Long road to a Thermal Cycler - trying to make a relay work

Projects General Guidance
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Hi, I'm posting because I am in need of some help for a project I have been working on. I have little electronics knowledge outside of some general physics courses, and messing around with the arduino a little, and sometimes trying to read circuit diagrams and design bread board circuits drives me nutty.

I'm trying to build a circuit to control a light bulb, for use in a thermal cycler (PCR machine) as found here

http://russelldurrett.com/lightbulbpcr.html

Now, on the way there, I stumbled upon another website

http://www.glacialwanderer.com/hobbyrobotics/?p=9

and from that, I am basically trying to copy this circuit

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I do not have the correct diode, nor the correct transistor, nor the same relay, but I think I have equivalents that will work for what I want.

transistor I have is a C9014 NPN transistor C9014 Transistor - C9014 NPN Small Signal Transistor

The relay I have is a 5V switch (so I can use the arduino) and rated for 1A. (60W light bulb @ 120V = 0.5A theoretically still good to use)

link to relay http://www.te.com/catalog/pn/en/8-1419130-3

and it's wiring schematic

I bought it from radioshack because I wanted to get going without ordering from the internet. It's a SPDT relay.

diode is just some schotkey or zener diode from a radioshack electronics kit...I don't know which it is, it's orange and black and has a 3 on one side and a 5 on the other.

So where I am right now -- i rebuilt the same circuit as seen here http://www.glacialwanderer.com/_blog/blog2008/04_April/hb_relay3.jpg

Actual question I am trying to figure out how to connect the relay...

I drew up this http://i.imgur.com/bkbzj.png to see if it was possible.

I don't see how a relay works on a breadboard because some of the pins are going to be connected on the same row, regardless of how you place it.

Can the coil be in the same row as one end of the other points? Could they share a common ground like I have written down?

I tried making the "secondary" power supply a 3 1.5VD batteries (4.5V in series) in order to try to turn on an LED...but whenever I try it the circuit is just permanently on and heats up really fast because so much current is flowing (I was holding the ends of the batteries and they got really toasty, really fast).

I think one of my problems was I wasn't keeping the relay "on". I did manually switch it a bunch with the arduino's 5V supply and heard the clicking, but in my head because it is a double throw I keep picturing the gate going back and forth between 2 different sports.

Do I need a single pole single throw to make this work?

Eventually I'd like to use it with the 120V light bulb but, I don't feel like killing myself or burning something until I can at least make it work with 3 batteries and an LED.

I'll upload pictures of my circuit later because this computer won't detect my camera.

Ok thanks for any info on how to hook up the relay to an LED circuit. I'll finish trying again in 30 minutes but I have to take the dogs out for a walk. This time I'll actually hold the switch "on" and see if the batteries still light up the LED.

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If you're doing this for the end result (rather than to get experience with electronics), you might want to take a look at this:
http://powerswitchtail.com/PSTIIU.aspx

Basically, what you want but in kit form, which might be easier :slight_smile: They also have assembled units, which is nice because you don't have to touch mains voltage.

The relay won't plug into the breadboard as-is. You'll need to plug it into a socket, and take wires from the socket to the breadboard. Alternately, you can just solder wires to the relay and run those to the breadboard. (Don't put 120v on the breadboard, though!)

You can use a double pole double throw relay as a single pole single throw relay - just ignore one "pole" (leave it disconnected). The "double throw" means the relay will switch two things at once independently, not that it cycles back and forth between them. Think of it like a knife switch:

where the thing that does the switching is not your hand, but a coil.

I don't think it matters if you have the grounds connected or not - assuming you have a separate power supply for the LEDs and the Arduino, the relay effectively isolates one circuit from the other.

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Alright thanks a lot for the reply. My main computer died (either the power supply or mobo) so I've lost my main computer for uploading videos/pictures. I do want to learn the aspects of relay control which is why I'm starting with simple LED circuits instead of the 120V light bulb. At the same time, that kit does look awesome, and probably safer then what I'd duct tape and solder together, so I'll give it a look.

I basically gave up trying to control the relay on the breadboard with the arduino and in my frustration, decided to turn to my old radioshack hobby electronics learning kit. Low and behold, I found a project entitled "controlling a relay" and built it on the radioshack platform. Having everything labeled and spread apart on the radioshack board really helped me, and the relay clicked inside my head, as well as the transistor control circuit. It all made sense.

So I re-built the transistor control circuit with the relay, modified the basic "Blink LED" code on arduino and got digital pin 4 to switch from HIGH to LOW every 5 seconds, and got the relay to open and close every 5 seconds. I was pretty pumped!

Mini little eureka moments are what make working with arduino and electronics so fun.

So yeah, as of now, I can make the relay open and close on a timer, but I see now it's impossible to make it work on the breadboard.

The only way to hook it up puts your 2nd circuit in series or parallel (I don't know which...but it's connected either way) to your main one. I used an LED as the diode across the relay coil, and happily watched it blink shortly after each coil collapse when the relay closed.

When I tried to hook up the 2nd circuit, using an LED, I seemed to mess something up and the relay switched back and forth 4 or 5 times a second so I just stopped. The pins on the relays have to be in the same row (if you use a breadboard), and they are pins that shouldn't be in the same row, so that's where my problem is.

I'll have to wait until I can find more solder and try to hook up the relay outside of the breadboard.

Ok, thanks for the help