RE: [CAD_CAM_EDM_DRO] Re: Proper pull up resistors for optical li mits

The resistor on the detector side is a "pull-up", not a current limiter. If
you don't have it, you may find that the circuit still works fine, because
the input pin on the Parallel Port may have a pull-up resistor built in.

And if you don't have an external resistor in series with the
emitter-side(LED), this may still work because some opto-sensors have this
resistor built in as well. If you DON'T have an external resistor, or a
built in one, then it will not work for very long before the LED burns up.

Andy Wander
Verrex Corporation


-----Original Message-----
From: Harvey White [mailto:madyn@...]
Sent: Friday, May 14, 2004 2:33 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Re: Proper pull up resistors for
optical limits

On Fri, 14 May 2004 17:12:45 -0000, you wrote:

>--- In CAD_CAM_EDM_DRO@yahoogroups.com, "fun2b432002" <fpaul28@h...>
>wrote:
>> Wondering if anyone could give me advice on the correct pull up
>> resistor for my optical limit/home switches. I'm placing slotted
>> optical switches on the + side of my X,Y,Z axis to use as limit and
>> home. My servos are WorldServo SST1500s and the manual suggests
>> using optical switches on the + axis side. There is a diagram in
>> the manual that shows connecting them to +5V (which is supplied by
>> the servo through the limit switch plug). They show a resistor
>> between the +5V and the optical interrupter switch but do not give
>a
>> value on the resistor. I assume this is a pullup resistor (I know
>> very little about this) and I'm wondering if anyone can help me
>with
>> determining which size resistor to use???? The optical switches
>are
>> transistor output type, Min Output 0.5 IC @ 20 mA IF. Another type
>> I may use are the same except Min Output 1.8 IC @ 20 mA IF.
>>
>> Thanks,
>>
>> Frank
>
>I still have my test setup of those laying in front of me.
>I had a hard time getting this to work because the first
>time I tryed to use it I could not get anything to work,
>found out much later that the emitter did not work on my
>optical interrupter switch.
>That was 1 out of 4 and I sure picked the bad one to start with.
>The way I found out was when holding it at the right angle
>to a external light source it started to show a current.
>The emittler is sort of like a Flashlight shining onto
>the detector, you want to prevent a external light from
>shinning directly onto the detector when choosing a final
>mounting position this could cause trouble.
>
>My test setup is wired as follows.
>
>(+) +5volt power (D) ground
>(E) ground (+) Pin 11 on LPT port
>
>This worked fine as is.
>The diagrams I have show a 4.7kohm resistor in line
>with both (+) lines.
>seams to work without these to I think they are just
>for protection.

No, you have to have the resistors. They limit the current through
the emitter diode and the pull down transistor's collector current
from the sensor.

Exceed the current ratings on the diode and it glows very brightly for
a very short time. Exceed the current rating on the sensor, and you
either get no guaranteed logic level low (so you get no response), or
you end up burning out the sensor transistor.

Harvey

>Good Luck
>
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