Optical interface for 3,3V and 5V

[Cristal]

Hello all,

I want to build an simple interface for my CNC, only step and direction, but must be opto isolated and able to work with 3.3 V and 5V.

For the opto part I'm using the 6N137, but I do not know how to do the interface for the 3,3V (for use with the laptop) and the 5V (for use with the desktop.)

Do you have any suggestions?

thanks in advance for your help

[pminmo]

The 6n137 can be driven directly by 3.3V logic with a 220 ohm resistor in series with the emitter diode on a parallel port pins 2 through 9 of the db25, the control output pins are the issue (1,14,16,17) as in many computers the don't have much power to source current. In that instance you probably will have to provide an isolated supply voltage for a resistive pullup to 3.3V or 5V depending on the computer. Now, you say 3.3V, there is a lot of confusion about laptops. Actually many laptops aren't 3.3V logic on their parallel port when they are powered by AC power. And you will find many older destops, that only put out 3.3V in the HI state. The 1284 spec for parallel ports wether laptop or desktop, specifies output highs as 2.4V or higher into certain loads. If the laptop says its 1284, then it will work the same as most desktops. Laptop issues many times are incorrectly diagnosed as being a port problem, when in fact it's automatic CPU power saving changing timing.

[Cristal]

Thank you Phil,

I'm looking for some kind of CI which may be a driver for such problems and I do not know any CI with this characteristics.

What would be the best way to try solve this problem?

[pminmo]

maybe this will help: http://pminmo.com/4axisopto/4axisDIYopto.htm

[Mariss Freimanis]

My suggestion would be the attached thumbnail. This circuit works with any standard logic level (2.5V, 3.3V and 5V logic) and easily passes 0 to 500 kHz signals. Only 2.2mA of LED drive current is needed.

Mariss

[Cristal]

Thank you very much Phill,

this is more that I need, but in the schematics there is nothing saying if the circuit can work with laptops and Desktops.

[Cristal]

Mariss,

This is exactly what I need, simple circuit, If the circuit works, my problems are solved.

I will test with the 6N137 beacuse here is very dificult to have the HCPL).

Thank you very much for your attention.

[Mariss Freimanis]

Cristal

You cannot use the 6N137 in my circuit because it's not a photodiode/transistor device. The 6N137 uses an IC amplifier in its internal circuitry. You can use a 6N136 instead of the HCPL-2531.

Mariss

[Cristal]

Ok Mariss, thank you again for your tip.

The 6N136 is easy to find here.

[cmoher3]

Greetings. In my quest to find a circuit to isolate a PIC 3.3v output to the 5V input of an H-bridge driver (LMD18200) I stumbled upon this post. The circuit in the thumbnail looks close to what I need but I'm a bit confused by it. Can I not just use a couple of resistors and two suplly voltages to do this with a 6N137 ? Thanks.

[Mariss Freimanis]

The 6N137 is waaaaay too slooooow.:-)

You are sending fast signals to the motor drive. You cannot afford the 10 to 50 microseconds it takes the 6N137 to figure out what it should do.

Mariss

[cmoher3]

Sorry, I forgot to add that I'm running a PWM at a frequency of 40 khz. According to a book I'm using, Building Robot Drive Trains, the 6N137 has a rise time of 50ns and a fall time of 12 nS. I couldn't decipher the Fairchild data sheet
for these specs. My PWM period is 25 uS, so I thought I'd be safe. Can you elaborate on where/how it would be too slow ? Thanks for your time and effort in responding.