Transistor biasing

[mailus]

I need to know how to turn on a opto coupler at 0V input.(the input is from some sensor)
so i need put PNP transistor to drive input for the opto coupler.
how to design this circuit?

my input in the range of 5V.
my optocoupler transistor side is used to drive a MCU pin.

i designed two circuit with small change wich is correct and WHY?

 

[audioguru]

The first circuit uses the capacitor to turn on and turn off the transistor slowly.
The second circuit has the capacitor turning the transistor off slowly but it turns on quickly.

 

[mailus]

The first circuit uses the capacitor to turn on and turn off the transistor slowly.
The second circuit has the capacitor turning the transistor off slowly but it turns on quickly.

so how to avoid noises?. the input are given from long distance about 10meters with single wire or multi core wire.

when i touch the input R2 resistor the opto-isolator partially on due not our body skin response.

how to design the any alterations please tell me..........

 

[audioguru]

so how to avoid noises.

Since the transistor has no series emitter resistor its gain is very high and is very sensitive to interference. Try moving the 100 ohm resistor to be in series with the emitter which reduces the gain and the resistor still limits the current to the LEDs.
A long single wire is a perfect antenna to pick up interference. It should be shielded audio cable.
Multi core wire has interference between the wires.

 

[mailus]

what about the capacitor C1? what is the function here.i don't know about this.
I saw in many boards the capacitor is used.

is this act as a time constant or filter?

In my circuit where should be the capacitor is placed to get maximum performance (reduce noises and on-off timing).