zero crossing circuit wierd delay

[andreas157]

i recently built this siple zero crossing circuit with input 5V 50Hz but i got a strange delay on the rising edge of the output pulse...im using 1megΩ resistors in series with my signal and an LM358, 1N4002 diodes. this delay can be seen here. any ideas? maybe noise causing this? how i could solve this issue? thanks

the setup
**broken link removed**

the result
**broken link removed**

 

[barry]

You need to put some feedback around that sucker.

By the way, that scope trace is useless. What's your timebase? What's your amplitude? What's your input?

 

[andreas157]

5V/div 2ms/div my input is a 50Hz 5V peak signal

 

[crutschow]

That photo address gives me an error.

 

[schmitt trigger]

I'm too getting a 404 error on the photo address

 

[barry]

I got it, but you're not missing much.

It would be mighty helpful if you included the input signal as well as the output signal. And tell us what your power supplies are.

 

[Audioguru]

A lousy old LM358 and those 60Hz rectifier diodes are pretty darn slow. The very slow slew rate causes the rise and fall of a squarewave to become a ramping triangle. The very high value series resistors feeding the high capacitance of those slow diodes also causes ramping and delay.

A REAL comparator IC will be MUCH faster. Some hysteresis can be used which makes the rise and fall of the waveform much faster.