Obsolete Cellphone Remotely Controlled Switch

Phone number dialing and processing relies on an audio signaling method known as dual-tone multi-frequency or DTMF. The dialing keypad on a touch-tone phone is a DTMF encoder. Each key press communicates the two-tone sound that corresponds to each of the twelve keys on a standard phone keypad. Phone network equipment can decode the tones into a digital representation of the phone number to control automatic switching. DTMF can also be used outside of phone networks to perform a variety of others functions.

In this article, I'll share a method to re-purpose an obsolete cellphone with a functional keypad, into a DTMF remote control encoder, for wirelessly activating/deactivating a relay switch.

Supplies

Electronics

• A working (service deactivated) cellphone with a physical keypad.
• MT8870 DTMF Decoding Module
• Bluetooth Audio Receiver
• Rechargeable Bluetooth Audio Transmitter (with volume control)
• 2 - 3.5mm Male-to-Male Stereo Audio Patch Cables
• Small Breadboard & Jumper Wires
• Dupont Jumper Wires
• Micro USB Breakout Board (power connector)
• 5VDC Power Supply (USB two ports)
• 4 - 2N3904 NPN Transistors
• 1 - 2N3906 PNP Transistor
• 4 - 1K 1/4W Resistors
• 1 - 10K 1/4W Resistor
• 1 - 1N4148 Diode
• 1 - 5VDC Relay Switch

Optional:

• 1 - 470 ohm 1/4W Resistor
• 1 - LED

Tools & Materials

• Spudger or Guitar Pick
• Precision Screwdriver Set
• Wire Cutter/Stripper
• Hot Glue Gun & Glue Stick
• Super Glue
• Soldering Iron & Solder
• Drill or Rotary Tool & 3mm/1/8" Drill Bit
• Multimeter

Step 1: Cellphone Disassembly

Note: If your phone has a headphone jack, you can skip to step 7.

Remove the phone's battery cover then the battery and the SIM card. Using a spudger or guitar pick, pry the phone's plastic chassis halves apart by sliding the tool along the seam to disengage the fastening clips. There may also be screws that need to be removed to allow the halves to separate.

Step 2: Modify a Patch Cable

Cut an end off of one patch cable. Strip back the outer wire jacket, then strip each of the three wire ends. Use a multimeter set to test for continuity, and identify the two wires that correspond to the 3.5mm plug's center and wire end segments. The wire that corresponds to the tip end of the plug is unneeded, and will be used as a strain relief.

Step 3: Install Audio Output Cable

Drill a hole for the cable in the chassis near the speaker, making sure there is clearance for reassembly. Pull the cable through, then coil the unneeded tip end wire around the cable jacket. Use a few drops of super glue to fix it in place as a cable-stay/strain relief.

Step 4: Add Lead Taps to the Speaker Terminals

I used a portion of a resistor lead, to tap off of the speakers spring terminals. Insert the lead end under each spring terminal, then heat the lead with a soldering iron until the lead melts into the plastic webbing. Add a drop of super glue to better secure it in place. Trim the lead, leaving enough length to solder a wire to it.

Step 5: Solder Wires to the Speaker Taps

Solder a wire to each speaker tap, polarity is not important. Push any excess wire into a cavity that wont obstruct the chassis halves from mating. Fill the cable entry point cavity with hot-glue. Make sure the speaker taps aren't going to short out on the circuit board when reassembled.

Step 6: Reassemble Phone

Reassemble the chassis halves by carefully squeezing the parts together until the clips snap in and seat. Reinstall any screws that were removed. Insert the SIM card and battery. Loosely fit the battery cover, then mark for a cable clearance notch. Notch the cover, then install it.

Step 7: Breadboard the Circuit

The MT8870 module decodes each of the twelve key's tones into a 4 bit binary code output on 4 data lines. I chose to build a 4 bit AND gate that corresponds to the number 7 key.

Labels Q1 through Q4 correspond to pins Q1 through Q4 on the MT8870 module.

Step 8: Connect, Pair, Use

Using dupont jumper wires: Connect pins Q1 through Q4 on the MT8870 board to the corresponding nodes on the breadboard. Connect VCC and GND to 5V and ground on the breadboard.

Connect the breadboard to a 5VDC power supply via USB cable.

Plug the Bluetooth receiver into the USB power supply, then link the receiver to the MT8870 module using the audio patch cable.

Plug the audio out from the phone into the jack on the Bluetooth transmitter.

Pair the transmitter to the receiver.

Make sure the transmitter volume is set to maximum.

Power on the phone.

Pressing the number 7 will activate the relay switch, it will remain latched until another key is pressed.

Press any other key to deactivate the relay switch.