit.

this one has the addition of a motor driver for £4 . it has a slider for the speed and works the locos on the test track really well and it is PWM . The arduino controls 4 points and i'm working out if it can also switch the loco shed doors open as well. Really cheap, combines speed of tortoise and PWM control all in one for the price of four £4 mini servos, the arduino and the switches

The points are driven by mini servo with a nice slow speed compared to the 555 before. All running from a reclaimed 12V wifi router plug/PSU combo and a simple 78010 voltage regulator. Slow speed is pretty good, and working on the code to give them a 50% boost to get them running before setting back to slow speed.

Will sort out some video

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Postby Thorness » Sat Jul 26, 2014 7:30 am

adamc wrote:i am also working on an arduino controller speed and points unit.

this one has the addition of a motor driver for £4 . it has a slider for the speed and works the locos on the test track really well and it is PWM . The arduino controls 4 points and i'm working out if it can also switch the loco shed doors open as well. Really cheap, combines speed of tortoise and PWM control all in one for the price of four £4 mini servos, the arduino and the switches

The points are driven by mini servo with a nice slow speed compared to the 555 before. All running from a reclaimed 12V wifi router plug/PSU combo and a simple 78010 voltage regulator. Slow speed is pretty good, and working on the code to give them a 50% boost to get them running before setting back to slow speed.




Pardon me for my ignorance of electronics but this is like a foreign language to me!

Arduino? 78010 voltage regulator?
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Postby adamc » Sat Jul 26, 2014 12:54 pm

:) Ah, I will prepare a better report and write it up. Arduino is bascially a little standalone computer, about £25 that you can program to read switches and turn LED's, Servos and motors on and off etc. I am using a motor driver board powered at 10V , hence the 7810 voltage regulator and eventually drive servos for point control. Hoping it will turn out cheaper and more adjustable than buying slow point controllers and a seperate speed controller.

I'm thinking it will cost me a total of about £75, plus it can also handle automated running as well I believe.

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Postby adamc » Mon Jul 28, 2014 1:35 am

advanced a bit on the controller this weekend. I've not got a direction switch and LED and also a drive LED. Plus control of two points.

Put one to the test on a small board, and its not a bad start, but think I need some more space below to try and work out a nicer way to throw the points.

Speed contol works really well on a BEC, but ive got a tenshodo spud that on normal DC ran well, then after some running in 30mins each way, seems to have slowed down. Both are a lot smoother, but the tenshodo doesnt seem to respond as well to volts, now preferring a higher voltage. I'd say the magnets have heated and de-magnetised. But the BEC has dual worm drives and seems sturdier. Also won some bachman bogies this week, so should see how they come on.

This looks a bit messy with the jumper cables, but basically hardly any components other than the driver module and a power regulator. Runs a little hot, so will probably have to heat sink it.

Image

the points work OK, but its very simple and would prefer to try a linkage like the tortoise, which seems to be a long type lever as this is just a short throw version. It works, but might have to tweak each servo.

Image

Image

http://youtu.be/mW40pdHKbVQ

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Postby adamc » Mon Jul 28, 2014 2:43 am

slow speed running BEC with PWM https://www.youtube.com/watch?v=EWDIx-j6wJI

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Postby adamc » Wed Jul 30, 2014 1:55 am

On the hunt for servos and these seem the cheapest by far http://www.giantshark.co.uk/product/171 ... ibre-gears , mad price of £2.50

also some cheap switches, http://www.rapidonline.com/electronic-c ... ch-75-0084 at just 54p !

still researching the best way to drive the points from underneath with an easy support/frame and rod.

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Postby tebee » Wed Jul 30, 2014 2:11 pm

I've been experimenting using the dirt cheap servo testers to control servos operating points.

The rotary control as oppose to a switch takes getting used to, but it does very precise control and you can vary the blade pressure.

They are very cheap 99p direct from China http://www.ebay.co.uk/itm/ESC-Servo-tes ... 4cf8891ad8

Plastic case is useless, am trying to work out how to panel mount them.

