is it possible to send 5v output from raspberry pi?

Irrelevant if the rPi is burnt up in trying to design your own interface.

I would design it to the requirements. I don't think we know what they are currently.

If I wanted to sense a mains voltage as input to a Pi, could I use opto-couplers, safely? If so, how?

Presumably, I'd need to rectify the mains, smoothy it, then drop the voltage, maybe via a resistor and zener diode, before applying to a opto-coupler. Would that work? Is there a device which would achieve all that on its own?

I'm presently using mains relays to provide a logic signal to the Pi, and that works well enough, but is a bit bulky.

Cheers, Tony

I would use a small mains relay (one with a mains-voltage coil), and sense the switch contacts.

Failing that, I'd transform it down to around 5v RMS before doing anything else.

Yes, an opto is a reasonable way to do that. I don't know you need to do all those steps. The opto *is* a diode. All you need to do is limit the current to a safe level that won't blow out the diode. But maybe I don't understand what you are trying to sense? Do you want to have the

Suitably rated ones yes. I looked at mains isolation for my heating system monitoring using an 8 IO/Switch 1-wire chip. Optos was one method I think I discounted those as it starts to get complicated running an LED on mains AC, 5 mA @ 230 V is over a watt per channel to get rid of and you have 50 Hz signal rather than a nice clean on/off indication.

There is a dedicated isolation chip out there, outputs a conditioned on/off signal. Very neat, low external component count, would need a PCB making though.

Guess what I ended up with... Mainly because I already had suitable relays and DIn rail for them from the previous heating system. Just needed a box.

It depends on how much power you want to waste. Resistor/LED will waste a lot of power. A small 5v USB charger with the 5v being used as the sense (via resistor divider) will also waste power, but I've seen them used. As will a mains powered relay.

The energy efficient way to do it is to use a high voltage series capacitor and resistor and possibly a diode to protect the LED from reverse bias.

See this page:

and read this thread on the forums from ages ago:

Gordon

an

I can tell you are not a bean counter. Getting $100 past a bean counter is easier than $200, even if you say it'll save you 10 hours work at $20/hr...

This is true, my impression is that this interface box will physically push buttons on a real CD machine of some sort to select the required disc/track.

Thanks, Gordon; those articles (especially the second link) look just the job. I'll start experimenting!

Just don't touch the live side...

Gordon

Too late...

All kidding aside, that is the down side of this circuit, the high and dangerous voltage inside the unit. These days most electronics has an external power pack which provides a low voltage to low power electronics like this. Some wall warts are really just AC transformers and can provide a low voltage AC signal to drive the sensing circuit as well as power the unit. Then there is no hazard from high voltage AC even with the unit open.

Yes, you're right to preach caution. I recall the days of working on live-chassis TVs. I also used to repair SMPSUs in VDUs. You need to be careful.

We're quite spoiled these days, aren't we.

Not always )-:

I was building a small 1U system last year and it had an open-frame PSU... And I had the lid off. It was propped under my desk and without thinking I reached into it to lift it up to check a module I'd plugged into it and grabbed something inside the PSU. I think it was the rectifier that took the 230v AC. That's the worst shock I've had. Hand an arm really felt weird for an hour or so.

Gordon

When I was a kid I played with parts from old TVs. I was doing something with a power transformer that had a 500 VAC winding. I reached behind the couch to unplug it and then grabbed the circuit and actually got burnt! Seems I had pulled the wrong plug... a lesson from the school of hard knocks.

In comp.sys.raspberry-pi message , Thu, 27 Nov 2014 14:42:55, Tony van der Hoff posted:

You should first say what sort of sensing you need. Do you want to sense just whether it is on or off, or do you want to sense whether the voltage is within EU or other applicable specification, or above or below that, or perhaps within the range that the equipment in use can accept, or do you instead want to find the actual value (and if so to what accuracy?)? What sort of response time do you need?

Once when I was a teenager, I picked up an open-frame linear supply providing 300vdc. My fingers curled under the edge and hit the bus. The next thing I knew, I felt like I'd been hit by a bat, and the power supply was about 20 feet away on the other side of the room!

That's how I learned about the value of bleeder resistors--and thinking about where my fingers were going. ;-)

And it's good to learn how to work safely around high voltages. We're turning into voltage wimps. ;-)

Not sure where I read about it, perhaps Pop' 'Tronics in the '67-7x era: An old lab or high-school electronics lab prank involved charging an axial lead tubular capacitor up to a couple hundred volts or so, holding it carefully by the body, and tossing it to someone else, while yelling, "Hey, [insert name here]!"