Changing polarity of a wire

Hi My Dear Friends,

I am designing a telephone interface. I have used an npn optocoupler in my ring detector circuit. I have connected telephone's red and green wires to pin 1 and 2 of optocoupler through some elements such as capacitors, resistors and diodes. Then I have connected emitter pin of optocoupler to ground of +5V power supply and I am using collector pin as the output. Now when telephone line starts ringing the output wire (connected to collector) continuously raises to -2.7V for 1 second and drops for 4 seconds. Now I wish to connect this to a micro-controller wgich needs to receive V+ to detect it as logical 1 and the output wire has V-. As I know from +2.5V to +5V range is detected as logical 1 but not -2.7V. Right now what I need is to change polarity the output from negative to positive in orde to get +2.7V which can be detected as logical 1 by the micro-controller. Now what I want you to tell me is how to change polarity of a wire. Kindly please hassle to help me as I need to complete this project untill tuesday.

Thanks in advace.

i think u are having doubt abt ring bdetector circuit also...
i have diven u some suggestion
u havent ackowledge yet on that...
have you completed that in proper fashion....
Can u mail me ur latest circuit that you have made...
i have worked on that...
i can suggest u the changes if u mail me that circuit....

As you have used the MCT2E or similar, the emiter Pin should naturally be connected to the GND and take output from the collector OR the collector may be connected to the +vcc and output may be taken from the emitter. Anyways posting your schematic will fetch better replies.
Cheers

The easyist way would to have the micro detect an active low, makes no real difference in programming, and is the conventional way to do things. You could then connect a resistor from collector to 5v logic, and then take the transistor collector to the micro pin. Normally the input pin is held high by the resistor and then gets pulled low when the transistor turns on. If you reaaly need active high inputs, then you will have to invert the signal, pnp transistor etc.

I do not understand where the -2.7V comes from. I wonder if it's a photoelectric effect in the opto isolator transistor because it has no pull-up resistor. This might also account for the strange timing. Normal ring detect circuits using opto isolators will produce a square wave pulsed at ringing cadence rate.

A schematic would be very useful.

The simplest ring detector would be a capacitor to block the DC a resistor to limit the current and the LED side of the opto-coupler in series across the incoming line. There should be a diode across the LED to protect it from reverse polarity. Just 4 components take care of the line side of things. On the micro side, ground the emitter of the opto transistor, connect the collector to supply via 4700Ω and take the output from their junction. You should not under any circumstances connect the micro circuit to the line directly, the opto is there to provide a signal path without having a DC path.

Brian.