Designing and Prototyping a Car Bluetooth Key LockBox

Project Info


The project was requested by a car rental company that rented its cars to its customers. Before the new design, a car was given to the customer at a given branch and the customer had to deliver it back to the company at the same or another branch. This process had many setbacks including the exceedingly high travel distance of the cars, their high fuel consumption, overusing of cars and wasting time for customers.

The new product was aimed at resolving the above issues by using a new key lockbox design. The new lockbox allowed the customer to securely store a spare key on the car and leaving it securely at any parking without the need to return it back to any of the rental company branches 

In this solution, once the customer finished using the car, he would park it in a parking lot and put the key inside the lockbox and hang it to the car’s window. When the window was rolled up, the lockbox was also locked to the window. After the customer informed the car rental company about this, the company could remotely change the username and password of the lockbox through its central control system.

Knowing the location of the car, the company could rent it to any other customer close to the location of the parked car by just sending its location and security information to the customer. This method would, in turn, save a lot of time and money for both the rental company and the customer.


This request was for designing a lockbox. The user would not need to carry the switch, and when he approached the car, he could unlock the lockbox through the Bluetooth connection and the APP.

As described above, this solution was to integrate a way that the original user (like the car rental company) would have permanent access to the Bluetooth lockbox; in other words, they will be able to always open the lockbox but will also be able to send out a code or access via the app that will allow the secondary user (customer) to access it. This device would attach to a car window when it was rolled down and would connect to the car once the window was rolled up.


The main requirements were the followings

  1. The device was to be at a minimum battery consumption
  2. The app was to be developed for both the Android and iOS platform
  3. The design had to have the capability of changing the password by the user


  1. Making the box resistive to any cutting
  2. Providing sustained power for the system


Having worked on the details, the block diagram was considered for the product.

  1. In order to meet the minimum battery consumption, the following algorithm was developed:
  • Microcontroller and Bluetooth would be in sleep mode in normal mode.
  • Every 10 seconds, the microcontroller would wake up and wake the Bluetooth up. It would then remain on for another 2 seconds until it received a pairing request.
  • If it did not receive any request, it would go to sleep for the next 10 seconds.
  • Once the pairing is requested by the app, the pairing operation would be carried out with a pin code and the microcontroller would wait to finish the process.
  • If there were no opening command or buzz, it would sleep for about 10 minutes.


  1. In order to provide a sustained power for the system, a solar PV panel was considered for the lockbox that would charge its battery during day time and while the box was attached to the window.
  2. To make the box resistant to any attempt to cut it, it was made of high strength zinc alloy body and hardened steel shackles, offering extra resistance to cutting and sawing.
  3. To keep the power consumption of the product at a low level, for its Bluetooth connection, an HM-11 Bluetooth module was used. This module has a low cost and is highly suited for ultralow power consumption systems.

The final design was finished, prototyped and successfully tested within 5 months. Based on the customer’s request, the size of the board was 75mm x 46mm, and a mini PV panel was placed in front of the panel. For is power a 3.7V battery was used with a boost converter, providing required voltages for different parts of the board.

John Davis
August 9, 2019
Montreal, Canada
Electrical and Electronic Engineering