Making a long sound cable

Using the sound from e.g. a starting gun is the most accurate way to start the clock in SprintTimer. A problem, however, is that the start often is so far away that the sound drowns in the background noise. To transfer the sound to SprintTimer at the finish you can use a microphone with an extension cable or a Walkie Talkie. If you have two devices and access to a network you can also use Start Sender, which is a flexible, and almost as accurate option as sending the sound. Using a Walkie Talkie is pretty common and usually the best solution for really long distances, e.g. in rowing and kayaking. But the most stable and accurate solution for distances up to 100-150 m is a sound cable.

The problem is only that 100 m long microphone extension cables for the iPhone/iPad are pretty rare (or rather, no-existent), so you have to make one yourself. Making a long sound cable, however, is not difficult or expensive. What you need is:

  • 100 m simple cable with 2 or 4 wires
  • 3.5 mm audio plug, male, with 4 connectors (TRRS)
  • 3.5 mm audio plug, female, with 4 connectors (TRRS)
  • Microphone (e.g. the headset from the iPhone)

The cable can be the most basic indoor telephone cable you can find. I paid around $20 for 100 m. A cable with 2 wires is enough, but 4 wires give some more flexibility and usually doesn’t cost more. A cable should be pretty easy to find in local electronics shops. One example

The audio plugs are a little more difficult item. It is important that they have 4 connectors (called TRRS), not the more common 3 connector version, otherwise they can’t transfer the microphone signal. There exists two types of plugs, either for soldering the cable or solder-free with screw terminals. The soldering version is widely available, but require that you have a soldering iron and basic knowledge of how to solder. Soldering tips

The screw terminal version makes it much easier to connect the cable, but is unfortunately more difficult to find. Two manufacturers are Velleman (items CD027 and CD029) and Shenzhen Flower Electronic (which I happened to find at my local electronics shop). See links at the end.


To transfer the microphone signal the two inner most rings of the plug must be connected (3 and 4 in the image below). So if you have a cable with two wires they of course go there. If you have four wires to play with you can choose to connect two wires to each of the two microphone connectors for increased reliability. Or you can connect one wire to each of the four connectors. The latter means that you also can use the cable to attach an external loudspeaker. Just remember to connect the same wire to the same connector on both sides, the wires are usually color coded to make this easier.


Below is the set up I used to successfully send the sound from the start in a 100 m race. The mic of the headset was taped to the starting gun.


Links to 3.5mm plugs with screw terminal (in some cases only sold in packages)
Velleman CD027
Velleman CD029
Velleman CD027
Velleman CD029
Kjell&Co (Sweden and Norway)