I was asked by @Alamar and @Atomic_Charge to turn this into its own thread, it took a while but here it is.
I recently started building my own effect pedals for bass, partly because of the 1-month challenge on here, partly because I'm studying electrical engineering and partly because it simply saves a lot of money (the fuzz I built costs 300 bucks new and I built it for less than 30, the actual components on the PCB cost a few dollars). Many effect pedals consist of very simple circuits that you can easily build on a perf board or buy pre-made PCBs for with the schematics openly available on the internet. I went a bit of a different route and designed and made my own PCBs, but that's because I have that option available to me, you don't actually need to do that in most cases.
The PCBs: I used Eagle to draw the schematic and design my boards, I've used it in the past and while it's not the most amazing program out there it gets the job done and is available with a freeware license (limiting you to a 100mmx160mm board but that's plenty). Here are the 2 examples of my boards + schematics.
Madbean Afterlife compressor:
Woolly Mammoth fuzz:
and the finished boards:
The process of building them:
I've heard on a few forums that many people are afraid of DIYing their pedals, honestly I'm very much biased on this because I'm literally studying this kind of stuff right now, but the process is very very simple. If you're decent at soldering (which I'm not, as you'll see I'm actually not that great at it) you'll be totally fine. A few things you'll need to keep in mind is that, depending on what you're building/doing, you might need to make your own bypass/switch wiring. Wiring true bypass with an LED is actually very simple, your power input jack already has a switch integretad to switch off the battery when you plug in the jack, etc., just keep in mind that you can't just wire the PCB up directly to the input jacks for a normal stomp box design.
true bypass:
starting the build:
preparing the switch wiring:
preparing the potentiometers:
a mess of cables:
the finished pedals (with external battery compartment because I built them for a friend):
Going with a kit/pre-made PCB:
I also tested that out since I wanted an auto-wah. It's, again, quite simple especially if you have a detailed manual.
The hardest part you'll face here is correctly identifying components (might actually be a bit harder if you haven't messed with electronics before), but besides that it's the same thing. I'd also like to talk about flexible vs stiff wiring here for a second, all my pedals have been done with stiff wiring, it has the advantage that it keeps the PCB in place without needing any additional support and it stays wherever you put/bend it and you can easily get it through holes (in the PCB and also on the input jacks or wherever), the only downside is that it can break at sharp angles and that you need to keep the wires short.
Flexible wiring is a pain in the ass to work with IMO, this kit came with flexible wiring and it took me forever to wire up the PCB in the end, maybe I'm just incompetent but i'll stick with stiff wiring for now. Flexible wiring has the advantage that you can use longer wires since they're easy to tuck away.
"I want to design my own circuits/come up with my own designs":
In that case you should look into simulation programs, LT Spice IV is free and works well, you can draw the schematic you designed, simulate a sine wave and see how it affects the signal. Obviously looking up and studying electronics will be your first step, but through trial and error you could probably come up with something interesting or come up with a modification to an existing design. After successful simulation you'd probably build it on a breadboard as a prototype so you can still alter it, and after that you could make your own PCBs or use a perf-board for your final version.
How do they sound:
I made a demo, clean, then compressor, then the fuzz, then the auto-wah and lastly a combination of the auto-wah and the fuzz. I literally built some of them yesterday so just see it as a rough example, with some tweaking and getting used to them you will be able to get a much better sound out of them. Also please ignore the actual playing, I'm still not very good at bass and fiddling with knobs while trying to play didn't make it any better: https://clyp.it/jjpjtfuz
and lastly all 3 of them:
