Introduction: Computer Control of AC Devices

Picture of Computer Control of AC Devices

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.

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.

Step 5: Mounting Lamps

Picture of Mounting Lamps

The lamp sockets are positioned and the screw locations marked with a pen. The screws and two washers are added and partially screwed in place. The lamp sockets loosely mounted as a "dry fit" just to see that they fit in place nicely.

Step 6: Mounting the U451

Picture of Mounting the U451

The U451 is mounted next to the lamps with small standoffs and #4 screws. Pictured are the two 3-screw headers - one for each relay.

Step 7: Wiring Parts

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Because of the high voltage and current involved with this example, you must use a wire of sufficient diameter. In this case 16 gauge will suffice.

Step 8: Hot Wire Connection to the U451

Picture of Hot Wire Connection to the U451

The pair of wires from the two-wire plug consists of "hot" and "neutral" represented by black and white respectively.

Hot connects to each of the two common "c" terminals on the U451. This example project uses a wire nut to make the connection from the single source hot/black wire to the two pigtail wires that lead to the U451.

Step 9: Connections to Lamp.

Picture of Connections to Lamp.

The neutral/white connection is split into two wire pigtails that connect to the lamp socket (silvered connection).

One black wire connects from the N.O. (normally open) screw terminal connection on relay 1 to this lamp socket (brass screw on socket) while the other lamp socket connects in a similar way to the N.O. connection for relay 2.

Step 10: Lamps and Bulbs.

Picture of Lamps and Bulbs.

Mount the lamp sockets, tighten the screws. Install the light bulbs. These test bulbs were incandescent. Other more energy sensible light bulbs could be used.

Check your wiring. Once you are sure about the 110V AC connections, attach the USB cable to the U451 and your PC. Plug the AC plug into an outlet.

Step 11: Very Simple PC Programming

Picture of Very Simple PC Programming

This project is brought to you by the magic of RobotBASIC. RobotBASIC (RB) is a free programming language and IDE (integrated development environment). It has support for controlling the USBmicro U451.

RB is free. FREE! You can write a program in the RB IDE and then save it as an .exe - a stand-alone executable that doesn't need the IDE to run. The USBm.dll is needed to go along with that file, but that is free, too.

I'm not going to get into the details of the program in this instructable. The entire program is the 35 lines in the image below. The program is easy to copy into the IDE and simply run. Here is the program as text: (note the spacing gets screwed up.)

// U451 relay control program

// If the DLL is found...
if usbm_DllSpecs() != ""

// And the device is found...
if usbm_FindDevices()

// Initialize the U451, outputs
n = usbm_DirectionB(0, 0xFF, 0xFF)

// Create title and two checkboxes for the relays
xyText 10,10,"Relay 1 Relay 2","",20,fs_Bold
for i = 0 to 1
addcheckbox "" + i, 10 + 120*(1-i), 60, " "

while true
for i = 0 to 1
if getcheckbox("" + i)
n = usbm_SetBit(0, i+8)
n = usbm_ResetBit(0, i+8)
delay 100

print "There are no Devices"
print "The USBmicro DLL is not installed"


Step 12: Running the Program

Picture of Running the Program

When the program runs you are presented with two boxes that you can click on or off. If everything works the bulbs follow suit!

Step 13: Let There Be Light!

Picture of Let There Be Light!

You now have control of two lights in this example instructable.

This setup could easily be modified to control the power to almost anything that plugs in. And RobotBASIC ( can be used to make much more complex programs that, for instance, turn on electric devices based on a complex PC schedule.

Let there be light bulbs. :-)

Step 14: Updated: Instead of Wiring the Lamps Directly...

Picture of Updated: Instead of Wiring the Lamps Directly...

...wire the relay into your own outlet.

Please see the specifications for the U451 for limits to the current for the devices you control.


Helma MariaT (author)2015-12-30

What is the easiest/cheapest way to controll 240 light bulbs?

The obvious solution of using 120 U451 is a bit painful.

Better solutions depend on your electronics construction skills. A single U451 can drive 6 more relays directly from the J4 connector. So now a solution would involve 30 U451s plus relays. J1 can't drive the coils of relays directly, but with some transistors could control 8 more relays, cutting the number in half.

You can buy boards on ebay that have 8 relays on them. The boards are $7-10. They can be controlled by a USBmicro board that is similar to the U451 and so one U421 could control 2 of these 8-relay boards. So 16 relays controlled by each U421 would be 15 sets of these controllers.

Instead of mechanical relays, you could also use solid state relays.

jonA4 (author)2015-01-02

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.

