Zero Crossing Detector Circuit Using a Transistor

Zero Crossing Detector Circuit use for check or detect zero cross of AC Power. This circuit is useful
where we need to make a AC Power Controller or Dimmer to control the speed of an AC Moter or Control the Intensity of an electric Bulb etc. with the help of TRIAC (Ex. BT136) than circuit is very useful. Here the circuit of Zero Crossing Detector

Zero Crossing Detector Circuit

Working - In this circuit first we use an resistance of 220K ohm for reduce the voltage level
than rectify this voltage u can use 1N4007 for bridge. We get a rectified pulsating DC out(shown in
fig. 2).
Filter it for supply +ve supply for transistor for pull up the collector of transistor with 10K ohm
than apply this pulsating DC to the base of transistor with 10K ohm. if the voltage at base is less
than base emitter voltage the transistor is in cutoff mode than output is come from 10K ohm pull
up is grater than 5v. so we use an 5v zener diode only for purpose of security of MCU.
but if voltage is greater than base emitter voltage of the transistor the transistor is in saturation mode
and the voltage at collector is zero.

OUTPUT - Output of this circuit is shown in fig.3. U can see that the output is high only
                            when the rectified out is near or less than Veb volts.