It only happened twice in the 9 years that I’ve been using these Z-Wave switches. Early one morning last week, my wife woke me up with those magic words – “Honey, there’s something wrong with the lights downstairs.” Everyone with a home automation system can probably guess what happened next. Not wanting my wife to lose faith in our system, I got out of bed and with dreary eyes, ambled downstairs to investigate. Sure enough, the overhead light in the kitchen was flashing… on, off, on, off… about once per second.
I could hear the familiar click of the relay inside the in-wall switch as the lights turned on and off, indicating that the problem was either with the switch itself, or with the home automation controller. So, I followed the standard procedure. First, I pulled the safety tab on the front of the light switch, which cuts off the 120VAC that it’s wired into. After leaving it off for a few seconds, I then pushed the tab back in, effectively rebooting the switch. To my dismay, the lights instantly started flashing again… on, off, on, off.
So, I moved on to the home automation controller. I thought it might be malfunctioning and running a rogue automation job to cycle the light switch every second. I checked the logs and found nothing unusual, so I rebooted it for good measure. All the while, the kitchen lights still kept flashing… on, off, on, off.
In a final desperate measure, I grabbed a spare Z-Wave light switch and replaced it. Yep, that did it. No more flashing lights, and everything was working again as it should. Despite the inconvenience, there was something good that did come from this situation; I now had a broken Z-Wave switch to tear apart and play with. So, let’s get to it!
Opening Up the Switch
The switch that failed is a GE-branded Z-Wave in-wall paddle switch, model # ZW4001. They currently seem to be going for around $50 on Amazon, though it’s been several years since I’ve purchase any. I have about a dozen of them throughout my home, and their track record has been pretty good. Like I mentioned earlier, only two have failed in the nearly 10 years that I’ve been using them. That said, I did decide to go with a different brand of in-wall switch for my basement, which is currently in the process of being finished.
The first step to tearing down this switch is to remove the plastic casing from the back. There were two screws holding in together. One was a standard phillips-style screw, but the other was a non-standard, two-holed screw. Fortunately, my trusty iFixit Manta toolkit had the necessary screwdriver bit to make short work of that.
Examining the PCB
After a little jiggling, I was able to lift off the back cover, which revealed the inner workings of the switch. Surprisingly, there were two separate PCBs connected with a pin header and a fuse, forming a “PCB sandwich”. Aside from that, much of it was as I expected. The big black box in the middle is a relay rated at 17A and 277VAC. This relay is used for switching the power to the light that the switch is driving. The relay is triggered one of two ways; either by pressing the paddle switch, or by controlling it through the Z-Wave interface. If the relay is the part that failed, replacing it would be simple enough. A new one would only cost a couple of dollars, so it might be worth salvaging this $50 investment.
Another thing I noticed on the top of the board is a chip, labelled as LNK304PN. After finding the datasheet, I discovered that this chip is a power switcher. I assume it’s converting the current for the DC side of the device, since I wasn’t able to find any sort of transformer on ether of the boards.
Moving on, I flipped the device over and examined the bottom PCB, closest to the face of the switch. As I expected, I discovered two tact switches – one for the top of the paddle, and one for the bottom. What I didn’t expect, however, was to see a microswitch used for triggering the kill tab that I mentioned earlier. As you can see in the photo below, when the tab is pulled out, the microswitch is open, killing the power to the device by physically disconnecting the line wire and the neutral wire. By disconnecting the line wire, power from the panel no longer flows through the device.
I also noticed a red wire coming out of the board and into the face place of the switch. After prying off the front paddle, I found the red wire running down edge into a cavity. This must be the antenna for the Z-Wave module. Since Z-Wave is RF based there needs to be some form of an antenna to give it some range for receiving and transmitting.
Next, I removed the board from the face plate and was able to get a closer look at the backside. There weren’t any big surprises there; a couple of resistors and test points adorn the board alongside the tact switches and the front-facing LED.
Now at this point, in order to go further, I needed my soldering iron to remove a couple of pins that are holding this “PCB sandwich” together. That, however, poses a problem. I’m in the middle of a remodeling project in my home, and my workshop is currently getting a much-needed upgrade in that process. So, my iron (along with most of my other tools) is packed away for the time being. But since I wasn’t planning on repairing this failed switch, I decided to go ahead and cut away the pins with some snips. They’re standard 2.54mm header pins anyway, so there’s always the option to replace them if I decided to later attempt a repair. I do have to confess, however, that it does make me a bit nervous to repair something that’s UL-certified; I wouldn’t want to create a potential fire hazard in my home. After a little snip here and there, I was able to deconstruct the PCB sandwich and get a closer look at its special sauce.
The Z-Wave chip was easy to spot right away. It was sitting on a postage stamp-sized breakout module on the board closest to the front of the switch. The part number on the module is ZW0301.
A quick search pulled up a datasheet on the entire module (part # ZM3102N), which showed that there’s 32KB of flash and 2KB SRAM on the chip. Reading through the datasheet further, the ZW0301 chip seems to have some nifty features. In addition to the flash, there’s UART, an SPI controller, an ADC, an interrupt controller, and several other micro-controller-like capabilities inside. And at the heart of the ZW0301 is an 8-bit 8051W CPU, running at 16MHz.
Needless to say, I’ll need to remove this Z-Wave module and spend some time playing with it in the future. This will have to wait for my workshop to get back up and running, as I unfortunately don’t have access to the right equipment to do it now. I’ve been wanting to build a native Z-Wave device from scratch for my home automation network for quite a while, so that’s a project I’ll take on this spring.
Turning my attention back to the bottom slice in the PCB sandwich, I noticed a few more passives, and no other ICs. The one thing that stood out to me, though, were the black diodes, labelled D1, D3, and D5.
These look to be rectifier diodes, so I’m guessing that these diodes are used for a full-wave center-tap rectifier to drive the LNK304PN chip that I mentioned earlier as a buck-boost converter. This would provide a way to transform the rectified AC into a switched DC output via an inductor. Here’s an example configuration from the LNK304PN datasheet which shows how to take a rectified AC signal and use it to drive an LED array.
This side of the board does seem to be where the AC current is converted to DC, and I do see an inductor on the board here was well. The other board in the PCB sandwich looks to be entirely DC with at least a 3.3v signal entering the board through the pin headers.
Overall, it was quite interesting to take a look at what’s inside of these Z-Wave in-wall switches. There are a couple of components that I can harvest from this failed switch – a microswitch, a couple of tact switches, a relay, and power switcher IC. And of course, the one component that I wanted – a Z-Wave module, which I’ll definitely make use of in a future project …assuming it still works!
I hope this article encouraged you to not just toss your broken devices into the trash can. A little bit of curiosity can go a long way to helping you demystify something a bit and hopefully learn something new in the process.