zero crossing detector

**broken link removed**

please does anyone know how the LM 3914 zero crossing detector in the above schematic work? the working of the 3914 remains a mystery to me, I understand as the input signal rises a 'coincidence' comparator lights up an LED when there's a match b/w the reference voltage and the value of the signal at that point in time. A comparator now we know, swings to Vcc when there's a coincidence at its inputs, wouldn't this then make the LED cathode just as positive as its anode?
How then does it light up?


No.2 How exactly does the full wave rectifier function? I understand the rectification process when the signal is positive, but for the -ve it just doesn't add up, from my analysis the rectifier would form an inverting amplifier of unity gain in the negative region, but it seems they used the equation for a non-inverting amplifier to come up with the solution...

thanks in advance!

The LM394 has nothing to do with zero-crossing .. it is just simple bar display driver .. **broken link removed**
On the other issue, the full-wave rectifier, that is, you should take into account the fact that the signal that is transmitted to the (-) input of the opamp is comming from Cathode-Anode polarised diode, and will be inverted by the opamp to a positive signal, just like -(-)=(+) .. plus the gain factor ..
Regards,
Ianp

IanP said:

The LM394 has nothing to do with zero-crossing .. it is just simple bar display driver ..
Ianp

Click to expand...

that's odd, for the author is actually using it for the zero crossing purpose chiefly and not for bar display?...

The LM3914 is an LED bargraph driver, but inside there are just a number of comparators with different thresholds (derived from a common reference), so he is using one of those to detect the zero-crossing of the input signal. The outputs of these comparators drive the LEDs, so their outputs are available. The author simply connected opamp A5 (used here as a buffer with adjustable output level) to one output of the LM3914 and that provides the zero-crossing function. The level adjustment is necessary because the LED's are powered from +9V, so when the LED is off, the voltage could be about 7V. You need just a TTL level so you divide down the 7V or so with the pot and buffer the signal with the opamp, in order to feed it to the parallel port.

But why pin 17? I understand the 3914 has an internal reference (default) of about 1.2V, so for 10 comparators that'll be 0.12v increment... :?

Each LED then lights up, as the signal increases for each 0.12v, right? But I thought the comparator would swing to Vcc / 9v when there's a coincidence at its inputs that would make the LED cathode about 9v thereby reverse-biasing the LED, how then does it light up...?

I still stand by with what I said yesterday: this circuit has nothing to do with "zero crossing". It has several comparators inside but none of them uses 0V as reference ..
And, on top of that the so called ""zero crossing"" signal is taken from pin17 .. ???

For examples of zero crossing detectors goto:
**broken link removed**

Regards,
IanP

well, unless we're missing something here, if the author could 'goof' on that point, then that casts a doubt over the whole project as well...

and would be very risky for one who wants to use it for school project...

or...?

I agree with IanP that the circuit is not a true zero-cross detector.
I think the author intended to detect the crossing through a threshold slightly above 0V, but taking into account the delay introduced, it might be possible for the resulting pulses to actually coincide with the true zero-crossing (though not for all frequencies).

I do not like the approach, I would not use it myself. I simply provided an explanation as to how this circuit might work. If you intend to build a zero-crossing detector, use a comparator with a 0V reference.

I was just thinking perhaps his intention was pin 10 and not pin 17. i believe since LED 10 is the first to light up and the last to go off, connecting the input of the buffer to pin 10 could give us an approximate zero crossing?

or what do you think?

Yes, you are right, the closer to zero the better.
But another puzzling fact is that he adjusts the reference, which means will be higher. So if the intention is really to detect a voltage as close to zero as possible, why not leave the reference at its minimum, why adjust it higher, which will also increase the threshold?
Initially I thought he wanted to actually detect crossing through 2.5V or so, since the input is marked 0-5V. But why do that? If the input was really limited to that range, then you could just convert it directly using the ADC, no need to rectify it and detect the negative part.

Perhaps the author has found some peculiarity if the chip, so that with a negative input, but a certain positive reference, it somehow produces an outpu pulse. It's hard to believe, but it migh happen. After all, there is a buffer opamp inside. If it suffers from phase inversion, then it could do that.
If that is the case, then it's not a normal mode of operation, not one I would rely on for proper operation.

The LM3915 is a monolithic integrated circuit that senses analog voltage levels and drives ten LEDs, LCDs or vacuum fluorescent displays, providing a logarithmic 3 dB/step ana-log display. One pin changes the display from a bar graph to a moving dot display.

Zerocrossing detector is nothing but a a comparator with reference voltage of 0V.