[time-nuts] Building a mains frequency monitor

[Charles Steinmetz]

Nick wrote:

>The instructable I wrote about it is at [link]
>
>There's code for the Arduino and the
>Linux side as well as schematics.

Several things to note about that front end circuit, from a time-nut 
perspective (the circuit was apparently created as a science project, 
and it may be fine for that):

1)  The LM358 makes a very poor comparator, even for a 60Hz ZCD, with 
rise and fall times of tens of microseconds.  Also, its output 
doesn't pull closer than about 1.5v to the positive supply.

2)  With the non-inverting input biased to +2.5v, the switching 
threshold is over 3 volts positive from the zero cross of the AC 
mains voltage, which guarantees that mains voltage variations will 
create timing offsets.  Ideally, the non-inverting input would be 
biased one diode drop below ground so the actual switching threshold 
would be near 0v.  However, that is not within the input voltage 
range of the 358 running on a single supply, so ground would be the 
closest workable choice (the 358 is a "single supply" op-amp, so its 
input common-mode range includes the negative supply -- ground, in 
this case).  Just remove R2 to implement this change.

3)  The unused section of the LM358 has its noninverting and 
inverting inputs grounded, with the output left floating.  This is 
not a good way to connect an unused op-amp.  Generally, one should 
connect the noninverting input to a potential that is within both the 
input common-mode range and the output voltage range (here, from 
ground to about 3.5v), and connect the output to the inverting input 
(making it a unity-gain follower).

A much better solution is to use a real comparator with the threshold 
at 0v.  Instead of a series diode on the input, use a series resistor 
and clamp diodes as necessary to keep the comparator's noninverting 
input within its allowable voltage range.

I didn't review the code, but anyone building the circuit should 
check it carefully to see if there are any similar issues on the software end.

Best regards,

Charles