Watcing Voltage through Analog..

Hey all - This is more of a general question - even though I'm using a N+. I'd like to watch the voltage of a 120VAC feed through an analog pin. What's the best way to go about handling this? (Yeah, I'm aware NOT to hook it straight up to the pin, unless I want to see fireworks!) Unless I'm looking for the wrong terms (possible), Google wasn't much help. SO I thought I'd get ya'll's pointers. Cheers! -Mike.

Hey all -
This is more of a general question - even though I'm using a N+.

I'd like to watch the voltage of a 120VAC feed through an analog pin. What's the best way to go about handling this? (Yeah, I'm aware NOT to hook it straight up to the pin, unless I want to see fireworks!)

Unless I'm looking for the wrong terms (possible), Google wasn't much help. SO I thought I'd get ya'll's pointers.

Cheers!
-Mike.

Mike,

There are several ways to do this, some would use electronics others simple electrical compontnents.

• If I wanted to keep it simple, use a transformer (something like a door bell transformer) in the range of 120vac to 6vac.
• Put a full wave bridge rectifier to convert the 6vac to direct current.
• Build a voltage divider to cut the dc voltage to 3.3vdc or less. For instance wire (3) 1000 ohm resistors in series (current through the network about 2.5 milli amps), and take the voltage between the first resistor and the negative connection (tie negative to Netduino ground) this will give about 2.5vdc when 120vac is applied (the output of the rectifier will be slightly higher than the ac input, RMS values to DC).
• Scale the reading to the applied AC voltage (120v).

Hope this helps,
Chuck

In case you do not really care about the voltage, but you want the signal for synchronization, have a look at "Zero Crossing Detector" circuit. While it is possible to connect the microcontroller input to the mains (AVR182 application note), I would strongly recommend galvanic isolation (i.e. transformer or an optoisolator). Keep one hand in your pocket when working near the mains.

Everything the above friends have written is fine, but my advice is:
do not play with voltage above 30 Volts unless you have enough experience in electric sciences.
I am sorry if I hurt you, but instead of giving you some guidance toward a potential danger, I prefer to keep you far from.
Hope you understand what I mean.
Cheers

Hey guys -
Thanks for the tips. (And warnings, tho they are unneeded. I work around 120v/220v feeds all the time, I'm safe there! )

In fact, one of these projects, the client asked if they could be notified when the voltage had a brownout or went out. While there are detectors out there, only the insanely expensive have any external communications abilities. My immediate thought was a simple ND+ (actually, an ND would work fine with an XBee or other communications shield..!) watching the 120v on an analog pin. I know mains electricity better than I know circuits and DC, so was lost on how to convert the two. (read: Get a 120v reading into the ND.)

The doorbell transformer idea makes sense, it lowers the voltage and gives me DC, which the ND can then (safely) read.

The doorbell transformer idea makes sense, it lowers the voltage and gives me DC, which the ND can then (safely) read.

Mike,

One thing that I thought of after I posted the first reply. Some of the wall warts (power supplies) used for telephones etc have an Low Voltage AC output (in the range of 3 to 12vac). I normally keep old ones around the house from various equipment that I have gotten rid of. Anyway they would would make a convient and safe way to step down the voltage and to protect the Netduino (and curious kids/adults). Also you may be able to use an un-regulated DC power supply with a voltage divider.

Chuck

Hi, If your looking for a lower DC Voltage, just use a regulator to obtain a lower voltage output example is LM7805 etc. or make a variable power supply to aid your needs.

Hi,

If your looking for a lower DC Voltage, just use a regulator to obtain a lower voltage output example is LM7805 etc. or make a variable power supply to aid your needs.

Brian,

We are trying to read a 120vac signal, to use an analog input the reduced voltage must be proportional to the input voltage. A regulator would provide a constant voltage to the analog pin such that a change in the input would read the same on the Netduino board.

But thanks for trying.

Chuck