Thread Starter


Joined Jul 10, 2008
i am doing my final year project. i use 4N26 as elctrical isolation. the zener diode 5.1V is used to protect microcontoller. my microcontroller max input volgate is 5.1V. let say the zener diode is ideal and i connect my circuit as shown. i choose Vce as output (7 to 14V ), but i not sure my zener diode is correctly coneected or not.. anybody can help me? thnak you very much.



Joined Oct 15, 2008
Your Zener is upside down in the schematic. Also you will need a load resistor from the collector of the photo transistor to the 7-14v supply. If you choose the correct value, then the Zener will be redundant as the voltage will never exceed 5v at pin 13.


Joined Dec 27, 2007
Yeah, a lot wrong with your circuit.

As mentioned, use a resistor from the collector of the transistor to your 5V supply. No need for input protection, since it is an open-collector output. The voltage will only be as high as your voltage applied to the pullup. I don't see a reason why you would use 7-14V..

Also, what's up with the 1.5K resistor? What voltage are you applying? Maybe if you're applying 50V or something, but if it is a 5V input, then the resistor should be around 130 ohms. I don't understand the 10M resistor, what is that for?