Saturday, January 1, 2011

Chapter 5 - Exercise 32 Still Drivin' Me Crazy!

This circuit is still driving me up the wall... I triple-checked all my connections, but I'm fairly certain the connections are done properly as the motor behaves somewhat correctly - it spins forward until a microswitch is pressed and then reverses direction... but only for a split second.

I used my logic probe to check the pulse on pin 3 but I'm honestly not 100% sure what I'm looking for...

One thing I did do that puzzles me - I unplugged the motor as it's quite loud and was making it difficult to hear the relay switching... when I did this and I turn on the power, I don't hear the relay doing anything, but when I press a microswitch, I hear the relay click (once) indicating that the coil is getting power and reversing the direction of the motor... but when I release the microswitch, I don't hear the click indicating the relay has switched back... but when I release the power button, I do hear a click. Maybe this is normal, but I'm wondering if somehow the relay is not functioning properly.

I've about decided to move on - I'll puzzle over it later tonight, maybe tomorrow... but if I can't find a solution, I'll move forward as I think I've gotten out of the experiment all I can.


  1. I have not got this far yet, and so this may be very useless advice, but:

    I've already been bitten by one bad pinhole in the quick protoboards... try shifting some of the leads into a neighbor hole on that row.

    Also, I would use the relay to power something else (an LED?) as an problem-cause elimination test.

    If you sourced your own relay, I would check if it is the type which remains on when triggered, or if it reverts to off.

  2. Good suggestions, all of them, Scott.

    I'll examine the protoboard and try out a few key wire changes... but the relay does match what is needed for the exercise if I read the data sheet correctly... but it's not supposed to remain triggered except that the coil is supposed to be powered for a few seconds (maybe 5 at most) as pin 3 provides a pulse to it and then turn off, allowing the motor to continue to spin forward.

    But I'm pretty certain the circuit is working as desired... motor spins in one direction until a microswitch is pressed, then it reverses. I still think it has something to do with either the capacitors and their charge time OR the 555 chip is bad (but I've substituted 2 others in with no changes) or I'm simply missing something.

  3. Oh yeah, capacitor could be off as you say (makes sense, and I'm not experienced enough to have immediately thought of that).

    I think caps will ground some current only until they are "full", so you could attach a speaker or (resistored LED) to the negative of the cap. If the noise or light cuts out too soon, that might narrow it down. I don't fully understand theory, but that could suggest the capacitor (or something leading up to it, such as the cap's input voltage?).

    Keep up the blog - you're doing a great job here. This is an awesome book and it deserves a walk-through like yours.

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