Seeing this article on Hack A Day (and another one where it is connected to an apartment entry buzzer button) got me thinking about a way to make mobile phone activated switch that is simple with as few parts as possible (and cheap). The idea I had was to utilize the existing smarts of a mobile phone coupled with its vibration motor to power a relay or press a button.
WARNING: This is hack will alter your phone and any servo’s permanently and may break them permanently. Perform these modifications entirely at your own risk.
To get started you will need the following bits.
- A mobile phone with vibrate (so basically any old cell phone will do).
- Obviously it also needs to be connected to a cell network.
- A hobby servo (smaller is better)
- Some wire (I salvaged mine from an old cat5 cable)
- Electrical Tape
- Double sided tape for mounting
- Solder (For Soldering Iron)
- Soldering Iron
- Needle Nosed pliers
- Wire cutters
- Mini Screw Driver Set
Extra Parts experimental tests
- Two LED’s
- A large capacitor around 3000uF
- Bread Board
- A diode
- Extra wires
- Remove the battery from the phone.
- Locate the screws holding it together.
- Unscrew all the screws (My phone had star shaped security screws, to open I used a flat head screw driver that fitted snugly and it unscrewed no problems.)
- Carefully remove bits until you locate the pager motor (see pic below).
The two black and silver things above are pager motors. If you need more reference do a google image search on ‘pager motor’.
Motor Removal and wire attachment
- Remove the pager motor taking note of where its connections are (My phone has the connections on the other half of the phone, so the motor connects when the phone is screwed together – your phone may be different).
- Fire up your soldering iron.
- Strip the ends of two wires, making them around 20cm long.
- Now solder the two wires to contacts where the pager motor was connected to the phone.
- Next make a hole in the case somewhere for the wires to come out (I used pliers to make a small hole next to the phones connector).
Now put everything back together how you pulled it apart. Except make sure the two wires you have attached poke neatly through your phones newly made hole.
Servo modification and attachment
Now that the modifications to the phone are done it is time to modify a servo.
- Unscrew the servo bottom and remove the motor and gear assembly (If you can access all the connections you don’t have to remove the motor and gear assembly).
- De-solder the motor connections from the servo electronics (red and black on my servo – some servo’s are different though so be careful – google your make of servo if unsure).
- Solder the connections directly to the motor connections within the servo (Should be two large solder points at the bottom of the servo’s motor).
- Re-assemble the servo and its gears.
- Now connect the wires from your phone to the servo and insulate with electrical tape.
- Test with vibrate mode on the phone. If the servo turns in the wrong direction reverse the connection of your wires between the servo and the phone.
This will depend on the phone, but what you need to do is set your phone to vibrate mode so that calls activate our newly installed servo.
If you used a newer phone you may be able to set it up to only vibrate for certain callers.
Now you just need to use some double sided tape to attach the servo to a switch or button so that when the servo turns it will press the button.
Call your cell phone! The servo motor should turn and activate your device. You may need to re-position your servo for it to work well.
Here is a video of my test:
Mobile Phone Switch Video.mp4
- No variable cost (eg does not cost each time you use it, as you are just pranking the phone). You still need to pay for the account though.
- Low parts count
- Easy to implement
- No need to modify the thing being remote controlled (eg no need to re-wire your apartment buzzer and lose your bond/deposit).
- Anyone who has the mobile number can activate it (on simple phone I used).
- Motor strength struggles and it is only just powerful enough to flip a light switch.
- Servo motor may be too big for phone and be damaging it (more circuitry could fix this however).
- Use of a phone that lets you customize vibration to specific caller id’s would allow activation by only authorised numbers. My Motorola KRZR lets you do this but I wasn’t willing to sacrifice it.
- A smaller servo would probably be more easily driven from the phone, which would result in more pressing power.
- Use the signal to trigger a relay and then drive higher power or other things.
- I could probably attach this to my kettle Cool!
I performed the following tests during development of this hack.
Initially I connected the phone wires to a bread board with two LED’s in opposite polarity to see the signal produced from the pager motor connections. By using two LED’s like this I was also able to ascertain the polarity of the wires.
I also used my multimeter to measure the voltage produced by the pager motor connections. The voltage was 3.3 volts.
Also not pictured I put a 3300uF capacitor into the circuit. This was relatively effective at smoothing the pulses produced by the phone. This meant you got a stable 2v for around 3 seconds which would be good for powering a relay or transistor.
Thanks for watching.