Connecting an electronic starting gun to SprintTimer

The most accurate method to start the clock in SprintTimer is to use the sound from the starting gun, either directly in Photo Finish or via Start Sender. You need a microphone placed close to the starting gun to get a sound that is significantly louder than the surrounding noise (cheering crowds, wind etc). But with an electronic starting gun, an even better solution is to connect the starting gun directly to the headset jack on the iPhone/iPad.

The main advantage of a direct connection is that it is completely insensitive to external noise since it is using the electronic signal, not the sound. This means that you can set the SprintTimer sound detection to ”always-on”. Electronic starting guns have a sound output port so they can connect to a loudspeaker. Since you still want the loudspeaker connected, you must first split that signal. Most starting guns have a standard 3.5 mm 3-pole plug (TRS), so you can use the standard headphone/sound cable splitters that are pretty common and easy to find.

Microphone adapters from Røde, Movo, Boya, Reytid, and Andoer

The second item you absolutely need is an adapter that connects the loudspeaker cable to the headset port on the iPhone/iPad. You should not plug the cable directly into the iPhone. But there are a few converters available, e.g. the Røde SC4 (more info below). If you have a newer iPhone, you will also need the Lightning-to-headphone adapter that comes with the phone.

I tested the setup with a starting gun from Jex Sport. It is also sold as Gill in the US and Hart in Australia. The gun has two ports, one for sound out, the other for connecting timing equipment. I only used the sound port. When plugged into the iPhone, the gun gave a signal that almost hit the max on the scale, so I could set the start level to essentially any level to trigger a start.

Scenario 1: Sound cable from start to finish

You can choose to connect the sound cable directly to the iPhone/iPad at the finish line. This is a simple and reliable solution, but you might then need a long cable, which is a hassle. You also have to make it yourself; see this post.

Scenario 2: Short sound cable and Start Sender

Alternatively, you can send the signal via Start Sender. A zero test can then be used to check that everything is OK.

Microphone adapters
The adapter mentioned above is absolutely essential and has three functions:
– Shift the connectors, so the sound reaches the microphone input.
– Make the iPhone turn off the internal mic.
– Reduce the signal to protect the mic port.

The Røde SC4 has been around for a while and I have used it for several years. There are also several alternatives available that cost less. I have tested a couple of them and they worked as expected (links to both the manufacturer and Amazon):
Røde SC4 — (Amazon)
Movo MC3 — (Amazon)
Boya BY-CIP2 — (Amazon)
Reytid SC4 replacement — (Amazon)
Andoer EY-S04 — (Amazon)
Headset Buddy ECM — (Amazon)

Testing the adapter
As mentioned, will the adapter turn off the internal microphone. This usually happens immediately, but, occasionally, you will have to restart SprintTimer with everything plugged in. Or reconnect the sound cable to get your iPhone to use the external source. To test that everything works, disconnect the loudspeaker. Then make some noise. The soundbar should not react at all. Then press the starting gun. You won’t hear anything, but the soundbar should jump. The icon also changes when an external mic is used.

You should also check your cables and all connectors. A bad connector might cause an electric noise which the microphone port interprets as a sound. You should be able to freely move and shake the start gun without any motion in the soundbar. A bad cable might otherwise cause a false start.

You can also connect the starting gun to a MacBook running Start Sender. Modern MacBooks has a headphone port that also can be used for microphone input, provided that you plug in a 4-pole TRRS plug. To connect a starting gun, you can, for example, use the SC4-compatible plugs mentioned above. Some more tips can be found here.

Appendix: Adapter schematics
The schematics below explains how the adapter works. It also enables you to make a dedicated start gun cable if you like small DIY projects.

The R2 resistor tells the iPhone that there is an external mic attached and that it should turn off the internal one. It should be around 2 kOhm (I have successfully used both 2.2k and 1.5k). The resistor R1 is there to lower the signal and protect the iPhone; 300 – 500 kOhm is usually suitable.