[SOLVED] need help with input buffer/protection circuit

I need help. I have a board with a MCU that has an input coming from off the board from a number of possible sources so the incoming signal will vary. Here is what I am trying to accomplish:

1) The input signal will be a DC voltage ranging from 0V up to 15V.
2) Any signal above about 3V should 'turn on' a 3.3V signal going to the MCU AND turn on a LED
3) The MCU runs at 3.3V so the connection to the MCU itself cannot exceed that.

I thought that a simple circuit using a 2n7002 Mosfet with the drain tied to Vdd (3.3V) and the source tied to the MCU pin and the LED (with current limiting resistor) would work but the voltage on the source pin is only about 1.7V (which is the voltage drop across the mosfet) but it seems like even with a small current limiting resistor the LED does not have enough voltage to turn on.

Any ideas?

this could be quite simple
Here is what you need to do:

> Make an OpAmp comparator which has a reference of 3V on the Inverting I/P and VCC of 3.3V and Output of Comparator connected to the Microcontroller I/P
> Give your I/P voltage to the Non-inverting Terminal of an OPAMP Comparator
> Now whenever the I/P Voltage is less than you get a 0 Volts at the output of the comparator
> When the I/P voltage exceeds 3V the comparator Output goes to 3.3V

makes sense? no idea of what a comparator,opamp is?

yes - I know what a comparator is but will a comparator work with such a wide input signal range?

why not. Have you checked the data sheet of any opAmp

Actually the bulk of the comparators that I found do not support such a wide input range but I did find some. I have ordered some TL331 devices from TI so I will let you know if this works - thanks.

So I got a TL331 comparator and it works great....except that after seeing odd behavior with higher input voltages I dug into the datasheet more closely and see that this device also brackets your input voltage essentially between ground and Vdd. So I cannot use this solution because I want the acceptable input range to go up to at least 12V and I only have 5V and 3.3V available on my board. This is why I initially was looking at a solution using a mosfet because it allowed the input to go way beyond my Vdd value.

Any other ideas?

So here is what I did to ultimately solve this. The incoming signal is connected to the gate of a 2N7002 Mosfet. This gives me protection for incoming signals up to 12V (actually much higher than that) and it has a max threshold of 2.5V. I connect the drain to 5V and source to ground through a current limiting resistor. I also connect the source to the input of a comparator with push-pull output. I set the threshold of the comparator at about 2.8V. So the input is protected from higher voltages with the mosfet and the comparator gives me a nice clean 3.3V output that can drive an LED directly and is also fed into the MCU for signal detection.