Quote Originally Posted by simmo993 View Post
when using this optocoupler: "If you are using the HCPL-2231 just do not insert R18 and R25 as this opto comes with internal schmitt-trigger and
push-pull-drivers and is already TTL compatible."
If he is right then it works like this:

Many opto couplers work just like a hand switch.
A switch does not give any voltage or current.

Imagine a hand operated switch between pin 7 and pin 5 of IC11 opto3.

If the switch is open, then there is no current from +5 volt through R18, through the switch to GND.
As a result the voltage on pin 7 will be +5 volt.

If the switch is closed, then there flows a current from +5 volt through R18, through the switch to GND.
As a result the voltage on pin 7 will be 0 volt.

The voltage on pin 7 comes from the +5 volt via R18.
Not from pin 7 itself.
R18 is in this case called a 'pull up' resistor.

This is how many opto couplers work.

With the HCPL2231 opto coupler it works different.
It does not work like a switch.

The pin 7 of the HCPL223 works as a small voltage supply.

If the HCPL2231 gets a ON signal, it will create its own +5 volt signal on pin 7.
Then there is no need for a R18 pull up resistor to create a +5 volt.
If the HCPL2231 gets a OFF signal, it will create a 0 volt signal on pin 7.

With a HCPL2231 you do not need the R18 pull up resistor.
And R25 is also not needed, it does the same for the other channel of the HCPL2231.

That is, if he is right.
I have looked at the HCPL2231 datasheet, with my limited knowledge i can not see if the pin 7 indeed works like a small voltage supply.
Hopefully someone with more knowledge can tell us :-)

On the other hand, leaving the R18 and R25 in is no problem with the HCPL2231.
It probably works fine.
Can someone "officially" confirm this ?

http://www.datasheetcatalog.com/data...CPL-2231.shtml

Vroemm.