I have heard of some other people incorporating payphone like props into their displays, so I decided to try my take on it. The cost of a used payphone was more than I wanted to pay, so I ended up buying a red phone with the push buttons located in the old rotary phone style. A push button phone was required because I needed to use the buttons as inputs to my microcontroller. The plan was to have all the electronics within the phone and use a cheap MP3 player to play the dial and busy tones, and the messages.
The old push-button phones are pretty straight-forward and are ripe for being reworked. I took the phone apart and there was one main circuit board in it with an HM9102D IC on it. It is hard to find info for this part, but what info I could find showed me that the push buttons are organized in a 4×4 matrix. So the things I need to monitor are: 1) the buttons pressed and 2) whether the phone is off the hook or not (which I can just treat as another button), pretty basic.
The Microcontroller and its Code
For this project, I went back to an Arduino platform, and the Nano in particular. This is a great little micro for an amazing cost. You can get them direct from China (so they may be clones, but that was fine for my task) for less than $3 each (including shipping)!! The Arduino is quickly growing on me and may replace all my PICs at some point.
On the Arduino, I am using 8 digital pins for the push button Matrix, one for an LED (for debugging purposes), two for hardware serial for debug, two for MP3 player software serial, one for whether or not the MP3 player is still busy playing, and one to detect if the phone is hung up for not.
Upon boot, the micro monitors the headset and waits for a user to take the phone off the hook. Once the phone is off the hook, I start playing a dial tone MP3 on repeat mode. Then I start taking turns strobing the 4 rows of push-buttons, one at a time with a ground (0V) and read the 4 columns one at a time. If I see a 0V (ground; the inputs have internal pull-ups on them) on one of the columns, I figure out what button was pressed and play that MP3. I then monitor the BUSY signal from the MP3 player as well as the headset off-the-hook button, if the user hangs the phone up, I stop the MP3 player and go back to the beginning of my program, but if the MP3 finishes playing its audop and the user is still listening, I play a busy signal MP3 on repeat until the user hangs up the phone.
The messages are a quick 7-digit dialing sound, followed by some amount of ringing, and then finally various voicemail messages.
Weather was going to be a major issue for this prop since it is an indoor phone. The plan (it is not finalized as of this writing) is to created a lighted box that has a plexi-glass door on the front. The user can then open the door and use the phone. When they are done, they can close the door to protect the phone from the elements. Nothing too fancy, but it should be good enough to keep most of the rain off of it. I will probably add a magnet to keep the door closed during heavy winds.
There isn’t too much to the power requirements here. The Arduino can run off of a USB plug, so I am using one of those standard USB cell phone wall chargers to power it. The MP3 player also takes 5V, so I power it off of the 5V output on the Arduino board. The spotlight to light up the enclosure is just a 120VAC LED spotlight, so that will be straightforward as well. All-in-all, this will be a very low-powered prop.