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Thanks for posting, Bean, we all go thru dark places from time to time, and just saying it out loud can help.  I appreciate all you do for us, both here on the forum and thru Madbean. 

On your original question: Something like an Alesis Micro Gate might be a good starting point.  One of these in a "Y" connection with your main signal chain could open the gate to the second branch of the "Y" at a certain signal level.  It wouldn't suppress your primary signal chain, but the secondary branch would be silent until the signal level threshold is hit, and return to silence when the level falls back below the gate threshold.  Or, you could run a separate signal in/out of the Micro Gate and trigger it with your primary signal source level. As a bonus, they're inexpensive and stereo, the two sides trigger together but signals are independent.
Manual: https://ia802808.us.archive.org/5/items/synthmanual-alesis-micro-series-owners-manual/alesismicroseriesownersmanual.pdf

After that 47k, there are two loopback paths, one to the gain pot lug 2, and one to the output of IC1.  I'd check everything in those paths, if the signal is good up until the 47k.  IC2 could also be faulty, try pulling it and see if the signal is good all the way up to it's empty socket.

The power on IC1 and IC2 are correct, IC1 runs on +9V and ground, while IC2 runs on +18V and -9V for more headroom.  The voltages on IC3 also look OK. 

I think the key is that the output is low both during bypass and when it's active.  That narrows it down to the following, which is the buffered bypass path, along with the foot switch wiring.  You might want to use an audio probe to see where the signal falls off.

• input jack
• 2 resistors and a cap on the input side of IC1
• IC1 which is the buffer
• 4.7u cap, 560R and 100k resistor in the bypass path (unlikely)
• 68k and 100k resistors at the output
• output jack

Rod Elliott addresses this at https://sound-au.com/articles/power-supplies3.htm

"A common recommendation is to use two or more capacitors in parallel, with each having a different value.  While you might think this could spread the resonance and reduce its effects, it does no such thing.  A 100nF cap (for example) in parallel with a 10µF cap behaves more-or-less identically to the 10µF cap by itself, even if one or both has a significantly higher/ lower ESR than the other.  You may read that the values should be 'non-harmonically related', meaning that they shouldn't be half, double, three times or any other simple multiple.

Just like any other caps in parallel, the larger (or largest) value will dominate, but the performance is much the same as you would expect of paralleled capacitors.  The total value is the arithmetic sum of the individual values used.  The only place where it's common (and useful) to use unequal value caps is when bypassing opamps and RF circuits.  A monolithic/ multilayer 100nF ceramic should be in parallel with the supply pins, and it's usually wise to include 10µF electros (usually between each supply and ground) where the DC enters the PCB.  This has nothing to do with snake oil, but compensates for the track and wiring inductance that may otherwise cause instability."

In other words, use your 100 uF for the DC supply and skip the rest.  Do, however, add 100 nF bypass caps from +V to ground right at each op amp.

Pretty much anything should work.  In the build doc you posted, the designer said they used LDR's from Tayda, and all the ones there are around 10k-20k resistance in the light, and 500k to 1Meg in the dark.  Most of them have peak sensitivity around 650 nm (red), with one peaking around 560 nm (green).  You could select an LED color to match, although the build doc mentions using orange diffused LEDs which are halfway between @ 600 nm.

If you have both signal and noise when using a power supply, then it's noise from your power supply.  In addition to mjg's answer, try adding some filtering to the pedal's power input.

But if you're not getting any signal at all in active mode, just that noise, and you get signal when using a battery, that sounds like your power jack wiring needs attention.  In that case, see below. 

The 3-lug DC power jacks have an internal switch, plus an internal-battery lug, a power-to-the-circuit lug, and a ground lug.  They should be wired as the name implies.  With no power plug in the socket, the switch makes, and the battery lug connects to the power-to-the-circuit lug, and you run on internal battery.  Putting a power supply plug in the socket breaks the switch, which disconnects the internal-battery lug, and connects the external power plug to the power-to-the-circuit lug. Visuals: http://beavisaudio.com/techpages/pedalpower/

If you have the wires to the DC jack battery lug and power-to-the-circuit lug reversed, it would work on a battery, but not on a power supply.  Try swapping the two hot wires.

Welcome to the forum!

The way you've wired power directly from +9V to the +9V point on the board, the rectifier isn't needed.  To use it, you'd input 9V AC or DC power to the A and C holes, the rectifier would ensure the correct polarity, and then it goes to +9V bus including the charge pump, and the ground plane.  Your wiring is fine, you just don't need the rectifier. 

For placement, the rectifier + and - legs (output) go south toward the +9V bus and ground plane, and the two wavy AC lines (input) go north toward the A and C holes.  I found the layout in the data sheet for the CBRHDSH1-40L.  If it's installed backwards, swap it around or remove it completely.

