Turn an RC control toy into a remote control to start and stop recording on your iPhone.
This instructable in combination with a free app allows you to remotely start/stop videos or time-lapse photography.

Background: We created a couple of apps that start recording wirelessly for use with our mountable iPhone case: http://www.iconic.am. We came up with this hack to start the camera from a distance.

required items:
1: the cheapest RC toy you can find (we used a $15 car from Target)
2: iPhone + iconic.am (video) or TimePress (time-lapse) app. Apps are both free and ad-free.
3: basic screwdriver set
4: soldering iron
5: hook-up wire, if you want to mount the batteries/radio away from the phone.

Step 1: Tear It Apart!

Remove all extraneous pieces from the toy. You will need only the radio, the battery box/power switch and one of the two motors.
Depending on the toy you use, you may want to cut off the front and back of the chassis using a hacksaw and great care to not damage anything vital.
Choose the motor based on which of the controls on the remote you want to use to activate the camera.
We chose the drive motor, which is the forward/reverse control on the remote.
Desolder the other motor or cut the leads to it as close as possible to the circuit board to eliminate the likelihood of a short.

Step 2: Make an Electromagnet

Unsolder the leads from the motor. If you want to extend the leads to mount the radio away from the phone then unsolder them from the radio side also and attach your longer leads to the radio.
Remove the motor from the gearbox. Pull the sprocket from the motor using pliers or by prying up on the gear with a flathead screwdriver. Whether you have to pull the sprocket before or after removing the motor from the gearbox will depend on the design of the toy you bought.
Bend the tabs away on the motor housing and remove the end cap. Pull the rotor out of the motor housing.
Connect the motor leads to the wires of one of the coils of the rotor. Most small motors have three commutator bars. Your electromagnet will be the coil between the two commutator bars you choose to connect to. The active electromagnet is shown with the 2-headed arrow in the cutaway below. It is easiest to solder where the end of the windings connect to the commutator. This soldering will take higher temperature and more time than normal, as motors usually have high temperature solder.
We cut (shown with X) between the two inactive coils to minimize losses due to current flowing through the other coils.

Step 3: Clean Up and Start Playing

We cut off the output end of the motor shaft (optional) and wrapped our newly created electromagnet in electrical tape. Mount the electromagnet right on the Apple logo on the iPhone for the best sensitivity. For our testing we simply taped it to the case.
We recommend mounting the radio assembly in a small enclosure and the electromagnet in a small plastic tube.
Download one of our two free apps from the app store: iconic.am (for video) or TimePress (for time-lapse photography).

While the app is running, it monitors the magnetic field. Any significant jump in the field will activate recording. We're planning on using this remote for a flyby cam and for making timelapses when the phone is mounted in some out-of-reach spot.

Here we are testing the remote with our app TimePress: