It can sometimes be tricky to find a payload even if you know the co-ordinates – if it’s landed in dense vegetation or high in a tree for example, and of course there are some rare circumstances where you won’t know the exact co-ordinates. For these reasons I’ve added software support for piezo buzzers and LED strobes to my Pi In The Sky software.

Piezo Buzzer

A piezo buzzer is a great way of locating a payload that you cannot see. I used this model from RS Components:

First we need to connect it to the Pi. I suggest that you use a “stacking header” on the Pi so that the pins go through the PITS board and you can then put a solderable pin header on top. To choose appropriate pins to connect the buzzer, refer to the PITS pin allocation table. I chose pin 16 (Wiring Pi pin 4) and pin 14 (0V) as those are free unless you are using an APRS board, which I am not. I used a dual-row right-angle header for a firm connection. Solder the buzzer to the header, fit the header to the Pi (when powered off, unless you’re feeling brave), then test to see it’s working using these commands:

$ gpio mode 4 out
$ gpio write 4 1
$ gpio write 4 0

The buzzer should sound after entering the second line, and then stop after the last line.

Now we can configure the software. Open the configuration file /boot/pisky.txt in the editor of your choice, and add the following lines:


The first line specifies which pin (Wiring Pi numbering scheme) is connected to the buzzer; the second line tells the software to sound the buzzer after descending below the specified altitude. You can omit that second line in which has the altitude will be taken from the existing “high=…” line used for image sizing.

When you start the tracker program you should then see a line like this:

Enabled Piezo Buzzer on pin 4 after descending below 500 metres

Strobe LED

The payload can also announce its position visually, using an LED strobe light as used on R/C aircraft; specifically the Strobon devices such as this one:


This the V1 model; we have tested with this and the V2 model. Both use PWM signal control rather than a simple on/off signal, as they are intended for use in R/C models where spare PWM channels are common. Both devices are incredibly bright and the makers say can be seen from up to 3 miles away at night; I don’t doubt it.

To connect to the Pi, you need to connect the control line, 0V and 5V; I used pin 12 (BCM pin 1) for the control. Note that this time we need to use BCM pin numbers since the PWM signal is controlled by a different I/O library and not WiringPi.

To configure the software, add these lines to /boot/pisky.txt:


When starting the tracker program you should then see a line like this:

Enabled PCM Strobe Light on pin 1 after descending below 2000 metres

The V2 (round) version of the Strobon has slightly different behaviour to the V1 model shown here, as it starts flashing on power-up until it sees a PWM signal, so you will see it flashing until the tracker starts; if it doesn’t stop then check that you specified the correct pin number.

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