Picture of Computer control of AC devices
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If you have ever wanted to control electronic devices from your computer to control or regulate your environment, this instructable will guide you. In this example of computer control, a USB relay device (USBmicro - U451) will control two 60 watt light bulbs from a PC program.
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Step 1: Safety First!

Picture of Safety First!
First off - Any time that you work with 120V voltages, safety is paramount. If you are not comfortable working with mains power, please seek the assistance of someone who is.

This instructable is intended only for someone experienced and confident in wiring high voltages. Do not attempt to do this if you are not. Household current can kill or badly injure you if you who do not understand the danger.

Only the U451 relay contacts should be used for control/connection to 110V AC. The relay screw terminals are isolated from the other circuit connections. DO NOT touch the U451 when there is 110V AC present.

Step 2: Major parts

Picture of Major parts
The major components used for this example instructable are:
  • USB relay interface USBmicro U451
  • two light bulbs
  • lamp sockets
  • 110V AC cord
The example project will control two independent 110V AC lights with the two relays on the U451. The U451 and lights will be mounted on a wooden plank.

Step 3: Project mounting board

Picture of Project mounting board
The lamp sockets are placed on either side of the wooden plank used for this project. The location of the lamps are copied to the plank and traced with a pen. A center line is drawn through the circle to aid in mounting the lamps.

Step 4: Board parts

Picture of Board parts
Four adhesive-backed felt pads are added to the bottom side of the board. This will prevent the board from scuffing the surface that the board is placed upon.

The lamp sockets in this example setup will rest on some washers to provide some space for the wires that lead out from the under side of the socket and to the U451.
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jonA42 months ago

Just a heads up. The plastic covers used are intended to be flush mounted against a wall since they aren't structural. For exposed boxes you need the metal covers that don't extend past the junction boxes themselves.

Jumprocks1 year ago
wHi, what woudl I need if i wanted to control 4 lights??
rusjane2 years ago
who is the person that make that device
amalkady3 years ago
what sholud i use if i want 220 v??
staniagas4 years ago
Here No steps are given about how to connect this with computer? Can you explain this!!!!!!!!!!!!!
shortw4 years ago
How much power can you put through the board?
indestructable (author)  shortw4 years ago
The U451 is limited to about 2 amps at 110V. So at most each relay could control about 200 watts.
then why does the usbmicron site say the relays are rated to 125v 10 amps ??
ToddR whiplash4 years ago
From their website:
PCB trace widths limit the connections to the relays to about two amps - this should not be exceeded.

The board is what's limiting it, not the relays. Probably not that hard to work around if necessary.
reoozeit ToddR4 years ago
My general knowledge of relays tells me that the 120v side is the side being switched on and off by the relay- essentially making it the same as the standard single pole switch in your house. And if those relay contacts are rated at 10 amps, they should be able to handle around 10 amps. The relays in components are typically purchased from other manufacturers and integrated into products by the electronics manufacturers. When they say differently about the connection to the relays being limited to 2 amps and are talking about the pcb, the 120v connection to the relays has some pcb trace going out to a separate terminal block- and that is true in this case (which would be a not-so-smart way to design a relay because it creates confusion about ratings, limits uses, and it could be done with terminals integrated into the relay to avoid all of this! ). In that case, a direct soldering of your wires to those contacts bypassing that pcb connection should suffice as described by author in comment below. Good luck, have fun, void your warranties, and BE CAREFUL! Don't hurt (enter your name here)!
indestructable (author)  ToddR4 years ago
A reasonable work around is to use a heavy gauge wire to connect to the soldered relay connections on the bottom of the U451 USB relay board (in pictures above). You make direct heavy connections to the relays themselves to overcome the limits of the printed circuit board.
maewert4 years ago
Very nice instructable.

