This instructable details how to assemble the open source Purple Squirrel Relay Kit; a small & simple kit for switching high current loads with a low current control signal (i.e. microcontroller or programmable logic device).

This kit will safely handle 8A without any problems. I know the relay is rated for 20A but the weak link in this design is the two position terminal block. The current design allows for only 8A which should be more than adequate for most designs; if you need more than 8A you will need to solder your wires directly to the PCB in place of the terminal block. We will be working on a higher current version terminal block but that will come later.

This project is suitable for beginners. Some soldering tools are necessary but even if you've never soldered before this will not be that difficult. You can buy this kit from the Purple Squirrel website or you can gather the parts your self and using the Eagle files from the website get your own PCB(s) made and go that route.

Step 1: Getting Started

Before starting check your kit to make sure you have all the parts.

Your parts list (if you bought this from Purple Squirrel) should include these 10 pieces:
(1) PCB purple in color
(1) Relay 20A rated
(1) 3 position terminal block
(1) 2 position terminal block
(1) LED, Red
(1) 2N3904 NPN transistor
(1) 10k ohm resistor
(2) 1k ohm resistors
(1) 1N4148 diode

Step 2: Assembling the Kit

Once you have all of the parts, assembling the kit only takes a few steps. Starting with the PCB.

Step 3: Installing the Resistors

Next we are going to attach the 1k ohm resistors. You may need to pre-bend the leads like this to get the parts to fit.

Once the resistor is in the location it is supposed to be turn the PCB over and bend the leads a bit as shown…this will hold the part in place while you solder it down.

After soldering the leads down, trim them off so they look something like images.

Step 4: Installing the Diode

Now we will attach the diode. The process is the same as for the resistors as shown.

Step 5: Installing the NPN Transistor

Installing the NPN transistor: Pay attention to the silkscreen marking on the PCB as it shows you how to put the part in. If you notice you will see a flat edge on the transistor. This flat edge corresponds to the flat edge on the PCB silkscreen symbol for the transistor.
The leads on a NPN transistor when looking at the flat edge are EBC (emitter, base, collector) and the leads on the PCB correspond to this.

Step 6: Installing the LED

The LED, a diode that gives off light, conforms to a polarity scheme as well. The longest lead is positive (the Anode) and the shorter lead is negative (the Cathode). The PCB shows a flat edge on the silkscreen marking for the LED. This flat side indicates the side that the negative lead or the Cathode should be installed in.

Step 7: Installing the Terminal Blocks

I installed the three position block first but could have installed the two position if had wanted. The thing to remember with these parts is that you cannot bend the leads to hold the part into the PCB for soldering. What I did was turn the PCB upside down, install one of the terminal blocks into its position and use the other to hold up the other side of the PCB while I soldered the leads down for the first one.

Step 8: The Relay

Installing the relay should be done last. Mainly because it takes up so much room and would make installing the other parts more difficult. However waiting till last also makes installing the relay easy. Insert the relay into the PCB from the top and then turn the PCB over so it is resting on the relay with the leads pointing up as shown. Then just solder the leads down.

Do some final clean up and trimming of any leads that are a bit long and you should be finished.

When it is all said and done your relay kit should look like this last picture when you are finished.