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gjestico

Optical servo extension

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Hello,

I would like to use a LED emitter and phototranstor to transmit the RX output signal over a certain distance, To operate a servo .

My limited electronics knowledge tells me that I should be able to make a simple circuit to transform the RX Pulse out put to a drive current to run the emitter, then a simple circuit at the other end to drive the servo. Servo end will require its own power supply obviously

First problem:

I have tried using a 2n2222 transistor , connecting the RX signal wire directly into the transistor base pin through a 1k resistor, then connecting the LED with a resistor to the transistors other pins.

No luck getting an output from the LED. This is a Futaba PCM RX so it has a known 3.3v pulse high.

Any suggestions for an electronic mental midget ?

Thanks

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I have tried using a 2n2222 transistor , connecting the RX signal wire directly into the transistor base pin through a 1k resistor, then connecting the LED with a resistor to the transistors other pins.

Those components would be fine. Post your schematic and someone will advise on what is wrong.

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Basically like this :

Note: I am using a PN2222 transistor, if that makes a diff. I had it lying around...

Edit : revised schematic in post down the page.

Edited by gjestico

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The IR source side is fine. You can't see the IR emitter in action. If you sub a standard LED, you should see it light up very dimly.

The IR detector will have more effective sensitivity if you use a grounded emitter arrangement (like the IR emitter circuit). An additional transistor stage will be needed to provide signal inversion.

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Troubleshooting, Im using a camcorder to view the IR output of the led. If i connect the Transistor base dreictly to V+ the LED turns on and I can see it on the camcorder screen. it wont turn on at all when run by the RX output.

Is this a good way to test things ?

Thanks for the help

Greg

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If i connect the Transistor base dreictly to V+ the LED turns on

Dose it turn on if you connect it to V+ with the resistor and without ?

Terry

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The Rx's servo signal duty cycle is only 5%-10% (~1.5mS on, 20mS off). It will probably be hard to see the IR using the camera trick.

If the IR emitter's switching transistor is wired correctly then your IR driver LED will work. However, are you absolutely sure that the transistor's collector and emitter leads are not swapped? Doing that would give some functionality, but at very low LED currents.

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Hi, Mr Cam

I've already used Totx and Torx 173 ( fiber optics modules from Toshiba ... classic use is ... HiFi ( easy to find !!! ) ) to drive some servos in difficult environments ...

Fiber w/connectors can easily be found in HiFi stores ... or fiber can be placed into the original covers of the modules ( just a 2.1 mm hole in the plastic ...)

Works great with plastic fiber, and if the "certain distance" :rolleyes: is up to 10 feet, it will be MUCH MUCH easier to recover pretty signals ... and no ambient light troubles.

Application was 4000 Hz PWM Twin RC controller for model boats ...

Alain

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Alain, where do you get your modules ?

Terry

Hi,Terry

I Bought them from www.CONRAD.fr ref 0181 404 and 0181 480 ...

http://www.conrad.fr/webapp/wcs/stores/ser...hLightId=213215

:P but there are departments for each country.

Note Bandwidth goes down to ... DC !!!

There are also Toslink Tx/Rx @ 12Mbps here:

http://www.selectronic.fr/article.asp?arti...ntier=70.4189-1

http://www.selectronic.fr/article.asp?arti...ntier=70.4189-2

verify the data given for DC use .

just on the other side of the Channel ...

Also have a look to Toshiba's page at Farnell's ... other good references !!!

Alain

Edited by Acetronics

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Ok I found some issues. The current through the emiiter LED was only 3ma. Must have got the math wrong its now ~20ma with a 150 ohm resistor.

It works now, but only when the detector is within a 1/4" of the emiiter. I need more range than that, I reworked the detector circuit as RC-Cam advised, i believe.

I got ahold of a scope, a problem seems to be the photransistor never seems to go fully "off" this leads to a pulse that never hits 0V. lowest it gets is 1V. So that seems to affect the range of the setup. Any hints on what to do on the recieve end to fix this ?

Revised schematic here. Please excuse the crappy scan of a hand drawing ! Please excuse the "smudges" where I erased stuff !

post-6-1166157546_thumb.jpg

Edited by gjestico

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Getting a 0 VDC level will be tough, due to the PN2222. However, you should be able to get a bit lower than 1.0VDC for the logic low, at least enough to satisfy a servo. Try changing the three 1K's on the IR Rx circuit to 10K's.

If it helps, but you need higher servo signal drive currents, then a CD4000 series buffer could be used on the Rx end to beef things up. Otherwise, if you cannot obtain good results then you should consider a different IR pair or reconfigure the IR detect end using a comparator IC instead of the transistor inverter.

Don't forget to do all your bench tests with the overhead lights turned off. Otherwise the naked IR detector may be biased by them.

EDIT: BTW, what exactly are you trying to accomplish? If this is just an optoisolated servo cable, then the popular 4-pin DIP opto IC's are much easier to use. If instead you are trying to control a servo remotely via IR, say over several inches, then you really need to employ a modulated carrier IR solution. There are many sample circuits on the net, and convenient IR modules are offered that do all the carrier modulation/demodulation dirty work for you.

Edited by Mr.RC-Cam

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I need to send the servo signal over about a 1" gap. cannot have any physical connection between the 2. I need to keep it as simple as possible, it's already at my electronic limits of knowledge !

Its for the 360' revolving mount I have fabricated to hang under my heli. In order for the mount to be truly unilimited there can be no wires between the heli and the mount. The circuit will send the cam trigg signal through the center of the pivot point. think 360 deg pan photo.

Edited by gjestico

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That sounds reasonable. The only hardship is having to shield the optics from environmental light sources.

There is another less sophisticated way to do this. The slip-ring mechanics from a telephone handset anti-tangle cord accessory could be used too. As long as you don't pass heavy currents through them they will offer low noise signal paths. There are four slip ring conductors, so you could double up the gnd and signal line to further improve the reliability. This assumes a local battery is on the 360 mechanics.

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unfortunatly , as it is built there is no room for slip rings. When I was making the pivot assy I planned from the start to use the IR type if setup. It is totally enlosed from outside light. I figgered i would come up with the electronics later, thats where I'm at now. Good idea though liberating the slip rings from a phone cord.

I got a different IR emitter led from the elec store, seems to work better, running it at 30ma getting 2 inches range in open air. Should be better when enclosed right ? This is almost good enough.

Greg

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Should be better when enclosed right ?

It all depends on the content of the ambient light seen by it during your tests. But for sure, it won't be any worse once it is shielded from the external light sources.

Glad to hear you are making progress.

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Hi,

There's something you could use to keep Led and detector protected from ambiant light : two tubes turning one into the other ... may be some heat-shrinkable between them to avoid metal-metal glitches.

the led fixed to one tube and the detector fixed to the other ...

Alain

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