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Postby adamc » Thu Jul 31, 2014 3:22 am

that is incredibly cheap !

I'm currently adding a rotary encoder to the above so as to be able to adjust/program each throw and store the result. The aim is then to be able to "tweak" each type of servo used.

Also turning off the servos after each throw so that they are not constantly on, but still the trick to discover is a reliable and easy way to mount them underneath without having to resort to a custom mount.

I've seen some options with a spring to pull them in each direction rather than pull them and will be trying that. Allows a much larger leeway in the servo throw. Also it means you don't have to be precise in lining up the servo horns etc. as the spring takes out all of the slack.

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Postby adamc » Fri Aug 01, 2014 1:51 am

One of the other aims is to create a welders flicker, I've got one of those modules, but the problem it seems with most of them is that they are on all time. I know from experience that the wire and gas would soon run out. So I've recreated one with a bit of code that will hopefully only come on every few minutes or so for about 10-15 seconds, so far more realistic and only the cost is to add to existing micro of a LED and a resistor, about £1 although I might a blue one as well. Looks pretty realistic so far

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Postby adamc » Sat Aug 09, 2014 4:29 am

now we have speed control, 4 slow points, adjustable point programming, random arc welding as well, and a direction and power LED

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Postby Nevadablue » Sun Aug 10, 2014 12:00 am

This is very interesting. I'm afraid it may be over my head, I'm rather of an electronic-dunce. I love the welding arc light. I have plans for a draw bridge, it would be neat if the approaching train could trigger the lowering function or stop automatically if the bridge were up.
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Postby chris69 » Sun Aug 10, 2014 2:55 am

Here is a question from an other ANALOG guy.
Is it possible to use and control a servo without the electronics?
For example a power source and a push button?
Thanks guys and a happy week end,
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Postby adamc » Sun Aug 10, 2014 4:17 am

its easily possible to control a servo with an arduino to lift and lower a drawbridge and detect trains. Once I've got the whole thing sorted, i will publish the code and circuit to do either the entire lot, or just to control servos. I'm writing it at the moment to start off at a default setting, then with a simple switch you can program the upper and lower points. Its one keypress at a time , so you might have to press it 50 times , but once programmed then thats it. Just a single switch after to move the servo to each end of the desired travel.

As to driving servos even easier, I am afraid you still need a rudimentary circuit. They basically work by sending them a series of pulses that the servo translates into the desired position. Like send a pulse width of 10 that sends the servo to one end, a pulse of 180 to send to the other end. I have made a circuit that uses a 555 chip to control a servo and it works out at under a fiver, plus the servo , but would require some soldering skills. I will revisit that and see what can be done by having a board made as we used to make these kind of things when I did robotwars. Perhaps there could be something there to make a simple servo controller that people might like to use for just moving a servo between 2 positions.

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Postby chris69 » Sun Aug 10, 2014 4:35 am

Thanks,
All I want to do with the servo is to control a dumping mechanism for skips.
Just Up and Down to dump them.
best
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Postby Nevadablue » Sun Aug 10, 2014 5:11 pm

I've been reading about the little (cheap too) servo 'testing units' that seem to work fine for 'manually' running servos. I'll look up the link...



Thanks Adam for the help. I'll be watching this unfold for sure.

http://www.hobbyking.com/hobbyking/store/__8296__Turnigy_Servo_Tester.html

Image

That one isn't fancy, but it is said to work fine and it only costs six bucks.
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Postby Larry » Sun Aug 10, 2014 6:22 pm

chris69 wrote:Thanks,
All I want to do with the servo is to control a dumping mechanism for skips.
Just Up and Down to dump them.
best
Chris


There are many simple circuits that will do that. Here's one that uses what is the old warhorse of integrated circuit chips, the 555 timer.
Image

Ten other parts. all cheap, half of them resistors. Almost any NPN general purpose switching transistor will work in the circuit.

One button will move the servo clockwise, the other counterclockwise.

