DIY ohm meter

[WarHawk-AVG]

https://www.fasttech.com/p/1494601

This thing is surprisingly small!

Now to do some modding (aka build my own USB re-chargeable ohm meter)! [the below was only a test for operational theory]

Ok, it's working, this is the 2x AA unit working..but not on 2x AA, it's working on a single 14500 (AA sized) Li Ion battery

Accuracy: on my MVP2 the head that is on it sometimes reads 2.6Ω and sometimes reads 2.7Ω, reading on the unit shows why

Ok...let me explain
A. The battery is a 14500 el-cheapo from fasttech, using the batteries in a Cree flashlights, the protection circuit went to crap so I removed it (NOT ADVISABLE)
B. Had to build a bypass to complete the electrical circuit to power the unit, it is simply a piece of pencil cut off to the length of an AA battery then wrapped in aluminum foil (again NOT ADVISABLE)
C. Unit works JUST FINE with a Li Ion battery in it

Now what I plan to do
I plan to make this a self contained USB rechargeable ohm meter..how do you ask?

Well by
A. Putting in one of these TP4056 5V 1A Lithium Battery Charging Board Charger Module in the center battery compartment, the specs say it is 15mm wide, however due to the "extra" on the PCB I am sure I can whittle off 1/2 mm on either side without hitting the traces (actually might not need to because the 14mm wide battery sits in the battery slot with extra room)...AND its a 1A unit which is WAY to high for a single 14500 battery, but knowing the schematics of the TP4056 IC Li Ion charging chip I can change the RProg resistor on pin 2 to a 10k Ω resistor and limit the CC/CV to 130mA, which should be safe to charge a small 900mAh battery.

B. Rewire the unit to not only charge the single cell from the above module, but to run from the single cell battery in the other slot

Without modifying the 1A resistor and completely removing the battery you could create a USB only run rig, power the unit directly from the USB module...but I want mine to be portable

What do you think? (other than the "don't try this at home" safety lapses)

Rechargeable ohm meter for sub $15

 

[yo han]

Great idea. But then again, a lot of work just to make a $10 item USB rechargeable (but that's the fun of modding right? )
But why lower the charging current so much? 450mA would be much better. With only 130mA it's going to take around 7 hours to charge a 900mAh 14500.

 

[WarHawk-AVG]

Great idea. But then again, a lot of work just to make a $10 item USB rechargeable (but that's the fun of modding right? )
But why lower the charging current so much? 450mA would be much better. With only 130mA it's going to take around 7 hours to charge a 900mAh 14500.

Yeah was thinking about that, and I think the 450mA would actually also be enough to run the rig without a battery making it a complete batteryless USB powered ohm meter if you wanted it I suppose.

I guess I am in the camp of slower longer charge stresses the batteries less but yeah...450mA would definitely charge the battery faster, good call

 

[yo han]

Oh but 130mA would also be enough to make it run without a battery.
The unit is probably taking less than 80mA

 

[Rader2146]

It does seem like an whole lotta work to make it rechargeable. You're also gonna cut the energy capacity by at least half by using Li-Ion vs NiMH (~2.8Wh vs ~6.0Wh)

On the other hand, a modder's job is never done. If it ain't broke, fix it until it is...and then fix it again.
Should be a fun project.