Probably depends somewhat on what you mean by "power supply." My laptop runs on a 120V A/C power supply, but the "power supply" is a transformer that sits outside the laptop. So, up to the power supply (my definition, in this case) the voltage is indeed 120V, but beyond the PS (i.e., the wiring inside the device) it is 19V (and various lower voltages, too).

If you're device is an appliance that plugs into mains power, and you don't know for certain otherwise, the safe assumption is that it's 240V. But if you're uncertain, it's perhaps ill-advised to be modifying the device...

Wow! I step away for a few hours, thinking we're done here, and there's a whole lotta discussion when I get back! Interesting!

Anyway, I can't tell you much more than I have, other than to say that my intent is to use this to make my porch light flicker on Halloween. I was looking for something like the PowerSwitch Tail but for a bulb socket. Never really expected to find something purpose-built, but this is pretty close. It's made to be connected to a PIR motion sensor as an add-on to allow it to control a second bulb, like if you were retrofitting a motion detector to a pair of floodlights.

Here's the device:
http://www.amazon.com/gp/product/B0009J1I2G/ref=s9_simh_gw_p60_d19_i1

And here's the sensor it's designed to be connected to:
http://www.amazon.com/Thomas-Betts-HS3100D-Sensing-Adapter/dp/B0009J1I26/ref=pd_cp_hi_0

So it turns out that it provides a pretty uC friendly interface. Now to find some zener diodes...

Thanks, folks!

D'oh! Sorry, Zener diodes. What type are you proposing?

Thanks, Mike. I thought the current was low as well (though I have no experience to base that on), but I was reassured that 0.012 * 250 = 3. Also, all I have is a multimeter, not a scope, so I can't "see" any transient spikes or waves.

Do the diodes then just provide a preferred path for a back emf spike, so the current wouldn't go through the uC? I received one other piece of advice to have 2 diodes, in series but opposite directions, across the coil. Would that be somehow equivalent? If not equivalent, do you think it would work?

I am looking to interface my Arduino with an existing device. This device is for turning on/off a bulb: it plugs into a bulb socket, and accepts a bulb on the other end. It has a cord connected to it with two wires. Those wires apparently connect to a single coil, latching relay inside the device. It's intended to be connected to a motion detector, and when activated the detector sends 2V down for just a moment to switch the relay to ON. Then, after a timeout, it sends 2V, but with polarity reversed, to turn the light OFF.

I want to control the light via the Arduino. The simplest plan would seem to be connecting the two wires to two Arduino pins, keep them LOW, then set one HIGH briefly to turn the light on. Set the other HIGH briefly to turn the light off. That would seem to work logically, but I understand that there are issues with connecting Arduino pins directly to things like coils. Back EMF is the danger, and there are all sorts of proper ways to deal with this, H-bridges and the like.

I've measured the resistance of the coil at 250 ohms, and the current draw appears to be 12 mA when I connect the relay to a 3V battery pack. At least Ohm's law still seems to apply. :-)

Is there some simple circuitry I can build to protect the Arduino? Are these numbers small enough that I don't have to worry?

Thanks again to everyone. I found a nominally 470 ohm resistor in my kit from college (makes it about 25 years old...) which measured 513. Quite a bit off for a 5%, but certainly good enough for this project! And the little LED is apparently green, btw.

Thanks for the advice!

Thanks, gents! Any clue what the 2.2V next to the LED symbol might mean?

Should I drive this directly from the 9V battery, or from the 5V regulated? Which would be more efficient?

How sensitive is that resistor value? Can I use a more common 470 (which would bring me closer to the mysterious PLUS 100 OHMS also)?

I've no idea what color the LED is, but will report back when I find out!

I have a switch on my project with a built-in led that I'd like to light. The packaging for the switch says,

LED Operating Voltage: 3 V DC
For different operating voltages, a resistor must be used in series with LED Lead.
How to calculate the resistor value.
USE OHMS LAW PLUS 100 OHMS
E = IR
E = THE VOLTAGE OF YOUR CIRCUIT
I = THE CURRENT OF YOUR LED
R = THE VALUE OF THE RESISTOR TO USE

There's a circuit diagram which shows two separate items: the schematic symbol of a switch (with the terminals labelled), and the schematic symbol of an LED, again showing the terminal labels, but also noting "2.2V" next to it.

Nowhere does it specify THE CURRENT OF (MY) LED.

Is this enough to go on, and if so, can someone point me to a page that describes how to calculate the proper resistor? I'd really rather have a pointer to the technique, rather than just a straight answer, though of course that's appreciated, too.

Thanks!

If you think you're going to somehow process/analyze the video signal natively on the arduino, you've got another think coming. The ATmega328 just doesn't have the memory or cpu power to do that. Is there a shield you're intending to use for the video processing? Without something like this: http://nootropicdesign.com/ve/, I don't think you'll have any luck.

You still haven't said what your goals are. There's a reasonable chance you're going down the wrong road already, and it would be better to know that now...

Ah, sorry -- I understand the question now. You're wondering what path the current is taking in order to complete the circuit. Hopefully someone more knowledgeable will chime in.

What are you hoping to do with it if you connected it? If you're hoping to deal with the image data coming from the camera, I think you will find the Arduino platform hopelessly under-powered for that. However, if there is some sort of control interface on the camera you'd like to interface with, that can almost certainly be made to work. However, you will need to find the specs for the camera module you've got.

Not sure I can answer, but you haven't provided much info. What pin were you touching? What code was running? From what you have stated, and what I know, it sounds like you were touching a pin whose state was changing rapidly, i.e. at a frequency that you can hear.