Even simpler would be to just hook up a dpdt momentary contact switch directly to the motor leads in the servo, bypassing the servo electronics. If you keep the voltage 5 v or below, it wouldn't burn out a servo motor.
Last edited by Larry on Sun Aug 10, 2014 9:04 pm, edited 1 time in total.

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Postby chris69 » Sun Aug 10, 2014 7:18 pm

Thanks,
I LOVE this solution the best:

Even simpler would be to just hook up a dpdt momentary contact switch directly to the motor leads in the servo, bypassing the servo electronics. If you keep the voltage 5 v or below, it wouldn't burn out a servo motor.

I will buy a cheap servo and start experimenting. Any good idea for a low voltage power source?
Thanks again!

Happy modelling
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Postby adamc » Sun Aug 10, 2014 7:19 pm

That's the very circuit in fact, just replace the fixed resistors next to the push button with 100k trim pots so that you can adjust the end points. I also used a center off DPDT momentary switch. So its On-Off-On.
I also used the other set of contacts on the switch to run the power through as well, so that it only powers the servo when needed and isnt on all the time as it can tend to buzz/hunt.

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Postby Larry » Sun Aug 10, 2014 8:24 pm

chris69 wrote:Thanks,
I LOVE this solution the best:

Even simpler would be to just hook up a dpdt momentary contact switch directly to the motor leads in the servo, bypassing the servo electronics. If you keep the voltage 5 v or below, it wouldn't burn out a servo motor.

I will buy a cheap servo and start experimenting. Any good idea for a low voltage power source?
Thanks again!

Happy modelling
Chris


Everyone has a bin of old wall warts. Now you have a use for them. Look for one that outputs 3-5 v DC and you should be fine. In fact, the lower voltage risks overheating the servo motor less.

A USB phone charger that outputs 500milliamps would be fine. USB uses 5v as the standard.


You can step down a slightly higher voltage ( like the 12v tap on your train transformer) with two resistors.

http://tuxgraphics.org/toolbox/circuit- ... lator.html

Use maybe a 330 ohm resistor (1/2 or 1 watt)for R2 and the calculator will give you the voltage for R1 depending on the output of your wall wart. In the 12v case, R1 should be 470 ohms or higher to get under 5v. The circuit will always be drawing a little current, however. The resistors will get warm.

A slightly better circuit uses a 7805 voltage regulator. Every electronics store has them. They will output 5v and scrub the rest of the wattage as heat when under load.

http://www.sminntech.com/techai.html

It won't draw much when you aren't using the servo. It might, however, need a heat sink if you are using the servo a lot. Most have a circuit in them that shuts them down if they get too warm.


I should have elaborated on the momentary contact switch. Motors use the most current and heat up the most when they are stalled. When you aren't actually performing the function you want, the motor should be off.

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Postby Larry » Sun Aug 10, 2014 9:14 pm

adamc wrote:That's the very circuit in fact, just replace the fixed resistors next to the push button with 100k trim pots so that you can adjust the end points. I also used a center off DPDT momentary switch. So its On-Off-On.
I also used the other set of contacts on the switch to run the power through as well, so that it only powers the servo when needed and isnt on all the time as it can tend to buzz/hunt.


Yeah, here's that circuit from the same site. It's what is in most of the servo testers you see. Very simple. Be aware that the leads in the schematics are NOT in the same order as the wires on most servos.

Image

The buzzing/ hunting might be from other causes, such as a poorly filtered power supply or too much load on the servo. Try adding caps across the power leads (.1 or .01 microfarad) as close to all the IC's and servo motors as possible.
If you look at the 555 IC Datasheet, or the 7805 Datasheet, you will see they recommend a whole range of filtering capacitors. You really can't use too many.


You might also try reducing the value of the 3M3 resistor. It controls how often the servo gets a control signal.


Worst case, you need to separate the power that feeds the IC from the power that drives the Servo (keeping a common ground). That's really best practice.
But I have run several servos off an IC with just good filtering and no jitter.
Last edited by Larry on Wed Aug 13, 2014 5:08 pm, edited 1 time in total.


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