Circuit to trigger a camera using a mobile phone, PDA, or MP3 player

Este artículo también está disponible en Español: Circuito para controlar la cámara desde un movil, PDA, o MP3.

On night photography, long exposures are the norm: sometimes several seconds, and sometimes several minutes; I also tend to employ HDR techniques, and that means that I often end making series of nine or even more photos all together. Spending half an hour (sometimes more) with the remote trigger on your hand can be quite boring, so I toyed with the idea of making this process authomatic... and I found a way to control the camera from a mobile phone, a PDA, or even an MP3 player.

The problem

One of my favorite 'recipes' stats with a one-second exposure, and then I double the time on each take, up to a final four-minute exposure; but, when the scene is specially dark, it may require more exposures, up to thirty seconds.

My problem is that cameras are generally not ready for this type of work: it is uncommon to find cameras capable of doing exposures longer than 30 seconds, and automated sequences of nine exposures are quite rare too. There are commercially available intervalometers, sure; but they are expensive, and I have never heard of one capable of doing this type of series.

The idea

So I began wandering on the idea of building one myself, as cheap as possible. My camera (a Canon 400D) has a specific connector for remote triggers, which can be used to release the shutter quite easily; in fact, there is a myriad of schemes on Internet to build your own trigger. And I came to the conclusion that the easiest way to make one would be to connect the camera to a mobile phone: this way I could control the timing by software, and have an almost infinite flexibility; all I needed was to make that connection possible.

And I figured I could use the earphone output from the mobile telephone: using the circuit shown here, it is possible to connect the audio output from the gadget to the remote trigger input from the camera. Basically, we just need to detect when there is a sound on the output, and simulate a press on the trigger release button while that sound lasts. Then, using a small JavaME application we could produce beeps of controlled length, and those beeps would control the shutter on the camera.

The circuit

WARNING: I must warn readers that I am not a professional of electronics; I cannot guarantee that this circuit will not fail, or that it will not break your camera, I cannot event guarantee that it will not produce alopecia: it works for me, and I tried to make it completely safe both for the camera and the mobile phone. But I cannot take any responsibility.

The output form the sound source enters through the connector at the left. C1 is just a decoupling capacitor. Then, the signal is amplified at Q1 and filtered by C2. IC1 is used as a comparator, and R4 is there to adjust the circuit to the signal level at the input (usually, it can be left at the intermediate position). D1 is an activity indicator LED, which should go off when the circuit releases the shutter. Finally, IC2 is an opto-isolator, and at the other side we have the connectors that go to the camera; in my case, I used a 2.5mm stereo JACK.

Conclusion

As can be seen, the circuit can be used with a 5v power source, because that is what I had available at the moment; but my intention is to adapt it to use a 9v battery: I do not expect having to change more that a couple of resistors. In fact, I think that none of the components has a critical value, I made this with what I had lying around. Soon I will publish the modified version, along with photos of the real construction of the device; I will also publish the software used to drive the camera.

Considering that the input to this circuit is the earphone output from the mobile phone, it could be used with almost any PDA, instead of the mobile phone. In fact, it could also be used with an MP3 player, if we previously record a sequence of beeps on a computer, and we put that 'song' into the player; then, reproducing that song with the player connected to the camera through this circuit, we would be triggering the camera at the rhythm of the sequence of beeps that we generated with our computer.

Links

• Homebrew Wired Remote for Canon EOS 350D Camera: Good instructions to build your own basic remote trigger.
• HDR-Jack - The ultra-compact interval and HDR trigger: A programmable trigger, using a micro-controller.
• Long-exposure night HDR photography with arduino (aka bulb mode bracketing): Another remote trigger project, with screen.