Regarding your trouble, the bypass signal is OK so the jacks and switch are OK, and the LED lights up so you have +9V coming to the footswitch daughter board.  The op amps need +18V from the charge pump, so check that you have +17 or +18V on pin 8 of the op amps.  If not, trace the +18V bus back toward the charge pump, checking the soldering of the components that come off pins 2 and 8 of the charge pump. 

This should do what you are asking:  Normally thru the pot to the output, but plug a cable into the first NMJ4 and it reroutes signal to that cable and to your 2nd box.  From the 2nd box output, you can come back into the second NMJ4, which will bypass the pot and hit the output.  Or you can just take the output cable of the 2nd box somewhere else and not come back at all. All cables are standard TS guitar cables.  The input jack on your amp may be switching something, depending on whether there's a cable plugged into it or not, but the input cable is standard TS.
 

Welcome back!  If you can post a sketch of how you want to connect it in your signal chain, we can make some recommendations on which jack(s) will do it for you, I've built a few boxes like you describe.  In the mean time, here are the various switching configs available in Switchcraft (pdf file) and Neutrik (png file.)

Hey, I'm sorry to hear about that.  I ship via USPS lot, and I've had some packages do strange things.  One day a substitute carrier picked up an outgoing box at my house, and mistakenly scanned it "delivered" instead of "picked up."  So for the entire trip, I couldn't see the status and had to call the helpline and have them look at it.  It was handled and delivered as usual, I just couldn't see it online.  Another time a package went back and forth between two cities several times, I called the helpdesk and they said yeah, we call that looping, it happens sometimes.  They called the postmaster at one of the cities, who intercepted the package and rerouted it directly to the destination the next day.  Try calling 800-275-8777 first thing in the AM or right after lunch, the USPS national helpdesk is quite customer-focused.  I used to live next door to one of their supervisors, and she was really focused on quality and customer experience.

Smallbear usually carries the NSL-7530/7532, but they're out of stock right now.  They do have the 9203 in stock which also works.  Both of these are listed in Bean's build doc. https://smallbear-electronics.mybigcommerce.com/photocells-cds-5mm-diameter/

The USPS website has a bad day once every couple of weeks, but it's vastly improved since a couple of years ago, and it's working fine at the moment.  Since 1970, the USPS has been a quasi-governmental agency, established by the Constitution but not funded by the government and not a part of the executive branch.  Revenue comes solely from user fees (postage.)  Since 2020, it's been run by a Postmaster General who's done his darndest to kill it so that the services can be privatized.  So far he's failed, but he has dealt some crippling blows including shutting down several major sorting and distribution centers nationwide, which has slowed down delivery in some locations.  Neither I nor any of my family work for USPS, but I'm on a first-name basis with my route delivery guys and girls, and they tell me stuff.   

I did a search at Mouser for SMD BJT that meet or exceed all the relevant properties of the MPSA18, and they have 47 choices.  Here is the link to search results.  The noise factor and beta are not searchable properties, so you'll have to sort those qualities for yourself, but this should give you a head start.  Once you identify the part number you need, Mouser has a presence in Portugal, pt.mouser.com.   
 
https://www.mouser.com/c/semiconductors/discrete-semiconductors/transistors/bipolar-transistors-bjt/?collector-%20base%20voltage%20vcbo=45%20V~~-&collector-%20emitter%20voltage%20vceo%20max=45%20V~~-&collector-emitter%20saturation%20voltage=200%20mV~~-&configuration=Single&emitter-%20base%20voltage%20vebo=6.5%20V~~-&gain%20bandwidth%20product%20ft=160%20MHz~~-&maximum%20dc%20collector%20current=200%20mA~~-&mounting%20style=SMD%2FSMT&pd%20-%20power%20dissipation=625%20mW~~-&product%20type=BJTs%20-%20Bipolar%20Transistors&transistor%20polarity=NPN&rp=semiconductors%2Fdiscrete-semiconductors%2Ftransistors%2Fbipolar-transistors-bjt%7C~Maximum%20DC%20Collector%20Current%7C~Collector-%20Emitter%20Voltage%20VCEO%20Max%7C~Collector-%20Base%20Voltage%20VCBO%7C~Emitter-%20Base%20Voltage%20VEBO%7C~Collector-Emitter%20Saturation%20Voltage%7C~Pd%20-%20Power%20Dissipation%7C~Gain%20Bandwidth%20Product%20fT

In your photo, there's not much solder on the north leg of R2, and the same with a lot of the other components.  I think I'd reflow each of your solder joints and add a bit more solder, there should be a small fillet around the legs of each component and many of them don't have that (R8, R5 jumper, etc), which could indicate cold joints = no connection.