# Making PCBs at home with photoresistive method

As part of my modular synth project my first step is to build a power supply unit that converts 12V AC to +/-12 V. There are quite a lot of circuits about this, I'm making one based on this schematics. I decided to build it on a PCB instead of a stripboard so I can learn how to make PCBs. Here's the super high-level breakdown.

### Tools and materials

(Note: There are chemical some stuff in there that can cause serious injury so wear protective glasses and rubber gloves.)

• A PCB design app (I used EasyEDA)
• UV light
• Sodium hydroxide (for developing)
• Sodium persulfate (for etching)
• SK10 flux as a protective layer

## Step 1: Preparation

I'm using EasyEDA because it's free, simple and does everything I need. The first thing is to draw the circuit, here's how mine looks:

I convert this to a PCB (Design → Convert to PCB) and then move the components to their places. It's important to use the actual parts in the schematics from the library so that the physical dimensions on the PCB is correct. I'm a beginner in this so it's a trial and error for me to find the right spot for the components. In this case I tried to follow the actual schematic's logic.

I used a full copper area for ground otherwise I would have needed to use jumper connections and the whole thing would have been a mess.

Next, I exported the bottom layer (the blue one above), printed it on plain paper and cut out with a modelling knife.

Some DIY PCB measurements
Ground hole size = 0.8mm
Non-ground hole size = 0.6mm (can be made larger when drilling)
Ideal pad size = 2mm, preferably not smaller than 1.8mm
Track width = 1mm
Clearing = 0.75mm

## Step 2: Exposing

In a dimmed area I removed the protective layer from the copper clad board. I dripped a few drops of regular cooking oil on the copper and gently put the printed mask on upside down. (This means that I didn't need to print the PCB mirrored.)

Then using my home made UV light I expose the board for about 6-7 minutes. I tested various timings, above 12 minutes it's going to be overexposed.

When I finished exposing, I took off the paper mask and washed off the oil from the board with soap.

## Step 3: Developing

Using the prescribed dilution with the sodium hydroxide, I developed the board until the layout started to be visible. It took about 2-3 minutes.

Then again, I washed the board in 40-50C water. I used pliers to move the board around. Safety first you know...

## Step 4: Etching

Using the sodium persulfate I etched the board. This takes time so be patient – for me it was about 15-20 minutes to completely etch the board and the most visible changes are only happening in the last couple of minutes. If I used warm water the process would have been faster.

## Step 5: Protection

Finally I used SK10 flux to protect the board from corrosion and to make the soldering easier.

(Note that I added a couple of islands to the final version because big areas on the mask seemed uneven. My printer is quite shitty.)