Some time ago, I've used an USB-based relay to control a XFD (extreme feedback device). Now, let'g go a step beyond that and let a relay dance.

Update: You can control and watch christmas lights yourself using Midi at http://heckenlights.org. Heckenlights are my publicly controllable christmas lights.

Today you get nearly everything. So I grabbed myself an Ethernet-based relay. This relay has 8 channels and is able to control each of that individually. The UK price is currently some £52.50. The big advantage to the USB-based model is, you do not need any drivers, only an IP-Address and a free RJ-45 Ethernet port. That's it. My plan is, to use the relay as basic controller for Christmas lights, but that will be handled in a future post.

As Java developer, i'd like to control the relay using a server-application, so this can run on my Raspberry Pi. I want to use Midi files to control the relay state, and for sure, I don't want to deal with C-code. Luckily Java provides a Midi API to access the Midi devices. On the other hand I have to bring the Midi-System and the Relay (non-Midi-device) somehow together. Why also not implementing a own Midi-receiver?.

The API is quite simple. A Midi-receiver implements two methods: close and send. Close is intended to free resources, send is used to dispatch the Midi message. Midi-messages are usually 3 bytes long. Sometimes there are longer messages, but for now they are out of scope.

The first byte consists of a Hi and Low-Byte. The High-Byte (also the first four bits) describe the command (Note on/off, Control change and so on). The Low-Byte contains usually the Midi-channel (0-15). There are also System Midi events, which use different Low-Byte information. For Note on and off, we have to see the second and third byte. The second byte is the note/key (C, G#, Ab), the third byte specifies the velocity/volume of that event. Now we have everything, we need to control the relay states.

The next challenge is to play/sequence the commands in the right order and time controlled. Therefore we can use the built-in sequencer. The trick is, to provide the custom Midi receiver as target, then you'll receive all the Midi commands in the right moment.

Receiver receiver = new Receiver() {
    public void send(MidiMessage mm, long timeStamp) {
        byte[] bytes = mm.getMessage();
        byte message[] = null;
        int t1 = 0;
        int t2 = 0;
        int t3 = 0;
        byte hi;
        byte lo;

        if (bytes.length > 1) {
            t1 = bytes[0];
            t2 = bytes[1];

            if (bytes.length > 2) {
                t3 = bytes[2];
                message = new byte[bytes.length - 3];
                if (bytes.length >= 3) {
                    System.arraycopy(bytes, 3, message, 0, message.length);
                }
            }
        }

        hi = (byte) ((t1 & 0xF0) >> 4);
        lo = (byte) (t1 & 0x0F);

        if (lo == 9) {
            // drum channel, we skip that guy.
            return;
        }

        if (hi == MidiHelper.NOTE_ON || hi == MidiHelper.NOTE_OFF) {
            System.out.println("Note: " + t2);
            System.out.println("Velocity: " + t3);
        }
    }

    public void close() {
    }
};

Sequencer s = MidiSystem.getSequencer(false);
s.getTransmitter().setReceiver(receiver);
s.open();
s.setSequence(MidiSystem.getSequence(new URL("http://www.bluegrassbanjo.org/buffgals.mid")));
s.start();

The last part is a mapping between the Midi notes and the relay commands. The Midi number for C4 is 60. For my use-case I map several half-tones to it's higher parent (so C and C# control port 1, D and D# port 2 and so on). I've made two videos, showing how it works.

Put all the things together and you can control your relay. Or see https://github.com/mp911de/midi-relay for a ready-to-run application.

You can also go beyond: Play live instead of providing Midi-files. Grab a Midi input device such as a Midi-keyboard to route the Midi message (live-played) to the relay.