Make your DIY LPM display more stable

First, I would like to note that this won't work in all types of DIY LPMs. I just happened to stumble upon the solution in a certain type of digital panel meter.

I have seen this panel meter get used in many Ophir LPMs since it is a 5 digit meter and has a decent accuracy.


The cause of the problem is the connector. Since it's a mechanical connection, over time (or many plug/unplug cycles later) the contacts get dirty and cause noise. Since this meter is very sensitive due to the 5digit display, the noise shows up as jumpy last two digits.

I removed the connector and cleaned the solder pads on the PCB


Then cut off the plug on the wires and strip the wire ends.


Solder them directly to the PCB for a solid, reliable connection


Put it back in and test


A solid zero at no input. Before, it would jump around at a few mW display and no matter what filtering and modification I did to the power supply, the noise remained. After this, the reading became quite stable.


This is the reason how I found out about this problem. I got tired of my DIY ammeter display (which used the same DPM) sometimes jumping up to tens of mA in the Amp range and never figured it out until I decided to just solder the wire in. After the mod, it displayed 0.000 with no input just as it is supposed to.

Re: Make your DIY Ophir LPM display more stable

Great post by you as always! Still cant +rep you again

Re: Make your DIY Ophir LPM display more stable

Great post, thanks for sharing.

It's interesting how connectors can cause noise... have there been scientific studies done on this matter?

Re: Make your DIY Ophir LPM display more stable

I'm guessing it has something to do with the different metals in the contacts. The effect is similar to thermocouples and TECs. I'm not 100% sure though

Very nice indeed, the noise is also because of an incompatible battery (for panel meter) you need atleast 6v for this panel meter iirc. I used the same one before upgrading to ARGMeter Ophir 1.1 add-on, before that my ophir reads +4mW on the panel meter. I will give this a try sometime and see if it actually zero's the panel meter or it is basically the ophir itself.

Another nice tutorial. Now I have to spread the rep around some more.

@Sinner - I thought these displays are supposed to be run off of 5V. The ones sold on ebay need only 5v

Yes, this display needs 5V since it doesn't have an onboard regulator. There are others with built in 7805 regulators that will need about 7V or more to work properly.

May also help laserbee A's. I've seen a similar connector for the screen in them.

Yup indeed it's 5v display I had thought 6v because I remember using 4xAA's to power the display.