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How-To: The Magic Phone, Part 2: The Circuit



This week we bring you Part 2 of The Magic Phone How-To: The Circuit. The Magic Phone is a project where both a DECT compatible wireless home phone and a GSM cellphone are placed inside an old rotary phone. In Part 1 of the How-To, we showed you how to reverse engineer the matrix on a phone circuit.

For the rotary mechanism to "push" the buttons on either of the two modern phones, we needed a custom circuit with a PIC microcontroller. If I had created a circuit without printing up a circuit board, the circuit would not have fit inside the rotary phone. So, to help me out, my friend Nicolas Leroy designed a circuit with surface mount components to keep the size down. Read on for the full text and photos of the creation of the circuit.

First, the finished circuit is saved as a high resolution image and the two sides are printed out on clear overhead sheets.


Next, those sheets are taped to each side of the circuit board, which is made from a special material. The circuit with both clear sheets is placed in a UV box which burns the intricate pattern into each side of the circuit board:


A series of chemical baths are prepared to permanently etch the circuit into the board. This process is similar to photo processing, with a primer, a main processing, and a finishing bath.


The front of the finished circuit after processing:


And the back of the burned in circuit:


Before soldering on components, drill all the throughholes with a mounted Dremel or precision drill press:


These holes need to be precise so take your time with the drilling process:


The components are next soldered onto the circuit with a precision electronics soldering iron. This circuit has a large half circle missing from the top edge to allow it to fit into the housing of the rotary phone. The circuit will be powered by the 2.8 Volts the DECT compatible home wireless phone provides.


Next the DECT phone is hooked up to power the circuit (two wires on the bottom right) and a programming interface is plugged into the ICSP port (cable at the center):


Here (in a lovely phone camera pic) is the circuit being programmed (on the left) and the DECT phone having its buttons pushed by the circuit. While testing it we chose to use the number 6 on the phone because on the decoded phone circuit, that number decodes to 1 and 2 on our extension cable to the buttons (please see Part 1 for an explanation of how to decode a phone?s button pad).


These alligator clips are clipped to the back of the DECT home wireless phone to pull power for our custom circuit. If you are unsure of where the power is on a circuit, simply test large areas near the battery casing. Our circuit was designed to be low power and run off of the DECT phone to cut down on battery needs inside the rotary phone.


Here?s the volt meter showing the DC volts we are pulling off the phone with the new circuit plugged in.


And finally a detail of the circuit powered up and ready to be programmed with the new 9-pin RS-232 serial port (upper right of the circuit). An LED was added to show the circuit is being correctly powered and to help with debugging.


In conclusion, a custom circuit was printed up to fit inside the vintage rotary phone?s housing. The new circuit can be powered off of the rechargable batteries of the DECT wireless phone which will also be inside the rotary phone. Tune in very soon for installment number three: Coding the Microcontroller.

Thank you to Nicolas Leroy for circuit design and printing.

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