split fractal
What's the best (ideally cheap and compact) way for a 3.3 or 5V DC device to detect whether 120-140V AC is present?
To add a wrinkle, the actual load may be physically disconnected with a switch (which my research suggests might not be an issue? but not 100%) sure.
rough owl
Most of the approaches boil down to an optocoupler of some sort. I've been known to do it with high value resistors and an off-the-shelf optocoupler, but you can roll your own with a neon bulb (and current limiting resistor) and a photocell. The usual warnings about working with mains voltage apply.
You can also use things like a 120V relay, or a small transformer connected to a voltage detection circuit, or even a capacitive dropper style power supply.
split fractal
Interesting. I also came across this, and it'd be nice to not touch the high voltage at all (but not a requirement). Not sure how reliable that is though https://www.instructables.com/Contactless-AC-Detector/
rough owl
It somewhat depends on what you're actually detecting. For example, there's a difference between "some voltage is present" and "there's power available to do something".
split fractal
Yeah makes sense. In this particular case, it's to detect when my dryer is done. Currently it has a really horrendous buzzer (that thankfully you can turn off), but it seems to be driven by a relatively brief AC circuit. My hope is to be able to detect when the wire leading to the buzzer switch is energized and be able to send my phone a notfication or something.
rough owl
This sounds a little frivolous, but I've actually done this in the past: pull the guts out of a cheap plug-in USB charger and use that. It'll give you 5V when power is available, which you can either use directly or as an input to a microcontroller.
split fractal
Ah yeah I was thinking about something like that. I've noticed that some of those chargers still take a second to start sending voltage after they receive AC, but I'm sure there's at least one that'd be fast enough for that.
rough owl
Do you even care if you find out a few seconds later when the dryer is done? Sending a notification to your phone is probably going to take a little time too. Or is the problem that the beep is too short?
split fractal
Right I think it may be too short, but will have to test
rough owl
I'm kind of leaning toward the optical approach if you want quick response. Replace the buzzer with a bulb (neon, incandescent, whatever) and have a photocell near it for a contactless signal transfer.
split fractal
I think the signal is less than a second, or a best slightly more
Yeah that may be the way to go. Thanks for the suggestion! I hadn't thought of building my own optocoupler for this.
rough owl
It is totally doable. You can have a sheet of glass or clear plastic between the emitter and detector to improve voltage standoff.
I've even seen it done with a fiber optic link to a remote receiver, worked great.
It's easier with thicc fibers like https://www.adafruit.com/product/4164
