Typical light switch instead of a relay

I am using the ArduinoDuemilanove ($45 from Arduino) and a PowerSwichTail ($20 from Adafruit). When the Arduino sends the signal to it, the PowerSwitchTail turns on or off one 75 watt lamp (about 120 volts with about 0.7 amps).

The signal comes from a wire on Digital Pin #9 (and of course returns to the GND) on the Arduino. That wire is carrying about 4 volts and 35 mA. Of course, that current is flowing all the time the light is on.

Now, let's say I wanted to control 6 more for a total of 7 of those lamps (of course I would also buy 6 more PowerSwitchTails). So in total, I would have 7 lamps and 7 PowerSwitchTails. The program in the Arduino would turn on and off the 7 lamps independently. Let's say separate wires would extend from Digital Pins #0-2-4-6-8-10-12 each to a separate PowerSwitchTail.

The program in the Arduino will never have more than one on-or-off activities at once. And in fact, only one pin at a time will be turned on or off with a minimum pause of at least five seconds between these signals to the pins. Now the problem, what if all 7 are on at once (albeit they are turned on at different times)? It will not work. There would only be 5 mA to each PowerSwitchTail. That's not enough.

Is there a switch that works like a light switch in a home? You turn it on and walk away. You don't have to stand there and hold it on. It would not be a relay with continuing power required to keep the light on.

Lamp A could be turned on by Digital Pin #0 and off by Digital Pin #1.
Lamp B could be turned on by Digital Pin #2 and off by Digital Pin #3.
Etc

That way there would always be 35mA available for the instantaneous activity of switching on and off a light one at a time.

That wire is carrying about 4 volts and 35 mA. Of course, that current is flowing all the time the light is on.

The current for keeping the light on seems high to me. How have you measure the current?

Let's say separate wires would extend from Digital Pins #0-2-4-6-8-10-12 each to a separate PowerSwitchTail.

Let's not use pin 0. It is part of the Serial port pair, and should not generally be used.

Now the problem, what if all 7 are on at once (albeit they are turned on at different times)?

The Arduino is capable of turning several of them on at the same time.

There would only be 5 mA to each PowerSwitchTail.

How did you arrive at this conclusion? Each pin is capable of supplying 35mA independent of what the other pins are doing, as long as the total current does not exceed 200 mA.

I liked everything you wrote... up to....

Now the problem, what if all 7 are on at once (albeit they are turned on at different times)? It will not work. There would only be 5 mA to each PowerSwitchTail. That's not enough.

No. If I understand you correctly.

You'd get 35mA flowining in each circuit, i.e. from digital Pins #0-2-4-6-8-10-12. The 35mA isn't a momentary thing that only happens when you first turn the relay on.

Now... as long as you can stick to what you said... only one of the 7 lamps on at a time, you are okay! You can even have more than ONE on at a time. Whether you can have SEVEN (7x35mA being run through the Arduino), I wouldn't like to say... others here can clarify that for you.

Each Ardino and Arduino clone has limits as to what can flow through any one pin, and also limits for what's going through the chip when you add up all the separate, per-pin currents. Mostly it depends on what microprocessor is at the heart of your Arduino or clone.