Zero crossing detector

I need a Zero crossing Detector(or polarity detector) that has an input voltage and gives digital logic 1 when input just goes positive and logic zero when input goes negative.

How about using an LM311 voltage comparator? Power it from ±12V or ±15V. Use a pullup resistor on the output to +5V (or whatever your logic 1 voltage is). Connect the inverting (-) input to ground and your input voltage to the non-inverting (+) input.

Feed Ac signal at the base of the transistor and tap out the output from collector....

try using simple voltage comparator circuit using opamp.(ie lm741)

hi fawad_hd97

Does this circuit(ZCD) can have dual power supply?
Can you state your specifications clearly?

fawad,

A simple comparator circuit works as polarity
detector. But it will not meet the requirements in all
applications.

For example
1) If the input voltage crosses zero the output will
go 'high' for this circuit. But due to noise in the input
signal,(mostly at zero crossing) If the instantaneous
value of input signal fluctuates between the positive
and negative values, the output will oscillate
between high and low logic states. To avoid this
you have to incorporate hysterisis by the way of
positive feedback.
2) If the comparator/opamp you are using powered
from single power supply(0 and +5V) you need to
take care of input pins from negative polarity input
signals.

Like wise, so may I can cite. So I suggest you to
clearly state your requirements and constraints.