Thursday, February 4, 2010

Chapter 3 - Exercise 15 - Relays the Same?

I may be wrong here - I hope Charles Platt will set me straight - but I think the relay used in Exercise 15 is slightly different than the one we used back in Chapter 2 Exercise 7.

The drawing for Exercise 7 shows the arm of the relay, when the coil isn't powered, pointing in the direction of the coil. When the coil is powered, the magnetic field it creates pushes the arm away (upward). This caused the 2nd LED to light up in Figure 2-60.

But in Exercise 15 the relay shown there has the arm in the "up" position, away from the coil. When the coil is powered, it pulls the arm towards it.

When I was building my circuit, I was using a relay where the arm was pointing towards the coil when not powered... I had to visually inspect the opened one (Exercise 7) to see this... once I saw that, I was able to properly wire my circuit. If I had to modify the circuit in Figure 3-93, I'd have to show the anode of the diode connected to the right-most pins (the one the arm is shown pointing at in Fig 3-93). I hope this makes sense... if we're to assume we're using the same relay from Exercise 7 here, then the circuit in Figure 3-93 would also need to have the arm pointing towards the coil.


  1. I'm losing track of your sequence, James! Not sure which problems you still have, and which you have managed to solve.

    Most basically...did you buy a relay different from the one that I recommended? Relay pin functions do tend to vary.

  2. Sorry about that, Charles. I'm solving them little by little, but I still think there's a discrepancy in the drawings for relays in the book - for Chapter 2, when unpowered, your drawing shows exactly what I see when I took a relay apart (the one you recommend) - when unpowered, the arm is pointing towards the coil.

    But in Chapter 3, the relay is shown unpowered and with the arm pointing away from the coil. When I used the relay from Chapter 2 and wired it according to Chapter 3, I had to change the wiring and connect the pins farthest from the coil to the anode of the diode... does that help?

  3. I'm having exactly the same problem. I purchased both component packs and took apart the relay (that's the one I'm using). According to chapter 2 the relay arms should be nearest the coil, but the diagram on page 133 shows the exact opposite, even though it's supposed to be the same relay.

    I have now been banging my head against the wall on this exercise for 7 straight hours, and have rebuilt the thing 11 times to no avail. I'm extremely frustrated and feel like tossing the whole thing. Mr. Platt's book is excellent but I've run into so many show-stopping errors that I now consider version 1 of the book inappropriate for a true beginner. It's just too frustrating to wire things up correctly when there are wiring errors in the diagrams. I'm sorry, Mr. Platt, but the errors are really bad.

    Please fix them!

  4. I have another comment to add to this relay discussion. Like Jasony I bought both Component Packs (1 & 2). When doing exercise 7 I couldn't get the second LED to light up when the coil was energized. I rewired the circuit a couple of times to no avail and then decided to move on. I thought my relays could be bad. When I got to this exercise I got everything wired up and it didn't work. I went back and got the data sheets for the relay that came in my components pack 1 (HLS-4078-DC12V) and the data sheet for the relay recommended on page 42 (FTR-F1CA012V).

    Eureka !! They're different !! The arms/poles of the relay are on the center pins of the relay shown in the book and on the data sheet for the FTR-F1CA012V Relay. The arms/poles on the relay that came with my components pack 1 (HLS-4078-DC12V) are on the pins closest to the coil.

    Once I took this into account and modified the wiring of my circuit everything worked.

    Like Jasony, I was a little frustrated at first. I assumed that the relay in the components pack would match what is used in the book. However, digging into it and discovering what was really going on was also rewarding.

    Now I am going to go back and set up exercise 7 again and get it to work right.


    1. Not sure if anyone's still reading comments on this blog but the posts been very helpful to me as I work through the experiments in the book, so thank you. However, I'm a bit stuck at the breadboarding stage of Experiment 15. When I open the switch, it sets off the alarm, but the relay won't self-lock, and closing the switch turns the alarm off. I'm using the Omron G2RL-24-DC12 relay recommended in the book.

      Could it be possible the pins on this relay are laid out differently to the schematic in the book? I've stared at the datasheet for the relay but I can't see an answer there. I've also checked and rechecked and rebuilt the circuit a few times. I'm also concerned that rewiring the relay wrongly might break it, as this particular relay was a tricky one to get hold of.

      Any ideas?

    2. An update: the relay is fine as it is. I spent the longest time puzzling over this and tried a load of variations, but in the end it came down to something utterly basic: connecting the two halves of the breadboard. Funny that I've managed to get this far in the book without having to do this. I won't be making that mistake again...