Introduction: Snail-Mail Mailbox Monitor
Like many others, I’m often left wondering if our mail has been delivered yet. Our local postal delivery varies from anywhere between 10:00 am to 5:00 pm, we have limited visibility out the front of our residence, and we can’t always hear the delivery vehicle unless we are close by. As a result, I find I’m often making trips out to the box, sometimes in the rain or snow, only to return empty-handed.
There are plenty of mailbox monitor projects out there, but I hadn’t been able to find one that fit my needs exactly. I had just a few simple requirements:
- Not too expensive (in case it doesn’t work out)
- No expensive electronics left outdoors (tampering)
- Minimum mailbox footprint (no visits from the bomb squad)
- Low maintenance (no battery charging)
- Works with my present metal mailbox (antenna/signal issues)
- Must be wireless (too many sidewalks to cross to run wires)
I hit on the idea of using either a wireless doorbell or garage door monitor for the wireless link. If one could extract some kind of signal from the receiver, it could also be used to trigger an input to a simple microcontroller setup. From there, the possibilities are endless…
With the previously mentioned requirements in mind, I began looking around for something that might fit the bill. I eventually decided to go with the SkyLink GM-318 Household Alert Garage Door Monitor Set. At the time I purchased it, it sold for $16.31 delivered (via Amazon Prime). Since I already had a TWINE, I realized I could further leverage the visual and aural notifications of the SkyLink by linking it to the cloud without the need for an additional microcontroller.
Here's a list of the parts I used for my project:
- 1- SkyLink GM-318 (Amazon.com)
- 1- Magnetic switch assy. (Amazon.com)
- 2- 1/8” audio jacks (Radio Shack)
- 2- 1/8” mono audio plugs (Radio Shack)
- Solid and stranded hook-up wire
- Rubber grommet
- Silicone sealant
- Duct tape
- Adhesive-backed Velcro
Parts used for the SMS notification piece:
The first couple of steps for the SkyLink modules were to verify they worked correctly out of the box, and then to set about disassembling them to look for connection points to break out inputs and outputs. The transmitter module proved to be the most challenging to open up due to the plastic bracket on the back that contained the antenna. The bracket prevented easy access to one screw on the back of the main case, and releasing the plastic clips that held the two halves of the bracket together was a real problem. I finally resolved this by shaving away the barbs just a bit with an x-acto knife so they could back out through the holes more easily. Once everything was apart, there was clear access to both sides of both PCBs.
Unlike other garage door sensors that utilize a tilt switch, the SkyLink uses an odd little sliding post with a roller on the bottom to sense when your garage door is fully closed. When the post fully extends, a magnet in the top of it activates a tiny reed switch to send the door open signal. I was pleased to discover that there was an extra solder pad on each side of the reed switch. All I had to do was add some wires and port them out to my audio jack. I found that a small cap off a hotel shampoo bottle fit the end of the case exactly, so it was epoxied in place after routing the wires through a small hole drilled in the case.
The receiver presented a few different options for outputs. To keep things simple, I decided to use the #1 LED. When the sensor is activated, the receiver pulses the LED about once a second at around 3.8v, which should be sufficient to trigger a microcontroller. I just piggybacked some wires onto the back of the PCB and routed them to another audio jack. There was plenty of room in the case to drill a hole and mount the jack on the left-hand side.
One complication I ran into was that the transmitter needed to be provided with a closed signal when the mailbox was opened, which is the opposite of how a basic reed/magnet switch works. Fortunately, I was able to find a nice magnetic switch set online that has both NO and NC terminals. Connecting the wires to the COM and NC terminals provduced the needed results.
The second issue was that the TWINE breakout board was expecting a grounding signal at its inputs, whereas my receiver output was providing a +3.8v pulse. I found that the folks at Supermechanical already offered a solution on their Breakout Board how-to that utilizes an NPN transistor to pull the input down when activated. Had I realized this earlier, I could have installed the extra components inside the receiver. Since it was already closed up, I just soldered the transistor and resistor together, put them in line with the wires and covered them with heat shrink tubing. I set up the TWINE rules with a one minute delay so that I didn’t get multiple notifications for the same event due to the pulsing of the input.
After thoroughly bench-testing the whole setup, it was time to install all the components. The magnet alignment on the mailbox door wasn’t super-critical, but I measured carefully so that they would end up as close together as possible when the door was closed. It should be noted that there is a right side and a wrong side to the mating magnet; be sure you verify the operation before permanently attaching it. The transmitter is mounted on the back face the mailbox with Velcro so that it could be easily removed for any future maintenance. The antenna extends through a rubber grommet and is sealed with clear silicone to prevent any water from getting into the mailbox. The sensor wire was secured with good old duct tape and the plug inserted into the phono jack.
For the interior, the breakout board was mounted to back cover of the receiver with small hardware to keep it from moving around. All the components were connected and placed in an inconspicious location with AC power available for both the receiver and TWINE.
I’ve only had the system in service for a few days now, but so far it’s worked flawlessly. As an added bonus, when I get a text notification on my phone, I also see it on my Pebble watch. Geek nirvana!
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