Sunday, November 10, 2013

Baby monitor fix

Our Samsung baby monitor has seem some hard use and stopped working due to a broken power connector on the screen unit. Here's the model:

When it broke we didn't really need it as much. Our son was old enough to be fine on his own in the crib; no worries about him not being able to roll over or push a blanket off his face. Everything changed last week when our son managed to crawl out of his crib. Today I gathered up the parts from the monitor and sat down to do my first bit of soldering in our new house.

I managed to find everything but one of the power adapters. A glance at the output voltage of the remaining wall wart let me know that the board is expecting to get 5V and 1A. That was great news since that's right in line with the power levels of most USB chargers. I ran off to my bin of spare cables and came back with a mini-b usb cable and set to work determining the pinout and cutting it apart. I was in full "mad-scientist mode" instead of contemplative "engineer mode"; this is when I make most of my mistakes. I'll mention how I screwed up later, but it started with chopping this cable up prematurely.

My collection of chargers range from 300mA to 1A. I ended up having an 850mA charger handy to test with; close enough (remember mad scientist mode here...). I verified everything on the monitor PCB by poking around with a multimeter on it's continuity setting. Most flexible cables, such as USB cables are made of lots of smaller pieces of wire twisted together. I hate trying to solder cables like that directly to a printed circuit board because there's always the potential of a stray wire strand shorting a connection. Digging through my parts bin I found some integrated circuit boards and cut one apart. I could then take my time soldering the stranded USB cable to this instead of directly to the monitor's PCB.

With my USB cable soldered to the IC PCB piece, I hot glued the IC PCB piece to the monitor circuit board with the IC side pin holes lining up perfectly over the broken power connector holes. Note in the picture below that I unplugged the lithium battery before I did this. Accidentally shorting rechargeable batteries can cause explosions or fires. Once the hot glue cooled, I placed a solid wire down through the hole and held the soldering iron against it. As the wire heated, it melted the solder it was touching below. After removing the heat, the solid wire was permanently connected to the monitor's board. I clipped the wires short, soldered the top to my IC PCB and verified everything was still good to go with the multimeter's continuity setting again.

A final inspection, a little more hot glue, and I plugged in the power for the big test. It works! Great!

I clipped a hole to allow for the cable to come out of the case, then hot glued everything like crazy so that strain on the cable won't pull on any of the circuit boards.

If you don't have a hot glue gun, they are cheaper than you'd expect.

I left 2 or 3 ft of cable since I didn't expect the battery to still hold a charge, meaning we'd only be able to use it when plugged in anyway. I was wrong, but it's only a matter of time until the battery craps out, so I don't feel too bad about it.

That mistake I mentioned earlier... here's the details.

In my haste to fix the monitor I didn't really take a close look at the rest of the board. I just assumed that the section of the board with the broken barrel power connector and battery connections would be the only way to power the board up. After the fix when I was putting everything back together I noticed a mini-b usb port on the opposite corner of the board! Unfortunately, the mini-b usb that I had just chopped up to complete this fix was the only one I had. This fix could have been as easy as cutting away the case around the mini-usb port and plugging in the cable, saving me the time and parts and making for a much cleaner fix. I may never know. I cut the case away anyway; maybe I'll find another mini-b usb cable somewhere and give it a try.