Go Down

Topic: Controlling a relay with a transistor switch (Read 1 time) previous topic - next topic


Hello first post here,

The reason I'm doing this is because the relay is going to be dealing with a very high ac voltage and I'm trying to keep the arduino as safe as possible - also the relay may need its own power source.

In other words, is replacing the LEDs in this circuit with a 120v rated relay feasible?


Any and all input is welcome.


Sep 23, 2012, 11:21 pm Last Edit: Sep 23, 2012, 11:25 pm by jackrae Reason: 1
Definitely not !

If you want to switch 120vAC and your level of expertise is low then the following relay shield, or similar, is recommended  - actually if your level is low then steer clear of mains voltage until you've developed some skill.
Note that this is only rated for 360VA.


Thanks for your response jackrae

Can you explain why it wouldn't work?

It seems that this relay shield would only bring that high voltage closer to sensitive electronics, which I'm trying to remove it from.

My skill level is not necessarily low with these kinds of electronics, I'm just new to the arduino controller.


A well designed mains-relay shield will keep all the relay contacts well separated from the relay coil connections (about a cm is good), preferably with a plastic sheet on the underside to protect the traces from touching anything accidentally.

And of course the relays must be mains-rated.
[ I won't respond to messages, use the forum please ]



With significant power being switched, I recommend getting the relay(s) physically away from Arduino (Close to the load if possible), and using Optical Isolation so you don't need any direct electrical connection between Arduino and the relay circuitry.

See the schematic diagram on this page: http://goo.gl/PcWJ2  You could do something like this yourself, or buy relay boards. (The one shown is only $4)..

Read up on controlling power with Arduino on the ArduinoInfo WIKI here - http://arduinoinfo.info   
See: http://arduino-info.wikispaces.com/ArduinoPower

DISCLAIMER: Mentioned stuff from my own shop...
Regards, Terry King terry@yourduino.com  - Check great prices, devices and Arduino-related boards at http://YourDuino.com
HOW-TO: http://ArduinoInfo.Info

Go Up