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Another look at a sound activated camera trigger.




In a separate article I showed a way to control your camera through an Ipod or Iphone (or any MP3 player really). After I published this article, I received a few emails with feedback. A reader from Finland noted that other people have been using a simpler way to accomplish this same task using an optocoupler instead of an active amplifier as in my article. I decided to investigate that option further and post my findings here.


Figure 1 shows the basic idea. Similarly to the previous article, you connect a standard 2.5 mm audio jack to the camera remote trigger input and an audio jack to the iPod or iPhone. Connecting these two jacks is a simple optocoupler device. In this case I used an FOD 817C that you can buy from digikey.com though other models could also be used. Google the datasheet for the optocupler you use and note the polarities as shown in Figure 1.


 Figure 1

Visit a local thrift store and you can most assuredly find tons of discarded film cameras and old slide projectors. For this project, I used a lens extracted from an $8 slide projector. Some gluing and drilling later, I had the following contraption ready. Figure 2 shows the completed project. After soldering the wires as shown in Figure 1, I encased the optocoupler inside a plastic pen cap and sealed it with hot glue.


Figure 2


Simple as this project is, it actually works! You can use most sound sources to trigger the camera. An Ipod or other MP3 players can do it but feel free to experiment with other sources. I suspect one could use a walkie-talkie pair for a simple remote controlled trigger for example...

One thing to be mindful of is that this circuit requires a stronger input audio signal than my original project (there is no amplification here after all). I've been finding that this often requires setting the iPod volume near it's maximum. This of course will somewhat depend on the sensitivity of the optocoupler you use but since they typically require that you forward bias a diode at the input stage, I expect this behavior with most implementations. If this is an issue and ends-up draining your Ipod batteries, then you can always implement my original project which is much more sensitive as it has a built-in gain/amplification stage.

And finally here's a tip: If you have some old AC adapters laying around (from old laptops, routers, cellphone chargers, whatever) you probably already have an optocoupler available to you assuming you are willing to gut one of these devices and do some desoldering. Infact, the coupler I used above was extracted from an old AC adapter I had kept around. Once you open the adapter, you should be able to identify the optocoupler by looking for a component resembling the one shown in Figure 1. In AC adapters (switch-mode supplies only) optocouplers are normally used as feedback from the DC side to the (switching) circuit on the AC side in order to control the resulting output voltage. Therefore, you will usually find them at the boundary between the high-voltage portion of the PCB and the low-voltage DC-side. See Figure 3 for an example (in this case we find two optocouplers in a PC power supply).

Figure 3