Relay Extension Cord Tutorial (and others)

I've recently gotten an Arduino board and must say I'm impressed with the hardware, software, and these forums.

I have plans to make some photography tools and an automated mini-green house, but first I've been doing a few simple tutorials as I get warmed up in this environment. Kind of like stretching before running a marathon (only in this case I hopefully won't collapse half way through). I just thought I'd post what I've done here so people can learn from them if they want.

Relays http://www.glacialwanderer.com/hobbyrobotics/?p=9 Servos http://www.glacialwanderer.com/hobbyrobotics/?p=7 SHT15 Temp/Humidity Sensor http://www.glacialwanderer.com/hobbyrobotics/?p=5

If anyone reads them and has ideas on how to make them better let me know.

This is very important safety information.

WRT your relay tutorial:

It actually does matter which wire you splice into. If that cord follows the standard, black is live, white is neutral. You actually should put the relay on live for safety reasons.

With the neutral wire switched, the device is still "hot" even when you think it's off. So if you touch something you shouldn't, you can complete the circuit to ground and electrocute yourself because the power is still present. It's also dangerous because the green ground wire is still a return, so if there's a fault in the device, you'll still have a short.

Your way is very dangerous.

i must agree, your tutorials are very nice, but adding a few warnings to the relay tut would probably do some good.

gotta be carefull when dealing with electricity... ( i'm sure you know that already, but the people you teach may think "sure it's all easy, whopdeee do!! ::::ZAP!!!:::" )

Thanks both of you for your input. Oracle, what you said makes perfect sense. I always treat anything plugged into the wall was as live, but I completely agree that putting the relay in the live wire is safer. I have updated the article with new pictures and text explaining this. I also prefaced the article with a general warning about AC electricity.

Thanks again!

The reason you can't just power a 5 volt regulator with a microcontroller is because relays are inductive devices.

I know you know what you meant, and I know I know what you meant, but this should most likely be:

The reason you can't just power a 5 volt relay with a microcontroller is because relays are inductive devices.

The next few sentences routinely refer to relays as regulators.

I'd also say statements like "..is because relays are induction devices." and "Inductive loads like motors and relays draw more current than a microprocessor can safely supply." are not entirely correct. Relays and motors are both inductive loads, but that doesn't mean they have to require large amounts of current, and high-current devices are not exclusively inductive. A better statement might use something along the lines of "Motors and relays shouldn't be connected directly to a microcontroller because they are inductive and require more current that a microcontroller can safely supply". Even this statement isn't 100% correct though,there are certainly many low-current relays with built in fly-back diodes that can be connected directly to a uC's output. In fact, your OJE-SH-105DM has a nominal current of 40mA (from it's datasheet); the Atmega168 at the heart of the Arduino can supply 40mA with a 1V internal drop (page 320 of the Atmega168 datasheet), so, assuming you clamped the relay with a diode, you could indeed connect your relay directly to the Arduino.

Thanks Spiffed! I've updated the article with your recommendations.