Fixing a bad STATE pin on an MLT-BT05 BLE module 4

This is a follow up article to the detailed review of the MLT-BT05 BLE module. In that previous post I mentioned that I got some MLT-BT05 modules with their STATE pin floating. No working STATE pin was an issue for me and I decided to investigate the cause.

Update: same issue was observed with Bolutek CC41 modules  that are delivered lately. The conclusion is that this is an assembly or a breakout-board issue and not specific to any type of daughter-board used. Could even happen with original HM-10 modules if soldered poorly to the breakout. Keep reading for a fix regardless of firmware/module.

The breakout board has a physical STATE pin and the main chip and daughter board are the same as in other modules with a working STATE pin. The only difference I could notice was the number of solder joints between the breakout and daughter boards. I suspected that this is the cause of the issue and that the pin on the breakout board is simply not connected to the chip.

Solder joints MLT (top) vs CC41 (bottom)

To support this theory further, the specification for the MLT-BT05 includes a schematic that describes pad 25 as a special pad whose signal behaves like a signal of the STATE pin.

MLT-BL05 diagram

Unfortunately in order to verify this, one would need to remove or cut the shrink wrap. I couldn’t do it when I wrote the original article, but got an opportunity to do so later.

Cutting through

MLT-BT05 fix – open shrink-wrap

Once the pads were exposed it was easy to confirm with a DMM that:

  • The STATE pin is routed to pad 25 on the breakout board
  • The round pad 25 on the daughter board has the expected high/low signal depending on connection state
  • The two are not connected 🙁

A quick solder job later

MLT-BT05 fix – joint added

And some sticky tape to patch things back up

MLT-BT05 fix – closed

And we now have an MLT-BT05 module with a properly functioning STATE pin!

Why???

I couldn’t find any good explanation for making it this way. Either the manufacturer doesn’t know what he is doing or they are intentionally saving on the labor/solder by not adding another joint.

It could also be that I got 4 modules from a “bad” batch and that there are also properly soldered MLT-BT05 modules out there.

Neither of those possible explanations is any good. Let me know in the comments if you have any idea.

Still, for only 3$ per module it is up to you to decide if it is worth bothering with the fix or if it is better to pay a bit more and get some other option instead.

 

4 thoughts on “Fixing a bad STATE pin on an MLT-BT05 BLE module

  1. Pingback: MLT-BT05 BLE module – a clone of a clone?? | Arik Yavilevich's blog

  2. Reply Mihai Tintea Apr 16,2017 3:44 pm

    I own the MLT-BT05 clone (the one that reports AT+NAME=MLT-BT05-V4.0, recognizes only the command AT+PIN and does not recognize the command AT+PASS, and has only one oscillator on the daughterboard). I cut the plastic protection and used the multimeter continuity test to see on all three sides of the daughterboard if the contacts are soldered. I cannot be 100% sure but some of them seem that are not soldered. Moreover, on the two long sides of the daughterboard, the contacts seem to be somehow aligned with the corresponding solder contacts on the breakout board, but on the short side of the daughterboard (the one on the opposite side of the antenna side), the contacts are obviously out of alignment.

    Do you think it would be a bad idea to desolder and resolder the entire daughterboard off/on the breakout board again ? I mean, ALL the contacts (2×13+8), just to be sure that nothing is left unsoldered.

    • Reply Arik Yavilevich Apr 16,2017 5:49 pm

      Hi Mihai,

      What you have to consider first, is whether your module is working correctly or not. If it is working ok I would suggest you don’t attempt to redo it. If it is not working ok, what is not working?
      You should be able to identify which pads are soldered and which are not by a visual inspection. No need to test with a multi-meter. See example of soldered and not soldered pads here: http://blog.yavilevich.com/wp-content/uploads/2017/03/Solder-joints-MLT-vs-CC41-tn.jpg . Feel free to post a photo of your module so we can see its joints.
      The pads on the short side of the daughter board are GND, SDA and some not-connected pads. None of these are usually used in a module like we are discussing here so it probably doesn’t matter whether they are aligned or not.
      The small “open stamp”-like pads can be difficult to solder. If your soldering skills are good then you might improve the connections and the physical strength of the module by re-soldering the pads, but otherwise I wouldn’t risk it unless you need to fix the module.

  3. Pingback: MLT-BT05 BLE module – a clone of a clone?? | Arik Yavilevich's blog

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