ngmedaboard
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As above mentioned,you can't do this.
it is not allowed to connect two different voltage level.
You should use a transistor or MOSFET to as level shifter.
Another thing is that you should take care not to power the 5V circuit, without powering the the 3V circuit. If you do that the 3V circuit will be supplied through the output back wards, and this may damage your microcontroller.
How did you work that out ???
I am amazed at the absurdities conjured-up by the readers of these forums.
For a start, the LED will have a characteristic drop of nearly 2v across it before ANY current flows and if a small current flows via the current-limiting resistor, it will flow through the protection diode (on the top and bottom of all inputs of a microcontroller).
If you read the PIC datasheet you'll see that the abs max i/o clamp current is +/-20mA. In my book the general picture also includes the specific cases, even if you get away with it, at times.
And there is no confusion of any issue here. What I try to advertise is a good design philosophy, even if the posters on the forum is not going to do this as a living.
Ok, I'm going to use NPN transistors to be safe. Play nice people.
Hi,
yes, the base resistor is a good hint.
* with a mosfet you don´t need a resistor. (but the problem is when port is floating - maybe during reset - the led state is not predictable)
* there are so called "digital transitors" with built in 1 or 2 resistors.
Klaus
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