September 27, 2021

Piezo Preamps with EasyEDA & JLCPCB

Would like to share two things here, first of all I am making a piezo pick-up for my classical guitar, and secondly this is my first time designing a PCB with EasyEDA , ordering and assembling with JLCPCB. So this is a bit of a beginners guide, to the things that I did not understand immediately or got wrong the first few times.
See below for a complete quick-guide on producing your first PCB with EasyEDA – from a beginner for beginners.

Piezo pick-up

I wanted to try out making a (cheap) guitar pickup with a piezo, as I had some lying around. I was also wondering what it would sound like and how high-quality it could get.

Make a Contact Microphone

Instructable : contact microphone – this guy just fixes a piezo to a guitar cable and jack.


In step 2 he puts together a contact microphone pre-amp with a JFET 6 more components. This is nice and simple and can be soldered tother by hand, see the circuit below.

How It Works

Elliot Sound Products

However I next stumbled across this great page sound-au.com. This site is incredible, there is so much content here you should check out. From the immense list of projects, to articles, application notes – ESP also sells PCBs of custom designed HiFi audio equipment.

Elliot Sound Products – Piezo Pickup Preamplifiers

Now it is immediately clear that he really knows what he’s doing, he simulated and fine tuned all these circuits and their behaviour. I immediately knew that this was the circuits I wanted to test out. I decided to start with the first two circuits for different capacitances, in Figure 1 – Bootstrapped Piezo Amp and Figure 2 – High Capacitance Piezo Preamp , both circuits are copyright of Elliot Sound Products. As he says, “With these circuits, tests are essential.  While all circuits simulate perfectly, that doesn’t necessarily mean that they will behave themselves in real life…” – so I will get these manufactured but am expecting to swap out a few resistor values if necessary. Good thing I’m getting 10 of them.

EasyEDA

Now I had come across EasyEDA and wanted to give it a try so I thought its perfect for this project. It’s an online editor but has an offline version too, that both look and behave quite as I expected from using Eagle previosly. In fact, the fuss with getting the correct symbols and footprints took up most the work for me, so an integration with online database of parts and footprints was what caught my attention in the first place.

There is a generic EELib of generic symbols for you to quickly build something

So I created my schematic which translates to PCB and lets you design and route your PCB with few frills but it does the job


















Electronic Components

Now when it comes to using real components instead of just the symbols you will find these in the Library.
Click on the Library button to add parts to the schematic – and now let me save you some time :

I first went for Library > ‘LCSC Electronics’ components tab, as this has a good range, and is easy to search with categories and Filters that you are used to from online distributors. (screenshot below)

LCSC Electronics

HOWEVER – If at the end you want to let JLCPCB assemble your PCB, then you need to chose components from the JLCPCB SMT Parts Library. This time click on Library > EasyEDA > ‘JLCPCB Assembled’ (screenshot below). As you can see below, this page does not have the nice filtering anymore. But in a way its not as necessary as there is fewer components. You will be happy to find the right one and just use it. I kind of like this restriction though in a way – it’s like a challenge to see what you can make from what there is. E.g. electrolytic capacitors in 33uF there was just one – and it was MASSIVE, see below.

Also, and this comes as less of a surprise, assembly is only offered for placable SMT components, so through-hole components are excluded, which means not pins, no buttons or switches, no variable resistors, no connectors. But this is fine as these are huge and you can solder them yourself.

JLCPCB

Still in the end I decided to remove the variable resistor which is a volume control anyway, as in this design I wont have a nice volume knob to control it anyway, maybe in the next one.
JLC also did not offer the NJM2068 opamp used in Figure 2, a quick research I decided to go for the NJM4580, if anyone has a better suggestion I am all ears.
So I also replaced the nice 6.3mm jack connector with a screw terminal I can solder on, or just solder a cable straight to the pins.
Also no more switch to turn on and off, I can just unplug the battery to do that.

PCB routing

Generate your PCB from the schematic with ‘Convert to PCB‘ and you get this.

Now in Eagle I think it is the ratsnest function that highlights all pads connected to this net / node. EasyEDA does not REALLY have that. But it does. When you are dragging a part here it will highlight all the connections on the nodes of this parts, but it is hard to see when which of the two pads is connected to which net, especially when they are crossing everywhere

EasyEDA will however always highlight all the pads connected to a node when you hover over a track/ wire connected to it. So I often ended up just connecting a short wire to a pad to highlight its node, see below :

So I ended up laying vague routes and connections to see what components and pads needed to be close to each other and where connections need to go, or just leaving short tracks connected to pads so I can see where they need to go.

With a bit of rearranging I ended up with this

Ground Plane

I decided to go for a Ground plane on the top Layer, which is really simple. See Copper-pour instructions here

1. You need a board outline. select track (W) and click on the BoardOutLine Layer for drawing

Now you can just leave all your grounds unconnected, I am highlighting them here below :

2. Then select ‘Copper Area’ (E) in ‘PCB Tools’ and select GND as the Net to connect to – this only works if you placed a ground symbol in your schematic.

3. Next just draw the same square as the board outline. Tipp : I left out one corner of the board so I could easily click on the Copper area later to hide it. By changing ‘Fill style’ from ‘Solid’ to ‘No Solid’ as it gets in your way while routing.

Tooling holes

SMT assembly requires tooling holes, now it gives you the option to add them yourself of let JLCPCB add them. I decided to add them myself following these intructions. – Now unfortunately these instructions seem really outdated as neither pads nor vias gave me the option of changing the drill size.

Now in the Library I found a few users’ tooling holes, but somehow none of these was the right size of 1.152 mm :

My workaround: you can add single pins from the EELib > Connectors’ menu to your schematic. Add them to the PCB, right click and ‘Attributes’ let you make all the modifications you need to, according to the JLC instructions :
Round
size: 1.152mm size
plated: no
Solder Mask Expansion: 0.148mm

That’s what I send off so we will see if it works or not.

Panelling

Now when ordering your PCB there are all these options for panelling / panelization.

This confused me as I could not tell if this asking if my design is panelled, or if I want them to panellize. However it did not matter since “Panel by JLCPCB” is not supported for SMT assembly. And actually you can panel perfectly in EasyEDA yourself.

Just select Tools > Panelize

V-cut cuts grooves into the PCB that later let you just snap it apart. This however is also not available in combination with SMT assembly, so ‘Stamp hole‘ is your only option.

Simply select the rows and collumns, and adjust the spacing if you need to. The result will look something like this






















Price

Now a note on the economics of panelling.
Firstly the minimum quantity is 5 pieces, so if you panel 5 PCB into your single PCB you immediately end up with 25 pieces.

Secondly it made hardly a difference in price whether I order 30 piece of a single PCB or 5 pieces of a panel of 6 PCBs. The PCB cost itself is the very small, for me the main cost were the components.

In the end, I went for 10 pieces of the single PCB this was just a first test. If they work well I will add switches and knobs and a casing and then I can reconsider volumes. But at 10 pieces I have enough for testing and some spare if somethign breaks.

See the Charge details below. Left : 5 pieces of 6 panel PCB – Right: 30 pieces of single PCB

5 pieces of 6 panel PCB
30 pieces of single PCB

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So this was my final result : )

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