Similar to mine at: Controller/

Mine does not use a relay board, so it is not for the faint of heart :-)

Best Wishes
maewert maewert4 years ago
sry for the broken link:
rohan98694 years ago
How do u add the USBm.dll and the program to the U451?
indestructable (author)  rohan98694 years ago
What are you asking? The program and the DLL don't get "added" to the U541 - the program runs on a PC.
rohan98694 years ago
From where can we download the software robot ide??
indestructable (author)  rohan98694 years ago

RobotBASIC comes from
ckundo4 years ago
Awesome! This might be interesting to you: we created an inverse of this idea where you can use the A/C device to control the computer. Some documentation here:
Hey nice!! Can you think of a way to add current load monitoring to this... It would be cool to be able to create software to mon current load and control lighting and other circuits based on time of day and power consumption??? Any thoughts would be appreciated!
You can build your own current sensor with this instructable:
Buzzsushi4 years ago
Where can I buy one of these controllers? I cant find out lol
indestructable (author)  Buzzsushi4 years ago
ludionis4 years ago
I could see hooking a few house lights up to a system like this, and when I'm on vacation, use RDP to remote into my home server, turn lights on/off to make people think house is occupied, or wire to a garage door opener and remotely open it from anywhere I have many fun possibilities!!!
drresearch4 years ago
Nice instructable, I voted for it and for your other instructable "eMail Light: LEDs show new e-mail". I finally bought the U451 and am waiting for it to arrive.
ipodgeek4 years ago
I just made the plug version of this project a few days ago after seeing this instructable. It is awesome. I've tried it with some lights in my house as well as a fan a sander and a metal polisher. It's very cool to turn on and off devices using your computer. I will post a few pics of it later.
indestructable (author)  ipodgeek4 years ago
That is great to hear! I'm glad that this instructable has helped you - I look forward to your pictures.
databoy4 years ago
I am a qualified licensed electrician. The electrical industry uses PLC's. (programmable logic controllers). You can buy a PLC off the shelf. They are dedicated stand alone micro-controllers and far superior for electrical switching applications than using micro-controller boards and writing software. The cost factor is about equal.
BTW my email is
Hey I am semi familiar with PLC's... Connecting them and wiring them. That said I follow the schematic provided by the engineers. Now i can figure out the schematic but I do not know how to spec the hardware. i have been trying for some time to set up a PLC to monitor the load on all forty circuits in my breaker panel and shut some off due to load, power consumption,( both peak, and cumulative,), or by remote. If you would be willing to assist me in the defining what hardware to purchase ,(PLC Lingo is greek to me,), I would greatly appreciate it.
insmac databoy4 years ago
An Ardweeny and a opto isolated triac for less than $15 USD and free software. Is there a PLC available for near that? Besides who wants to program in ladder logic...a language that was obsolete 30 years ago, but refuses to die? ;-)
mani5224 years ago
wat is d relay for 220 volt
Thenwcp mani5224 years ago
look for one with a rating larger than 220V
ipodgeek4 years ago
Great instructable! I really like it. I think I'm going to make something like this when I have the time. Thank you very much for posting it!
stackerjack4 years ago
I would love to make this project, but where, on my computer, do I enter the program?
indestructable (author)  stackerjack4 years ago
You would download RobotBASIC from and use that for program entry.
fly_boy_bc4 years ago
How to buy somethng and use it. This is an instructible? If you had to buy relays and a microcontroller it would be an instructible. If I show you how to build something using Mindstorms is that an instructible? NO IT IS NOT. Very very dissapointing. I was hoping to learn how to build something. I was not expecting "go out and BUY a controller and hook it up as per instructions".
thaCrab4 years ago
just wondering if it would be at all poss to hook a spare cell ph to your computer so you can turn on lights etc via txt or call when you're on your way home ???
Sagacity614 years ago
This is an advertisement from Dontronics perhaps?
munkey9064 years ago
I thought about doing something like this, and then found that the Insteon and other home automation equipment will end up being similar in price (when it's all said and done for a useable and professional install). Plus many of the home automation equipment will carry the on/off signal over the house wiring (no need for USB) and there are even options to hook them to your router and control from an iPhone or other web device. Very cool though... and handy if you don't want to drop cash on home automation stuff..
DAG10304 years ago
Very cool! I have a lot of application for this. Thanks for posting it.
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