Particle Photon Controlled Gate Lock

One of the most common ways homes in my community are burglarized is by entering through the side gate (thieves know they're never locked) and breaking the window pane that's built into the back door. One of the top security tips from police departments is to lock your home's side gate. Despite this advice, most people, including myself, don't lock our side gates due to the inconvenience. Most gate latches have provisions for keyed or combination padlocks that are only accessible from the inside. My family uses our side gate almost daily to take out the trash and recycle, or to take dogs in and out of the backyard and often need to open it from the outside. People who use landscaping and pool services also need to give workers easy backyard access. In any of these cases, using one of these locks is not ideal.

This problem inspired me to develop an IoT gate lock that can be controlled from anywhere using a smartphone. One key requirement was to utilize the existing gate latch and leverage the same hole feature that it provides for a traditional padlock.

These latches are ideal due to the fact that they rely on gravity for automatic latching (pretty clever), can accommodate misalignment from sagging gates, can be unlatched from both sides of the gate, and are relatively inexpensive. My idea was to drive a steel pin through the same padlock hole and rely on the shear strength of the steel pin the same way the padlock does. This seemed like the best choice as it removes any stress on the mechanism and device overall.

Since I have good Wi-Fi signal strength where my side gate is located, I wanted to be able to control the lock from my phone.

Basic Requirements

I wanted the locking device to meet the following basic requirements:

1. The device must drive a locking pin through the existing hole in the gate latch.

2. The pin must move into the latch hole when the device is put into the "lock" position, and out of the latch hole when put into the "unlock" position.

3. The locking pin must provide enough shear strength to prevent someone from breaking the pin and allowing the latch to open.

4. The locking pin's driving mechanism must prevent manual operation. In other words, the pin cannot be moved in and out of the latch by hand.

5. The device must be battery powered and should operate for several months on one set of batteries.

6. The device will be controlled remotely by a smartphone app.

7. The device must be somewhat resilient to outdoor environments. If this were an actual product it would need an ingress rating of IP65 or higher and would need to withstand extreme temperatures. My first goal was to make it work and not fill with water after a decent rainshower.

Design

For the electronics I decided to go with the Particle Photon as my controller so I can securely control my lock from anywhere. What I really like about Particle is they offer more than just a Wi-Fi SOC in the Photon. Particle offers an end-to-end cloud platform that allowed me to quickly prototype my project and focus on my solution instead of the IoT infrastructure behind it. Don't get me wrong, infrastructure and security are critical to successfully developing an IoT product, but using a platform allowed my to build and test my project quickly. Particle provides all the pieces you need for an IoT project including device management, security, IDE, and APIs. Particle also has some great libraries and resources for building mobile apps on their platform.

Building the Proof of Concept

I started by building a breadboard proof of concept with a makeshift gear drive which allowed me to quickly learn the Particle platform and test my idea.

Electronics

I used 3 AA batteries to power the Photon which is providing approximately 3.5VDC, well below the specified upper limit of 5.5VDC. I chose the DRV8833 dual half bridge to drive the DC motor as the half bridge allows me to drive the motor in two directions.