how to check whether AC is on or not?

hi all ,we are working on a project where we want to control AC from a computer.The issue is how can we detect whether the AC is on or not?(we are using LDR to detect the same for lights).One way is to use step down transformers in the main line and then use that voltage level as an input to the computer parallel port.But we want to make the circuit as compact as possible and step down xformers are bulky in size.So are there any better alternatives???
Thanks in advances...

The first thing that occurs to me is to use a simple AC-DC converter (the capacitor, resistor, diode) circuit often used to power very small things like LED's from the mains, linked to an optoisolator.

Hi,

If you are trying to control AC, you will be having a control circuit having a Relay which uses the parallel port of computer.
I think the relay gives you the status of AC, weather it's on or off.

I think this helps..

Can you give me the circuit??? pls...

For my idea, there's a good explanation and diagram here... just add in your chosen opto-isolator where the LED is:

Capacitor Power Supply circuit

Warning! If you not experienced working with AC power, messing with AC power can kill you!


Use some optocoupler, see the diagram:

Hi,

Please go through this link.
Control Electrical Appliances using PC - CodeProject®

I think this is the exact circuit you require..

Thanks,
Suresh

mitesh.aghera said:

Can you give me the circuit??? pls...

Click to expand...

How much power consumption does it make? Because we are working for project on saving energy.

---------- Post added at 18:00 ---------- Previous post was at 17:57 ----------

Is the circuit giving on codeproject.com site able to give the feed back that whether the device switched on or not?

mitesh.aghera said:

Is the circuit giving on codeproject.com site able to give the feed back that whether the device switched on or not?

Click to expand...

No, that one just controls the device, there is no feedback to see if power is actually applied.

A very simple idea... use another LDR and whatever circuit you already have devised for it, connect an old-fashioned neon panel indicator light across your mains power lines that you want to monitor, and put them both in a bit of tubing at opposite ends to block out ambient light. Simples!

Well, nearly simples... you will need to 'buffer' the output of the LDR beacuse its resistance will vary with the AC. Just use a reservoir capacitor in your circuit for that. You could also use a phototransistor instead of an LDR, but if you already have a design for using an LDR, that's fine. Basically, you are making yourself an optocoupler.

I don''t know off the top of my head what current a neon panel light draws, but it won't be much and probably a lot less that even a capacitor power supply. If you get one from a supplier, it should tell you the power requirement. I think it's generally one mA or less.

Don't forget that a neon indicator light needs a series resistor to limit current, suitable for whatever mains voltage you have. A full panel light will usually have one inside already, but if you get just the bare neon bulb, you'll need to add a series resistor.

A neon indicator: Link.

Have you any circuit for this? Give me.