Wednesday, January 13, 2010

Chapter 2 - Exercise 7 Part 3

This is the last of 3 parts for Exercise 7. This part covers the circuit that you'll build using a relay.

UPDATE: I've added a video at the end of the post of my circuit working.

First off - a bit of advice... this circuit will be much easier to build if you have some patch cables. Gator clips are also required, but having just a few patch cords will make getting everything hooked up a little easier... not much, but a little.

This is not a difficult circuit to understand, but it is a difficult circuit to build. The posts on the relay are quite close, and you have to make sure that your gator clips or patch cables do not accidentally touch... I think the only risks are you might burn out an LED (12 volts is applied to this circuit) or short circuit somewhere causing the relay to continually power one LED over another.

Also, in the book it shows two yellow LEDs... I used a red and a green to make it easier for you to see which is lighting up when the button is pressed. Speaking of the button, this is another tricky little bit of wiring, especially if the posts on your button are close like they are on mine. One of the photos I have here is a closeup of the button wiring, but even then it's a bit difficult to tell that the two clips are NOT touching... they look it, but I pushed one as close to the plastic bottom of the button as I could... the other is just pinching on the tip of the other post. Probably about 1mm distance between them.

The cool thing is hearing that relay actually click-clack as you press the button. My red LED starts out lit, but as soon as the button is pressed, the red LED turns off and the green stays lit, but only as long as I hold the button. (It's a bit fun to press on-off-on-off as fast as you can... for those of us who like results, the small sound the relay makes really makes the frustration of building this circuit worth it.)

I also had to use a 1k resistor (but the book says this is okay).

Okay, so I understand this... really, I do. But I mentioned yesterday that I'd be posting a question for my readers to try and answer (and the best answer gets the winner a new Maker's Notebook). So, here it is:

On page 59, you can see the schematic in Figure 2-60. Now, with all my past electronics experience (not a whole lot, really), I do know that in order for a circuit to work, an LED to light, or some other reaction, the circuit must be closed. Take a look at the symbol for the push button... it appears open, doesn't it? But when you plug in the AC adapter, the red LED lights up. Now, do what I did - try and trace a closed circuit path from the positive DC to the negative DC symbol. Can you find a closed loop? I can't. But I know how this works... and I know why the LED lights up... I just can't explain in better words WHY! Is there a better way to draw this schematic? How does the current run through the circuit based on this drawing? (I'm pretty certain it's related to the bottom portion of the relay drawing, but that's just my guess.)

So, readers... care to try and explain this to the rest of the class? Try to keep your explanation to less than a page of text if possible, okay? And if you need to provide a drawing, just post it on Flikr or some other image hosting site and give us the URL link. What I'm looking for is a good explanation for why the circuit is working when the button is not pressed and apparently not closing the circuit. I'm going to post another item after this one where I ask you to post your comments for the contest... that way people can comment on the circuit above and ask questions without wading through all the contest replies, okay? I'll take all the responses on Jan 31 and make another post for votes as well as invite a couple guest experts to chime in and help me pick the best response.

By the way, after I unplugged the AC Adapter, the red LED stayed lit for about 10 seconds. What does that tell us about the relay?


  1. I don't have the book (YET) so i don't have a schematic, but the relay coil should not be able to hold a charge for a noticeable (human eye) amount of time.

    If the red led is not being powered through the relay coil but is connected to the contact points (perhaps the Normally closed posistion) then it should be impossible for it to remain lit.

    So how is it happening? I suspect that the filter capicators in your AC->DC adaptor are providing power to your LED after you unplug it from the wall AC.

    Try removing one of your power connectors on the DC side of the power supply while the circuit is running and see what happens.

  2. Great answer, Tsaavik... thanks! I don't know why the AC adapter didn't occur to me... I knew I didn't have any capacitors in the circuit but forgot about the AC adapter.

  3. The puzzling part of this experiment for me is that I cannot get the second LED to lite up when the coil on the relay is energized. I have check the wiring 3 or 4 times and cannot get it to work. The first LED lites up when the relay coil is not energized. When I push the switch the first LED turns off but the second one does not turn on.

    I am using the Relays from the MakerShed Components Pack 1. Over the weekend I am going to go out to my local electronics store and buy another DPDT 12v Relay and see if I may have just gotten a bad relay.

    1. Solved it! My kit was supplied with a relay by Kest part number KS2E-M-DC12. Wish I could have helped poor Stephen out but at least the newbies won't have to suffer too long. I had the same problem. I checked my connections over and over again. I switched out relays, leds and power supply. It took me about 1 hour but I decided to do what one should always do when a problem is difficult to solve. Verify all information taken as given. I looked closely at the operation of the relay while energized. This was convenient since an earlier experiment required you to remove the outer housing of one of the relays. Using a large magnifying lens I could see that the copper contact arm pivoted from the contacts adjacent to the coil contacts. While viewing the diagram on page 59, Fig 2-52, the "pole" on my relay is the lowest contact of the group of 6 contacts. Remember, the contacts at the bottom of the housing are the coil contacts. So the left diagram "not energized" is correct but the right diagram "energized" is not correct. The lowest contacts,of the group of 6, are the "poles" and these are connected to the highest contacts. If you refer to page 59 and this time figure 2-59 simply reverse the wiring of your LED and switch. Move the LED to the middle contact and the switch to where the LED was connected. Again, this correction applies to relay part number KS2E-M-DC12. Have fun!

  4. Stephen, check to see if your DC adapter is delivering 12 volts

    1. This is exactly what was happening to me.. I had it set to 6volts (from the previous steps where we were measuring the voltage from the stripped adapter cables). The magnet inside the relay wasn't strong enough (I noticed I could tap the opened relay, and it would THEN click).. so I flipped it up to 7.5 volts, and then it started working.