Building a starting gun

An electronic starting gun is a useful tool since it is entirely risk-free and lets you control how loud the sound should be. It can also be connected directly to SprintTimer, as described in an earlier post. This makes it insensitive to external noise, and you can set the SprintTimer sound detection to ”always on.” But while a gun is not very expensive, it still costs from around $200. So if you are on a budget, or like DIY projects, you can build one yourself. That might sound hard, but it actually just a question of pretty basic wiring and you can get the components for under $10. It will also be more flexible than commercial guns since you can decide what type of sound you want and how to trigger the start.

I will go through the necessary steps and components. The heart in our starting gun project is a small sound recording board called ISD1820. It costs around $5 and can record a 10 s sound through an on-board microphone and play it back at the push of a button.

Testing the board
If we start by looking at the board itself, there are several switches and connector, some that are relevant for our project, and some that we can ignore. The first steps are shown in the figure below (click on the image to make it larger).


You need to supply power to the board. It requires around 3V, so two AA or AAA batteries are suitable. You should connect the + on the battery to either of the pins marked VCC and the – to either of the GND pins.
Next, you should attach a loudspeaker to the speaker connector. Any speaker with max 8 ohm impedance (e.g. from your stereo) can be used.
The two switches for ”repeated playback” and ”play through” can be turned off by pushing them left. 

You are now ready to test your ”gun:”
Push and hold the Rec button while you say something or make a sound.
Release the Rec button and push PlayE to play your recording. 

Did you hear something? If so, congratulations, you now have a working sound-player (more about recording later).

Improving the volume
As you already have noticed is the sound not very strong, and it is far too weak for outdoor use. So, unfortunately, you need an amplified speaker, but that is also the case with the commercial electronic guns that you can buy. You can use a portable stereo/boombox, a megaphone with aux port or similar. This means that you need a cable with audio plug suitable for your amplifier (usually 3.5 mm) at one end. You can either buy a plug separately and attach it to a cable, or buy a ready-made cable and cut off one of the plugs. Your connection should now look like this:

Attaching SprintTimer
The next step is to get your device to trigger SprintTimer. One might think that you can plug in the loudspeaker cable into the headphone port, but that is NOT possible. The signal is too high, and the connectors are wrong. You, therefore, have two choices: One is to make another loudspeaker cable, for example use the other half of the cable you just cut, and attach in the same way as the first. This means that you will have two identical cables connected to the speaker outlet. Then use a converter cable from, e.g. Røde or Headset Buddy as described in the Starting gun post. I have used the Røde adaptor in my starting device showed in the top image above.

Alternatively, place two resistors before the plug, use a 4-pole (TRRS) plug and connect the two inner bands. The resistor R1 is there to lower the signal and 300 – 500 kOhm is usually suitable. The purpose of R2 is to tell the iPhone that there is a microphone connected and it should be about 2 kOhm (I have successfully tested both 2.2k and 1.5k). This solution costs less than an adaptor cable.


You should now be able to plug in the cable in the iPhone and get the sound bar in SprintTimer to react when you push the play button.

A better start button
Pushing the PlayE button on the board is fine while testing, but it is not very practical in the field. You, therefore, want to attach an external button. You can use any type of button, but preferably a push button that returns to off when you release it. You simply connect it to the PlayE and GND pins close to where you attached the battery. So your final wiring will look like this:

Note that using an external switch opens to using other types of switches. You can for example attach at flip switch to the starting gate used in alpine skiing or dog racing.

Sound recording
It can be a little tricky to get optimal results when recording the sound. The microphone is not so sensitive but does on the other hand overload easily. So you have to experiment with how loud the sound that you are recording should be to get a good result. Also, remember that this not is a hi-fi equipment, so don’t set your expectations for sound quality too high. It is also important to push the recording button at the right time. If you push it too early, you will get an initial silence when pushing the play button. Start recording too late, and you might cut off the sound. You can make the sound yourself, or search the net for a suitable sound that you play back on you computer. If you want the sound that is used in SprintTimer, you can play it here.

The ISD1820 board is sold in many different versions, a search at Amazon gives over 300 hits. The layout of some of these boards is different from the one shown above. But the functionality and connections are usually the same, so using a different board shouldn’t be a problem. The boards are often sold with a small loudspeaker that can be useful for testing, but it is not enough for real use. The 3.5 mm plugs are widely available, usually in a soldering version, but screw terminals are also available, (see an earlier post). Alternatively, you can cut a ready-made cable with a male plug at each end. Finally you probably want a case to enclose everything in. But there are literally millions of standard electronics boxes, so finding one shouldn’t be a problem (unless you want one that looks like a gun 🙂