TTL Buffer

[kabilan]

Hi guys,

I am into parallel port programming. I want to use this to control the LED through VB6. From my research, I found out that before doing this I need to have a protection circuit to prevent any damage to the motherboard. I was told that an optocoupler was needed. However, optocouplers are a bit expensive if I had to have many outputs. So I was again told to make use of a TTL Buffer.

Now the problem is that I don't have any idea of what TTL to use. I found 7407 or 7417 but don't have the idea of what the 30V/15V output indicate. Moreover, the 74367 is not available on the stores.

Can you suggest what buffer should I use? Or, is there any better protection circuit other than the above mentioned?

Thank you for the ideas.  ;)

[ante]

Hi kabilan,

Welcome to our forum,

These buffers will not galvanic separate the computer from possible damage, only opto couplers or relays will do this.

[kabilan]

Hi Ante,

Thank you for the warm welcome, I appreciate it.  :)

Do you have any URL link to any simple relay circuit which I can take as reference? Assuming I made a LOW STATE input, will the relay react with it and produce a 0V-0.2V output? I am going to connect the relay's output to the HT640 encoder's inputs.

How about the ULN2803 is it safe to use it?  ???

[MP]

kabilan,
The ULN2803 will buffer the port just fine. Limiting the port pins with zener diodes to ground will also protect your parallel port pins. If you are only connecting to LEDs, you really do not have a worry, though. Just use the correct resistor to limit the current.

MP

[ante]

Hi kabilan, MP,

Well, it is pretty safe but to be absolutely sure there is only the galvanic separation which will allow 100% safety. On the other hand, if you know what you’re doing you don’t need it. Just double-check everything before you start things up and you’ll be fine. ;D