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. But a stable and accurate solution for distances up to 100-150 m is a sound cable.

The problem is only that 100 m long sound cables for the iPhone/iPad are pretty rare (or rather, non-existent), so you have to make one yourself. Making a long sound cable, however, is not difficult or expensive.

You obviously need the cable itself, but it 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 more flexibility and usually doesn’t cost more. A cable should be pretty easy to find in local electronics shops. One example

Audio Plug
The second item is the audio plug. There are a couple of parameters to consider when picking the right one:

  • Diameter (usually 3.5 mm)
  • Male/Female
  • Number of connectors (TS, TRS, TRRS)
  • Soldering or screw terminal

The number of connectors you need depends on what signal you want to transfer (more below) but there are essentaill three types:

How you choose to connect the cable depends on what you find convenient. The soldering version is widely available, but require that you have a soldering iron and basic knowledge of how to solder. The screw terminal version makes it much easier to connect the cable but is, unfortunately, more difficult to find (see links below).

3.5 mm TRRS with screw terminals

There are two scenarios/setups when using the cable. One is that you have a microphone that you place close to the starting gun (or whatever makes the sound). The other is you have a device (e.g. an electronic starting gun) that sends out the sound electronically.

A microphone cable
In this case, you obviously need a microphone (not necessarily a high quality one). If you have an iPhone headset of the older type with an audio plug at the end, that is perfectly fine. Otherwise, you need a microphone that is made for the iPhone and has an audio plug (not Lightning connector). If you have another type of mic, you must check how it is wired.

Then you need two TRRS (four rings) audio plugs. One 3.5 mm male for the iPhone side. The other side depends on the microphone but is usually a 3.5 mm female. To transfer the microphone signal the two innermost rings of the plug must be connected (3 and 4 in the image below). Just remember to connect the same wire to the same connector on both sides, the wires are usually color coded to make this easier. If you have a newer iPhone you also need the small audio connector that comes with the phone.

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.

Two TRRS (screw) and an Apple Headset (old version)

A sound out cable
If you have a device that is making the sound itself, like an electronic starting gun, you need a cable that is more like a loudspeaker extension cable. That means that you should have a TS or TRS audio plug, you don’t need stereo so both are OK. Again a 3.5 mm male on the iPhone side and a plug that fits the starting gun on the other (usually 3.5 mm male).

However, you can not plug it directly into the iPhone. The wiring is wrong and it won’t be recognized as a microphone. You, therefore, need an adapter like the Røde SC4 or the Headset Buddy ECM between the cable and the iPhone. If you have a newer iPhone you also need the small audio connector that comes with the phone. You can read more about connecting an electronic starting gun in this post.

Two TRS (solder), a Røde SC4, an Apple Lightning connector and a Gill gun.

3.5mm plugs with screw terminals
Velleman CD027
Velleman CD029
Kjell&Co (Sweden and Norway)

Loudspeaker to iPhone/iPad mic converters
Røde SC4 (Amazon)
Headset Buddy ECM (Amazon)