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.
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.
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.
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
And some sticky tape to patch things back up
And we now have an MLT-BT05 module with a properly functioning STATE pin!
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.