Relay advice please

[John J]

I'm struggling to understand what you are trying to achieve. I'm running cctv systems around the farm and another in our village hall which record continuously day and night onto hard disc drives in standard dvr's. The 16 channel hd kits record over 7 days before overwriting while the 8 channel lower definition units have longer retention times. All the cameras and the recorders are powered by 12 volt dc supplies. The camera signals to the dvr's are 1 volt peak to peak into 75 ohms. The dvr has its own smps rated at 3A, the cameras are split over multiple smps units for redundancy but most of the cameras are well below 1A each

[Dave Liquorice]

Or pop the cover off and pull a strip of paper between the contacts while they are manually pressed together.

[The Natural Philosopher]

Oh. I'm not struggling. It seems clear that he has some PIR triggered lights and when they get triggered he wants to start recording. He wants to use the mains across the lamp to generate a signal that will pull down a control input to ground, that is fed by a quite high value pull up resistor from 12V.

The circuits that have been linked to will do *exactly* this.

[The Natural Philosopher]

I'd go reed switch, but getting a mains coilled one is harder than the solid state alternative

[Dave Plowman (News]

Why would you use any relay on the output of a PWM generator?

[John J]

I take your point but it's perfectly possible to use a more or less standard system to generate alarm messages to a phone. Having received a message the timing is readily available. Still you pays your money and you takes your choice.

[Dave Plowman (News]

Quite. I have an el cheapo mains coil relay switching my AV system via the amp on off switch and aux mains output. As the aux mains output is limited to an amp or so.

It is an open frame no name device bought from Maplin in the 70s or 80s. Obviously fitted in an enclosure for safety. Used at least every day and on for hours. Still works just fine.

Cars have relays too that can outlast the car. Not always, of course.

[Paul]

They make 12V PIR sensors, which could run off the 12V supply feeding the camera. Some experimentation undoubtedly required, but the nice thing is, you're no longer fiddling with mains voltage.

Paul

[Dave Liquorice]

So a micro controller can be isolated from, and switch, mains power? Doesn't take a true optimist to guarantee a SSR will last longer than a traditional relay in this application.

[The Natural Philosopher]

Of course it can. Opto isolators or double wound transformers - all of these meet very high isolation specs. As can of course the relays.

Nope.

Relays have their place. If they are off, there is no leakage and there is true isolation

But in switching awkward loads everyone has gone MOSFET these days Not even triacs, much really not even GTOs

semicoinductors do in fact get better from year to year.

>

[Dave Plowman (News]

Most would isolate the mains input to the device - not its output?

Well, yes. But then not many used a hammer where a screwdriver is needed.

BTW, would you guarantee your SSR on the output of the PWM device will provide total galvanic isolation when off - in the same way as a mechanical relay would?