Introduction

A little over a year ago I purchased a mains voltage sensor module from from a local reseller of cheap electronics from China (eparnters) for $4.99 NZD. This is a small transformer (model ZMPT101B) with an amplifier on a small PCB.

Yesterday I returned to working on the project I bought this for, and after wiring it up, I found no signal on the output pin.

Sadness 😭

Photo showing the ZMPT101B voltage sensor module with the incorrectd OP07 op-amp installed

Yeaaaaaah… that’s the wrong part.

No signal

After a few minutes of frustration watching the scope and adjusting the gain pot, I pulled up the data-sheet for the OP07 op-amp on this board, and started probing the pins with my DMM and noting down voltages.

Pin-out diagram for the OP07 op-amp, showing pins and signals.

Pin-out for the OP07 op-amp

What I found made no sense. My first thought was maybe the package was soldered the wrong way around, so I redid my notes with the 180 degrees rotation, but that still didn’t make sense…

Photo of a note-book showing SOIC-8 packages with their pinouts and measured voltages

Confused notes and atrocious hand-writing…

Getting a bit desperate, I started doing web searches for the ZMPT101B module, and came upon this site where I saw the following image of the same module.

Photo showing the ZMPT101B voltage sensor module with the correct LM358 dual op-amp installed (sourced from the web)

Same module, different op-amp?! electropeak

Notice this module looks identical but the op-amp is an LM358. Pulling up the data-sheet, I realised the measurements would make sense if the dual op-amps of the LM358 were chained together:

Pin-out diagram for the LM358 op-amp, showing pins and signals.

Pin-out for the LM358 dual op-amp package

The fix

So… let’s fix it. I picked up an LM358 at Altronics…

An image of a loose LM350 dual op-amp SOIC-8 package

This is the correct chip!

…and set about removing the OP07 with some hot air:

Photo showing the ZMPT101B voltage sensor module with the op-amp removed (showing bare pads)

PCB after removing the OP07 SOIC-8 package

Getting the orientation correct for the ST Micro LM358 SOIC-8 package was a little tricky for me. Here is the drawing:

describe what it is

ST Micro LM358 SOIC-8 package drawing from the data sheet

Note the slighly more beveled edge on the pin 1-4 side. It is actually visible in the photo if you look real close:

Drawings of the ST Micro SO package showing the beveled edge on the side with pins 1 through 4

Orientation indicated by the beveled edge

Some more heat and solder and the new op-amp is installed:

Photo showing the ZMPT101B voltage sensor module with the replcement op-amp soldered.

ZMPT101B with correct (LM358) op-amp installed

And happily it worked! I immediately got a signal, and it was easy to adjust the gain to prevent clipping with the scope attached::

A screen-shot of a digital osciliscope showing a 50Hz sine wave signal.

That sweet 50Hz hum!

Conclusion

In countries where buying hobby electronics supplies from the likes of Adafruit and Sparkfun incurs expensive shipping, online resellers buy bulk items on alibaba.com and re-sell them in-country with a stiff mark-up for impatient people (like me).

I suspect the factory (probably a CM using spare capacity to churn out these modules) made a mistake and loaded the wrong reel of op-amps into their pick-and-place machine. This might also have been a grey-goods (i.e., workers running the line at night) situation. Either way, the modules were faulty and probably offloaded at a steep discount to the NZ reseller (more profit!).

The seller probably didn’t know any better (I wouldn’t expect them to test everything they resell) and I fixed the module anyway (and had fun doing that) so all is well that ends well.