Simple Lock Combination help<picture included>

Discussion in 'General Electronics Chat' started by okg0591, May 12, 2012.

  1. okg0591

    Thread Starter New Member

    May 12, 2012
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    I need help with simple combination lock. I used the All About Circuit site to guide me through the process. The main issue i'm having is that my green Led light "Go" shine no matter what. But i cannot get the red LED Light to show at all.[​IMG]

    http://www.allaboutcircuits.com/vol_6/chpt_7/7.html

    I've rebuild the circuit several times and still no luck. No matter what the switches are on the green light still shows.... Please help anyone...Thank you very much
     
  2. P-MONKE

    Member

    Mar 14, 2012
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    5
    Unfortunately I had to give up attempting to trace the path back from the green LED through your circuit as I kept getting confused as to where the wire jumpers were going. It would be possible if I could move wires around way to see where the others were going, but this is not possible in a photograph!

    You seem to have inadvertently stumbled across the ancient amateur construction technique colloquially known as "bird-nesting" :D

    Is the any chance that you can tidy up your layout to make it clearer and post another picture?

    Also I see that you appear to be using TTL chips rather than CMOS - there's no issue with the logic here, but there's a possibility that the pull-down resistors may be to large. Try 1KΩ instead of 10KΩ. If that solves the problem then there's no need to do a rebuild apart from good-practice.
     
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  3. dataman19

    Member

    Dec 26, 2009
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    P-Monke is right...
    The CMOS Logic uses the higher value pull down resistors.
    Most non CMOS 4070 applications use 5.1K and more use 1.0K pull down resistors.
    ...
    Dave
     
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  4. okg0591

    Thread Starter New Member

    May 12, 2012
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    Thank you for your replies i'm going to try the 1k instead of 10k and also try rebuilding it tomorrow when i go back to the lab. thank you very much.
     
  5. okg0591

    Thread Starter New Member

    May 12, 2012
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    so the problem was that the XOR chip is TTL so i had to switch my diodes out with an OR chip. Also instead of using 10k resistors i used 1k like i was advised thank you again, now my simple lock combination does work. and it looks like bird nesting is something i'm going to have to work on still lol :)
     
  6. P-MONKE

    Member

    Mar 14, 2012
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    I'm glad it now works!

    But why did you swap the diodes out for an OR chip? I agree that it achieves the same functionality, but I see no problem with using the diodes as the signal would only drop 0.7V, leaving approximately 4.3V for the inputs to the NOR gates, which is plenty to be seen as HIGH by the inputs?

    Maybe my inexperience here is showing - can anyone with more explain why the original circuit designed for CMOS not work with TTL (with the exception of changing the pull-down resistors of course)?
     
  7. okg0591

    Thread Starter New Member

    May 12, 2012
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    Well i asked another person and they told me that i should switch the diodes out with the OR gate since my XOR Chip is TTL technology.
     
  8. dataman19

    Member

    Dec 26, 2009
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    Glad you got it working...
    ...
    We had a similar problem with a crypto lock system when I worked for the government.
    ..
    Contractors did all the wiring, and the NSA Approved digital lock was installed...
    Plans called for low voltage wiring, but never gave enough details about the NSA Approved Cypher Lock (The NSA is like that - Secretive)...
    ..
    Anyway, everyone was in a ******* match and cou;ldn't get the keypad to work. I just swapped out the high value resistors on the "contractor supplied keypad" and the lock system worked.
    ..
    Told everyone I would have to "Kill Them" if I explained it to them.
    ..
    It seems that the NSA likes TTL and not CMOS. Most "commercial and consumer electronic locks use CMOS Logic in the switching schemes.
    ...
    Everything worked great until one day when the Security Chief was reprogramming the lock.... He asked me for a random Combo and told me not to remember it - I gave it to him. He reprogrammed the locks, then went outside to test it. Locked himself and everyone else out of the bunker. At least everyone except me, I was in the back working on a Cypto Terminal. Later on I heard a lot of faint banging up front, went in to control and saw several people standing by the front door. No one else was in control (a security breach on its own) Called them up and asked - What's up? He asked me what the combo number I had given him earlier was - I told him "beats me - you told me not to remember it, so I didn't."
    ...
    He was upset... "Great, how the hell am I gonna get back in. No one knows the combo, and you have to punch in the combo to get out or even open the door????"
    ..
    Three minutes later I opened the door. When he asked how - I told him I shorted pins X and XX. The lock reset to 99-99. He was pissed, how did I know that? Told him it was a crypto cypher, that pin combo generated a control signal and did a master rest (thought everyone knew that)... Only works on the inside lock keypad, the outside keypad won't reset.
    ...
    Ten days later we go another new cypher lock system installed. I guess they don't like back doors - too bad, all cypher locks have back doors...
    Or he was pissed that I knew the back door and he didn't (Security Chiefs can be like that).
    ....
    Anyway, glad your lock works.
    ..
    Dave
    Phoenix, AZ
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The TTL ONE level is typically only about 3.5V. The diode OR gate drops it to about 2.9V. That's too low for a 4000 series CMOS gate.
    The 10k resistors could still be used the the switches if one were to swap the positions of the resistors and the switches: Resistors to +5V, switches to GND. In fact, this is preferred for TTL.
     
    Last edited: May 20, 2012
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  10. P-MONKE

    Member

    Mar 14, 2012
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    The OPs NOR gates were also TTL, but I see your point. Like I said, my lack of experience was hindering me here.

    Thanks for the tip!
     
  11. Ron H

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    Apr 14, 2005
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    How did you determine this?
    EDIT: I see what looks like PCF74.. on the chip that appears to be the NOR gate. Is that your clue?
     
    Last edited: May 21, 2012
  12. P-MONKE

    Member

    Mar 14, 2012
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    I saw "74" on it in two places - I'm wrong aren't I? I can't get any sensible answer by Googling "PCF74" - is it CMOS then?

    Luck this isn't my day-job :D
     
  13. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I don't know if you are wrong or right.
    There are CMOS parts that start with 74: 74HC, 74HCT, 74C. I don't know where the PCF comes from.:confused:
     
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