Wednesday, February 17, 2010

Chapter 4 - Exercise 18

I was hoping to have more to post today but I'm a little frustrated with this exercise. I was able to test the three numeral LED (see video below) but after wiring up the very first 4026 chip, I got no response on the LED. I double and triple checked all my wiring using Figure 4-35. I made sure the pins that went to negative voltage went to negative... and the pins that wired to positive voltage were wired to positive.

I checked my pushbutton with an LED - I had it wired correctly.

I checked the 9V voltage... all columns were registering the 9 volts and I properly wired up the columns as seen in Figure 4-34.

So, I grounded myself (again) by touching my metal table... pulled out another 4026... wired it up and pushed the button. Nothing.

Ditto with a 3rd 4026 chip. At this point, I went back to my wiring. The notch on the chip is pointed up (away from the LED) so that would mean pin 1 is upper-left. I again verified all my pins were wired up but nothing. Either this 4026 series of chips I bought isn't working with 9V (and I bumped it to 12V but still no response) or I'm doing something seriously wrong.

I spent over an hour wiring and rewiring just a single 4026 chip... I've got to take a break from this and come back after I've not thought about 4026 chips for a while.

Anyone else have luck wiring this up using Figure 4-35? I just find it hard to believe that these 4026s can be that sensitive... I may have to go buy more before I can finish this exercise... but I'm hoping someone has a suggestion because I'm flat out of ideas.


  1. can you post the schematic? (I don't have the book)


  2. Hey, Damien... didn't know you were following this thing.

    I don't have permission to reproduce the circuit or anything from the book, really - it's all copyrighted. Is there something specific you wanted to know about it?

  3. Of course I'm following. I haven't done any serious electronics for a few years, but it is always good to keep thing refreshed.

    Without the schematic, I can't really help. check out

    and make sure pins match the schematic. It is not uncommon for mistakes to be made in drawing up the wiring.

  4. Thanks for the tip about Wikipedia - I never thought about how useful it would be for details on the chips.

    Unless I missed something, my 4206 pins match the ones here...

  5. I don't mind if you reproduce a digital photo of the schematic, James, and I don't think O'Reilly would mind either. A photograph of a book page is not the same thing as a book page.

    I'm puzzled that you are having this problem. I actually reconstructed this entire exercise a month or so ago, just to make sure. In Figure 4-35 each little arrow indicates that the pin of the 4026 should be connected to that number pin on the 3-digit LED display--right? When you press and release the button, you will get "key bounce" (erratic counting) but you should certainly see something. My guess is you may have one of the control pins on the 4026 connected the wrong way, to pos instead of neg or vice-versa. One of those pins has a blanking function, as I recall.

    CMOS chips are vulnerable to static, but not THAT vulnerable.

    You do have the 3-digit LED chip the right way up? Decimal points between the digits should be on the left, as in Figure 4-33.

    I have not received any errata about this experiment (yet!) but there may be something I forgot to mention, which seems obvious yet isn't really obvious.

  6. pin 2 - 0v ?
    pin 3 - 9v ?
    pin 15 - 0v ?

    Do any of the segments light up at all? random numbers or nothing at all?

    Switch connected straight to pin 1? any debounce?

  7. Don't worry, everybody - this circuit isn't going to get the best of me!

    I've made enough mistakes in earlier experiments to realize that this is likely user error... so I'm going to take a break and come back at it tonight or tomorrow. I'm sure it's something small and hard to see... I'll figure it out.

    And Charles... thanks! I'll take a photo of the schematic and post it shortly for those without the book.

    And for "those without the book" - why?! Get it and get to work :)

  8. I read through the exercise and came across the note on floating pins. Is each and every single pin connected to something? The book particularly notes the "clock disable" pin as one you don't want to leave disconnected.

  9. Hi, Aaron.

    Yes, I verified that EVERY pin of the 4026 chip was either positive or negative connected.

    I'll rebuild the circuit later... it'll probably work like a charm.

    In the meantime, here's a link to download a photo of the circuit:

  10. I can't see any issue with it.

    I would probably try disconnecting the LED display, and then running the 4026 by itself. Use a multi-meter to see if any of the output pins are going high.

    Probably a long stretch, but is your power supply up to scratch? If the LED display draws too much current, it might dip the rails and cause brownout.

  11. I tried the experiment at both 9V and 12V... no luck with either. The LED I purchased is the one recommended by the author, so I'm going on the assumption that it's been tested thoroughly.

    Like I said - I'm 90% certain it was user error... I hope not, but that's my case history :)

    I have a logic probe... not quite sure how to use it properly, so I may have to dig out the manual and give it a spin if I can't get the thing working properly.