Great, good luck with that :)

Another thing that comes to mind: for audio purposes another technique used in ring modulators for audio effects is to use a mosfet switch to mix the signal with a square wave. This has more byproducts than mixing with a pure sine, but is a lot easier to do. Since you are downconcerting, it should not matter at all if you use a square wave, since the byproducts will all be (higher-order) harmonics of the local oscillator, which you’ll filter out anyway.

I’m not sure I understand what you are trying to do… do you want to build a radio? Or are all your signals in the audio range?

Anyway:

• w2aew has some great videos on diode ring mixers, like this one about winding the transformers: https://www.youtube.com/watch?v=a8ViWS61hsU
• You can also get ready-made diode ring mixers (including transformer and everything) like this one: https://www.minicircuits.com/pdfs/SRA-1W+.pdf

Regarding Gilbert cells, the two popular chips are MC1496 and SA631. The 631 comes with a built-in oscillator, so it’s quite handy. Unfortunately both are hard to come by these days.

Growing plants with LEDs certainly works, I’m just wondering if the power of a USB port is enough

there’s no need for relays if you have low voltage

That’s a good point. Also makes me wonder if those lights will actually make a difference when it comes to plant growth, given the low power.

It’s not completely stupid. One pissible issue: when the light level is around the threshold, slight variations (think: cloud passing by) will cause the relay to switch back and forth quickly. This can be solved with some additional parts (keyword: hysteresis).

I would recommend using a microcontroller, it makes this problem easy to solve, plus you can have more logic to trigger the lights (eg time based).

Another thing is turning off: if it’s purely based on light level, you need to make sure the sensor does not “see” any of the LED light, otherwise it thinks the sun is back up

The only winning move is to cook a nice curry, because plain rice is just boring.

It’s like if farmers were just letting plants do all the work, instead of manually assembling the potatoes themselves

Try searching for “coreless motor”, gives me lots of ebay results with tiny 7mm diameter dc motors looking like this: https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.0qbVZk69iUxzXpp-87VHFwHaHa%26pid%3DApi&f=1&ipt=34626bb8832b480226ede8e0a44a8013a56d9217b7cb576ec699024a06297899&ipo=images

Afaik that’s a myth. There is no testing or certification associated with the CE logo, it’s just a self-declaration of the manufacturer that certain standards are met. You can just buy rolls of these stickers, or print them yourselves. That’s what manufacturers do, legit ones and shitty ones.

You could probably increase the 82K and 10K resistors to be much bigger

That’s what I thought initially, but this stackoverflow post dissuaded me. The argument there is that the measurement will be wrong, if the input current is not enough to charge the internal cap within the measurement period. But I’ve done some testing now, and measurements done with 820k and 100k agree well with what my voltmeter measures, so I’ll go with this solution!

a fresh alkaline 9V battery is actually 9.6V or more, not 9V.

Indeed! 9.6V * 10k/92k = 1.04V is still below 1.1V, so I should be fine in this case :)

9V battery voltages droop noticeably when under load because of their high internal resistance. Make sure to measure under the same conditions.

This is a good point!

My firmware will be pretty monotonic though, basically:

1. wake up
2. measure battery
3. measure some other sensors (the actual task of the device)
4. turn on a transceiver, send all the measurements (including battery voltage)
5. turn off transceiver & go to sleep

So, the load should be always the same at step (2).