How many testers does it take to install a doorbell? Isolating a defect in real-life

By: Mike Trites

As testers, we tend to be analytical thinkers by nature. This is a valuable quality in our work, but how often do we take note of how we exercise those skills in other areas of our life?

The following is a true story of three software testers attempting to install a wireless doorbell system and the parallels between this task and a typical software development process.

Requirements
The requirements for this project were pretty simple. In an office with an exterior door to the street, without a dedicated receptionist, a system was needed to notify staff whenever somebody entered the office.

Implementation
To satisfy these requirements, an off-the-shelf product was selected. The chosen product was advertised as a “portable wireless chime & push button” which included the following features:

• 6 chime tunes with CD quality sound and fully adjustable volume control
• Individually coded bell push button to avoid interference with other users
• 450 ft. range (137 meters)
• Low battery indicator
• Easy installation - no wires and no need to program the included bell push button

Easy installation was the biggest selling point for this particular unit. (It was also the source of what would prove to be a false sense of optimism.)

With the product chosen and purchased, the integration work for this system (installation) would be done by internal staff. Because installation was a multi-step process, we made sure to test it at each step.

Unit Test
The first step was to test each of the individual components while they were laid out on a table. These components included:

• A set of door magnets connected to a wireless transmitter
• A separate wireless push button for the exterior
• A base unit with a wireless receiver and chime

Initial unit tests all passed. Pressing the exterior push button unit activated the chime as intended and so did separating the door magnets. It seemed we were ready to proceed with the integration.

Integration Test
The next step involved mounting one of the two door magnets, the wireless transmitter and the exterior push button, then testing that they were still able to activate the chime on the base unit. At this point, everything continued to work as expected, so it was time for the full deployment.

Smoke Test
After mounting the second door magnet and the base unit, deployment was complete and the system was ready for an initial smoke test. Since the door magnets were the highest priority items, they were the first component to be tested. To perform the test, we would simply open the door and confirm that the chime on the base unit activated.

Unfortunately, the actual behaviour that was observed differed from the expected behaviour. In this case, nothing happened at all. It would seem that we had a defect.

Isolating the Defect
In order to correct the issue, we first needed to investigate and determine the root cause. Our first hunch was that, when attached to the door and frame, the magnets were no longer making contact. Adjusting their positions and forcing contact proved that this was not the cause after all.

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