In a Nutshell

The Phone Call Monitoring Facility (PCMF), or Phone Call Monitor for short, is a device that detects the ring signal on a telephone line and provides visual indication of incoming calls, separately showing the number of those that potentially could have left a voice mail. (The somewhat strained acronym was meant to capture the initials of the gadget's two designers.)

Background

There was a time in telecommunications history, when telephone land lines were still prevalent, but the "local" answering machines (often playing a memorable role in movies from the 80's and 90's, just think of Office Space) were being replaced by the voice mail feature from phone service providers. One obvious difference is that this "remote answering machine" is invisible to the user, therefore there needs to be another way to show that new messages have been received. One typical solution was a "Message Waiting" indicator on the phone equipment itself, which would blink until all new voice mails have been played.

At least in theory... In my office at Lawrence Technological University, where I was a faculty member for a period of time, this indicator light was not working, which made me miss important voice mails on a regular basis. (Granted, the phone line also gave the usual stuttering dial tone upon unread voice mails, but that required explicit polling, i.e., me remembering to pick up the phone and listen to the dial tone - not a big help!)

As an engineer, who enjoys tinkering, I decided to attack the problem with a home-grown solution. Coincidentally one of my students, Monica, was looking for a good senior project idea just around that time, and since she was showing a particular interest in microcontroller-based design, all the pieces fell into place. The project turned out to be the perfect size in terms of its complexity and scope, and gave my student the opportunity to experience a complete product development cycle, from inception to deployment. Best of all, unlike many student projects, this one had a very practical application in the real world. The picture below is an artist's rendition of the development process.

Features

Through its RJ11 jack, the PCMF is connected to the telephone line, where it monitors the ring signal, the high-voltage AC waveform the phone exchange puts on the line to make the phone equipment ring (originally spec'd for the very ancient ones with a mechanical bell). It then prints the total number of calls, as well as the number of calls that rang long enough to leave a voice mail, on an elegant dot matrix vacuum fluorescent display. The device also has a second RJ11 jack, so it can be "daisy-chained" like the answering machines and caller ID displays of the time.

Once acknowledged, the counters can be cleared by pressing the Call Count Reset button. Since the number of rings, after which the voice mail picks up, varies across the phone exchanges, this value needs to be adjustable. On the PCMF the user can do this from the Setup screen, which is entered by pushing down on the Call Count Reset button during power-up; the selected number is then stored in non-volatile memory.

Press

Monica and I wrote an article about the project, which was published in the June 2008 edition of the Circuit Cellar electronics magazine. It shares the full technical details of the device's operation, and also recounts Monica's fun-filled learning experience.

Click on the image above to open the article in PDF format (posted on this website with the permission of Circuit Cellar).

Current Use

Two specimens of the PCMF were built, one for each designer. My copy remained in actual service at home for many years after my move from Lawrence Tech, as my telephone set didn't even have a Message Waiting indicator. The device got decommissioned when I finally relinquished the "land line" service from the cable TV provider. However, the beauty of this gadget and all the hard work Monica had put into it would have made it a sacrilege to just throw it in the bottom of a box - instead, it is now being converted into a clock.

Project Details

The technical details of the project broken down into categories can be found in the following sections:

	Hardware
	Firmware