Make: Electronics Experiment 18 (first part)



Hi all,

I'm working on the first part of Experiment 18 in Make:Electronics, following the circuit diagram in Fig 4-37 (image attached). I managed to get the steps before this to work perfectly fine.

Here I'm using a 555 timer to drive three 4026 chips that power a 3 digit common-cathode display.

Strangely, the first time I tried it, the circuit works fine in counting up the numbers, except that it freezes whenever there were at least two "2"s in the number. For example: 122, 222, 322...922. And also 22X, and 2X2. To get past that, what I did was to disconnect the power supply briefly and reconnect it, then it goes to the next number.

I did this twice, and got the same effect.

Then on the next day, I tried again. This time it only freezes at 222. Again, disconnecting and reconnecting power resumes counting. Interestingly, if I physically bump the circuit a few times, it resumes counting.

Other things I have tried:
using only IC2 and IC3, or IC1 and IC2 to control the display. i.e. using only the first two or last two digits -- it doesn't freeze at any number.

Any thoughts on how I should troubleshoot this? I just find it really interesting that the freezes involve only the digit "2" but doesn't impact the other digits.

Thanks!
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  • Sorry, I forgot to add, the circuit consists of what is shown in Figure 4-37 plus 4-38 (555 timer) added to it.
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  • Brian Jepson (Editor, Maker Media, Inc.) June 29, 2013 01:26
    Hi Edmund,

    Sounds like you've done some isolation of the problem. There may be one or more bad components; you may want to try a different 555, fresh chips, and even a different display (if one of the diodes in the display is damaged, you may get erratic results).

    For example, I recently built a calculator from a kit, and because I was sloppy (either held the soldering iron down too long or zapped a pin), the first decimal place doesn't light up. So I can type 1.11, 1.111, etc. but not 1.1. In my case, it's clearly a fried pin, but I'm not sure if that applies here.

    Best,

    Brian
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  • Hi Brian,

    I replaced the 555 chip with another, and the circuit now works fine. However, when I put the old 555 back in, it works fine too! So I'm pretty sure it has nothing to do with the chip. But what I find puzzling is that in theory any loose connections / damaged pins should manifest in counting of numbers besides "222"! I guess it has to be one of the 4206 chips, and somehow the error goes away with time!
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  • Brian Jepson (Editor, Maker Media, Inc.) June 30, 2013 19:30
    That is mysterious! It could be that the power draw utilized by 222 was enough to max something out, or that having those particular gates open resulted on a short. But you'd think that would have also happened with 888.
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  • Hi Brian, I've managed to solve completely solve problem of freezing at 222, and it has nothing to do with the chips. I noticed an error in connecting a few components -- this led to unstable voltage at some points, and I think is why the circuit showed the error some times and not others.

    I've moved on to the final part of Experiment 18, and I bumped into another problem.

    The "Start Delay" switch does not work -- it does not start the count.

    I have verified that each individual 555 chip does what it is supposed to:
    IC5 keeps the digit count running,
    IC6 stops the digit count with S3 (and for testing purposes, if I ground pin 4, the digit count starts running again)
    IC7 sends pulses that last about 5 seconds when I press S4 -- I used a voltmeter to probe pin 3, and also tested this using an LED.

    I have also tried something else -- with the entire circuit constructed as in the figure, I additionally placed a wire with one end connected to pin 3 of IC7. Then I connected the other end to a positive power terminal for a second, then quickly grounded it. I basically wanted to mimic the switch from positive to negative voltage applied to C4. In this case, the count is able to restart.

    I replaced C4 with another 0.1uF capacitor, it doesn't work. I have used 0.1uF capacitors from two different manufacturers (I don't know who they are, but one is brick color and the other is yellow). I also tried a 0.01uF capacitor, and also some electrolytic capacitors (1000uF, 100uF, 33uF).

    Any other things I could try to troubleshoot? :)
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  • Brian Jepson (Editor, Maker Media, Inc.) July 05, 2013 13:00
    Hi Edmund,

    This one may be difficult to troubleshoot from afar, but I'll ask the author if he has any suggestions. My only suggestion is that if you've established that the building blocks of the experiment work, that you try replacing the components that "glue" those building blocks together in case one of them is damaged.

    Best,

    Brian
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  • Brian Jepson (Editor, Maker Media, Inc.) July 05, 2013 18:55
    Hi Edmund,

    Charles asked that you post a photograph (or preferably two or three photographs) of the breadboarded circuit from different angles, so that he could attempt to diagnose the problem.

    Thanks,

    Brian
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  • Hi Brian and Charles,

    I have attached some images of the circuit:



    Hope that helps, and let me know if you need any more photographs. Might it also have anything to do with the 555 timer I'm using? It's the NE555P from Texas Instruments.

    Thank you all again!
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  • Brian Jepson (Editor, Maker Media, Inc.) July 06, 2013 01:35
    Hi Edmund,

    We had a look at it, and it's a little hard to evaluate the connections to the display module because the jumper wires are so long and loose. But we weren't able to see anything that jumped out at us. We do know the circuit is well tested if that helps; in the development of the book, the author and two others built the circuit and we are confident in it.

    I'd suggest rebuilding the circuit with shorter component and jumper leads, to keep the breadboard neater and clearer. You may be able to more easily find a missed connection that way. You should also check the orientation of the tactile switches on the breadboard; if it's rotated the wrong way (it looks OK to me though), it will always be conducting/closed.
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  • Thank you for your help so far! I rebuilt the circuit so that the connections are clearer:


    I can't seem to spot any wrong connections yet though...
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  • Brian Jepson (Editor, Maker Media, Inc.) July 08, 2013 13:39
    Thanks, Edmund. There isn't anything in the photos that jumps out at us. Here are some suggestions though:


    • Make sure your electrolytic capacitors are OK. If you ever accidentally reversed them and applied power in this or another circuit, they are likely dead.
    • Confirm that no IC pins are bent underneath them. When this happens, they may appear to be inserted into the board, but aren't.
    • Make sure you are using an adequate power supply that can supply enough current for the circuit.
    • With your multimeter set to measure current, break the connections from chip to chip and use the multimeter to measure the current at that point in the circuit. Do the chips appear to be signaling one another OK?


    Aside from that, I can't see any obvious defects in the implementation. You may, however, want to make sure you solder the connection to the leads on the pushbutton. I see that the wires are wrapped around the leads, but I don't see any solder. You could try touching those two wires together to see if it behaves differently than the button before you bother soldering it.

    - Brian
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  • I’m impressed
    A promise to check with the author... a request for photos for a closer look... Now this is customer service!
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