Jumprocks (author)2013-10-15

wHi, what woudl I need if i wanted to control 4 lights??

rusjane (author)2012-06-22

who is the person that make that device

amalkady (author)2011-07-04

what sholud i use if i want 220 v??

staniagas (author)2011-02-18

Here No steps are given about how to connect this with computer? Can you explain this!!!!!!!!!!!!!

maewert (author)2010-09-09

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 (author)maewert2010-09-09

sry for the broken link:

rohan9869 (author)2010-07-22

How do u add the USBm.dll and the program to the U451?

indestructable (author)rohan98692010-07-23

What are you asking? The program and the DLL don't get "added" to the U541 - the program runs on a PC.

rohan9869 (author)2010-07-21

From where can we download the software robot ide??

indestructable (author)rohan98692010-07-21

RobotBASIC comes from

ckundo (author)2010-07-16

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:

Stevens_Carl (author)2010-06-29

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!

ckundo (author)Stevens_Carl2010-07-16

You can build your own current sensor with this instructable:

Buzzsushi (author)2010-07-15

Where can I buy one of these controllers? I cant find out lol

ludionis (author)2010-07-15

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!!!

drresearch (author)2010-07-12

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.

ipodgeek (author)2010-07-07

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)ipodgeek2010-07-07

That is great to hear! I'm glad that this instructable has helped you - I look forward to your pictures.

databoy (author)2010-06-25

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.

Stevens_Carl (author)databoy2010-06-29

BTW my email is

Stevens_Carl (author)databoy2010-06-29

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 (author)databoy2010-06-25

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? ;-)

mani522 (author)2010-06-19

wat is d relay for 220 volt

Thenwcp (author)mani5222010-06-25

look for one with a rating larger than 220V

ipodgeek (author)2010-06-25

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!

stackerjack (author)2010-06-24

I would love to make this project, but where, on my computer, do I enter the program?

You would download RobotBASIC from and use that for program entry.

fly_boy_bc (author)2010-06-25

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".

thaCrab (author)2010-06-24

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 ???

Sagacity61 (author)2010-06-24

This is an advertisement from Dontronics perhaps?

munkey906 (author)2010-06-24

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..

DAG1030 (author)2010-06-24

Very cool! I have a lot of application for this. Thanks for posting it.

WVvan (author)2010-06-24

Nicely documented.

Thav (author)2010-06-21

A note to people that might want to use this for something like a strobe light or dimmer application, (or even just bothered by the klackity klack you'll get) you will need some type of solid state switch. The simplest method for this is a TRIAC, which is basically a semiconductor device that will start conducting in either direction (important for line AC applications) when a pulse of current is driven into its gate, but will stop conducting at a current zero crossing, you can then gate it on again in the next AC line cycle. This is how some in switch dimmers function. Fully controllable devices would be IGBTs or MOSFETs, and you would need two of those devices per controlled device.

jhd04 (author)2010-06-20

Also, you could achieve higher amperages by using the relay output to feed power to the inputs of a contactor, which then is just a much beefier relay. It's how I control router and vacuum power on my CNC, so the machine can flip them on and off, but they take about 12-15amps, much too high for any relay. A contactor can go up to almost unlimited current ratings (though large ones are mad expensive). You can usually also add 1 or 2 circuits to each contactor, allowing you to power several devices off of one relay, though they all turn on and off at the same time. This one from Tyco can do up to 50A! That's beefy! Just make sure your supply circuit can deliver that before you turn it on, or you'll blow a breaker. Anyway, good instructable!

indestructable (author)jhd042010-06-20


Yeah, there is always more than one way to skin a cat, so to speak.

The relays can control a few amps of AC (or DC). The other outputs can switch a couple hundred milliamps to ground, which can run LEDs ot other DC devices - including other relays.

The "A" port on the U451 can connect to Dallas/Maxim "1-wire" devices (one wire plus ground...) that read temperature. If you are familiar with micro electronics, then this is also a SPI and/or I2C interface to USB. The U401/U421 similar devices can interface LCDs to a PC.

faytaliti (author)2010-06-18

This is a great project. But, back home in my place, we have 220 V AC @ 50Hz. So, can you update the instructable with the required mods for a 220V supply as well? Cheers.

indestructable (author)faytaliti2010-06-19

No difference - the relay is rated for 220V. Different wire colors and plug types, but otherwise pretty much the same for 220. Current needs to be limited (a 100W bulb is probably the max) as per the specs on the USBmicro web site.

amacgregor (author)2010-06-18

Would it be possible to make it Wireless instead of USB. Cheers

rdk (author)amacgregor2010-06-19

I forgot, but if you'd like to see datasheets check out the digikey page.  The top datasheet gives a good comparison of the various modules.

rdk (author)amacgregor2010-06-19

You could always use an Arduino for that. Last I heard, there's a module by AdaFruit that can handle point-to-point/-multipoint over a 300' radius (1 mile if you buy the PRO module). However, it isn't 802.11 so you have to have one for a connector to your computer.

artworker (author)2010-06-17

Its Awesome. I was also looking for device control using USB and got the same USB micro site. 5*

indestructable (author)artworker2010-06-18

Thanks! The relay board is the core of a lot of project ideas that I have. Not that all of them will become instructables, but some perhaps.

makeup 1 (author)2010-06-18

Cool idea

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