ThirdRail Build Instructions

Congratulations! You’re a few simple steps away from owning a great little music-tech gadget that will serve you for years and that you’ll have the satisfaction of having built yourself.

Building this project takes about half an hour if you’ve done some DIY electronics before, complete beginners should expect to take a little longer. The instructions assume that you can identify components, solder and do simple voltage and continuity tests with a multi-meter. There are lots of resources available to help you learn these skills – just hit Google or YouTube and you’ll soon be ready to start!

But before you do… we highly recommend that you

  • Take the time to read through this guide a couple of times
  • Check your parts against the Bill of Materials at the bottom of this page to ensure that you have everything that you need

Enjoy building your ThirdRail, we hope you make great music with it!


To complete this build you will need

  • Soldering iron
  • Solder
  • Wire cutters
  • A voltmeter or multimeter

You’ll also need a clean, well-lit workspace with a heat-resistant surface to work on.

Build Instructions

Putting ThirdRail together is very simple, as there are only a handful of parts of just a few types.

All components go on one side of the PCB – the side with the part numbers printed on it.

Solder parts into the marked positions; where a part has to go a specific way round there will be markings on the PCB to guide you, and a clear note in the instructions.

We add parts to the board in height order, starting with the shortest parts (this makes the PCB easier to manage and handle during the build).

D1 and D2 – rectifier diodes

_MG_7279_thumbDiodes have to be inserted the correct way round – so make sure that the white bar at one end of the diode matches the bar on the printed symbol. Set the diode into place, check it, then flip the board over and solder it into place. You can tidy the wires now with a pair of cutters or do them later (I tend to do mine as I go along as it makes the PCB easier to photograph).

C2 – ceramic capacitor

_MG_7280_thumbLocate C2 and solder it into place. This type of capacitor can go either way round.

U1 – voltage regulator

_MG_7282_thumbThis component takes the incoming voltage of 9 to 12 volts DC and brings it down to a nice, steady 5V. It’s important that it goes in the right way round, but if you take a close look at the photo you’ll see there’s only one way it will actually fit. First thing you’ll need to do, though, is make it bend at the knees.

Hold the component so that the flat, metal “back” is away from you, the black resin “front” facing you. The components legs are wider at the top – about a millimetre below where the wide part of the leg ends, bend the leg 90 degrees away from you. Do this for all three legs – you can use pliers or even just the straight edge of a ruler to do this. You should now find that the legs fit straight into the holes in the PCB and the flat “back” of the component sits flush on the exposed area on the PCB – the holes in the component and PCB should line up, too.

Although it’s not essential to bolt the component to the board, it’s worth doing – if only to help keep it still during soldering. Use an M3 machine screw and nut if you choose to do this.

J1 to J4 – 4-pin headers

_MG_7284_thumbYou’ll find one of these in the middle of each edge of the PCB. Working with one at a time, locate the pin header in the holes, flip the PCB over and solder into place.

You might find it useful to just solder one pin, then check that the header is straight, making any adjustments before soldering the remaining 3 pins.

J5 to J10 – box headers

_MG_7285_thumbThe Eurorack power headers have to be correctly oriented in order to function correctly and not damage your modules, so take great care to ensure that the gap in the header shroud matches the gap in the printed outline on the PCB silkscreen.

Position all of the headers, then lay a piece of thin card over them to help you to hold them in place while you flip the board over. Slip the card out from under the headers then solder one pin of each header into place.

Before you continue to solder the remaining pins, flip back over and check once more that all of the headers are correctly oriented.

If you are happy that everything is laid out correctly, continue to solder all of the remaining pins.

C1 – electrolytic capacitor

_MG_7290_thumbElectrolytic capacitors are polarised – they have a positive and a negative connection and these have to be the right way round. Find C1 on the PCB – you’ll see that one hole has a “+” symbol next to it. This shows that the positive pole of the capacitor should connect here – the other hole is, therefore the negative.

There are two ways to tell which connection is which on an electrolytic capacitor:

The positive lead will be longer than the negative lead

The negative lead will be marked by a white stripe down the side of the body (or “can”) of the capacitor

So, put the long leg of the component into the hole marked “+” – this should mean that the white stripe on the body is on the side that faces your 10-pin box header J5. Push it down snug to the board and solder it into place.

That’s it, your ThirdRail is ready for testing and use.


Before we connect the ThirdRail to a power supply, we’ll make a quick test for short-circuits. You’ll need your multi-meter for this step.

With your meter set to detect continuity

Pick any one of the edge pin headers J1 to J4. Using your meter, ensure that there is NO continuity between the four pins.

Next, choose one of the 16-pin IDC headers and ensure that there is NO continuity between any two pairs of pins. J6 is a good candidate for this test, as it has the pin designations printed clearly below it.

If your ThirdRail passes this test, it’s safe to connect it to your power supply and continue with the next set of tests. If it fails this test, take a good look over the back of your PCB for shorts between soldered connections. Rectify and try again.

With your meter set to measure voltage

_MG_7383_thumbConnect your ThirdRail to your modular power supply by connecting the supply to the 10-pin IDC header. Select one of the edge pin headers J1 to J4 to measure. Touch your meters BLACK probe to pin 3 (GND).

Touch the RED probe to pin 1 (+5V). You should measure approximately 5V here (a small amount of drift is acceptable). +5V here means that all of your components are working correctly.

Next, touch the RED probe to pin 2 (+12V).  You should measure approximately 12V here (a small amount of drift is acceptable).

Finally, touch the RED probe to pin 4 (-12V).  You should measure approximately -12V here (a small amount of drift is acceptable).

Repeat these tests on one of the 16-pin IDC headers. J6 is a good candidate for this test, as it has the pin designations printed clearly below it.

IMPORTANT NOTE: These tests are designed to ensure that ThirdRail

  • is safe to use
  • will not damage your power supply
  • will not damage your Eurorack modules

Skipping these tests, carrying them out improperly or ignoring the results can carry serious risks and consequences for you and your equipment. In proceeding to connect ThirdRail to your equipment, you are assuming liability for any consequential loss or damage.

Using ThirdRail

_MG_7381_thumbTo get power into your ThirdRail, you must connect your Eurorack PSU to the 10-pin header J5.

If you want to power more than 5 modules with your ThirdRail, you can use the edge pin connectors (J1 to J4) or one of the 16-pin box headers (J6 to J10) to connect your ThirdRail to one of our MiniBus boards. See the MiniBus Build Instructions for more information on how to join MiniBus boards together.

Bill of Materials

In addition to your PCB, you’ll need parts. The full list of parts for the project is given below, with Mouser part numbers to aid you in identifying compatible parts. Although we’ve used all Mouser parts in the example build above, you may be able to obtain cheaper compatible parts by shopping around.


Reference Value Description Qty Mouser Part Notes
C1 470uF Electrolytic capacitor 1 661-EKYB250E471MJC5S
C2 220nF Multilayer Ceramic Capacitor, 50V 1 594-K224M20X7RF53H5
D1, D2 1N4004 Rectifier diode 2 621-1N4004
J1, J2, J3, J4 4-pin, male IDC header 4 571-6404524
J5 2×5-pin, male Shrouded IDC header 1 538-70246-1001
J6, J7, J8, J9, J10 2×8-pin, male Shrouded IDC header 5 538-70246-1601
U1 L7805 Voltage regulator TO-220 1 511-L7805CV