Noob trying to design 4-way stop-light-like system

Discussion in 'The Projects Forum' started by drteeth, Nov 18, 2010.

  1. drteeth

    Thread Starter New Member

    Jun 25, 2005
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    I am quite a newbie in the electronics world. You will find me listed in the contributors section in the DC book linked here because of some edits I suggested way back in 2005, but I studied electronics for about a week and then quit back then. I'm back at it and further in today.

    I'm trying to design a simple (though not to me) design for my kids for Christmas and would love your help in making sure I'm not making any stupid mistakes. My kids love Thomas the train and have a bit of track that they are always making elaborate designs with. I was hoping to make them a stop light for the tracks for Christmas.

    The idea for the design is that there would be a main on/off switch (probably a slider) that would turn the system on. On the north/south sides, it would show a green light. On the east/west sides, a red one. When they press a momentary button, they would flip: north/south would now be red and east/west would now be green. When they press it a second time, it would revert back to the initial state.

    I am diving into all the books and tutorials I can find and have learned about resistors, capacitors and transistors, but that's about it. I'm not fully comfortable creating circuits with all those present quite yet. However, I have come up with a design that seems to work in Yenka. I've attached a screenshot of it. Please look it over and correct any mistakes I've made. I found the basic idea for the 4013 side of the design on the web and added my own idea for the transistors to the LEDs.

    I'm trying to keep costs down and the design as simple as possible. I'm not quite sure I know exactly why it works in the simulator and that frightens me. =) Please correct, guide, point, chastise, make fun of, lead or congratulate me on any aspect. I'm trying to learn any and everything I can.

    Thanks, all! I'll be sure to keep you all updated as the project progresses.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    In your circuit, you have one PNP and one NPN transistor being driven by the Q output. The PNP is being used as a saturated switch, but the NPN is being used as a voltage follower. The NPN transistor should be on the low side (cathode side) of the LEDs that it's driving, so that it will operate as a saturated switch.

    Your base resistors are rather high in value.
    If you are going to be using TTL logic, you will need to use 5v for the supply.
    CMOS 4000-series devices can operate from ~3v-16v, but have limited current source/sink capability.

    Have a look at Rob Paisley's site for Model RR electronic projects:
    http://home.cogeco.ca/~rpaisley4/CircuitIndex.html

    Lots of project ideas, including several for traffic lights that sequence by themselves.
     
  3. drteeth

    Thread Starter New Member

    Jun 25, 2005
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    I've attached an updated schematic that I think reflects what you are suggesting. Please forgive the layout -- I'm sure I've broken all sorts of rules and standards on that thing.

    If you wouldn't mind, could you describe the benefit(s) of connecting the NPN on the low side of the LEDs? I did a quick google search looking for transistor saturated switches, but didn't really find an explanation of why they are laid out this way. It's entirely possible that I just haven't gotten that far in my reading yet....

    I believe you are referring to the 1k ohm resistors connected to the transistor bases, right? What would be a better value?

    I hadn't considered having enough voltage to drive the 4013 itself. That's what I get for trying to design a project that I don't fully understand, huh? :eek:

    So let me ask this: is there a different/better way to create a simple toggle switch like I'm trying to do here with the 4013? I'm trying to make the system toggle with a push button. I would also like to avoid loading this little thing up with batteries to try to get it up to 5v. Are there any suggested changes in the overall design?

    Wow, those are quite a number of projects. Most of them are a little more than I would need and/or want for now, though. I've seen some projects of lights that sequence themselves, but don't want that for my kids. The trains this project is for are 3 and 5-year-old hand and imagination-powered so I would like it to stay at whatever setting they'd like it to be at for as long as they'd like.

    Thanks for the link, though. I'm always interested in looking at what other people have designed. I'm not quite up to understanding exactly what's going on in his designs by just seeing the schematics, though. Someday (soon). :)

    Anyway, thanks for the reply! I look forward to what suggestions are made next.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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  5. drteeth

    Thread Starter New Member

    Jun 25, 2005
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    Thanks for the link, Bill. However, as stated above, I don't want them to cycle automatically. I had found a number of designs in which they did automatically cycle but for some reason couldn't find any that were only cycled when a button was pressed. That led me to try to design this circuit.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    So you replace the 555 timer with a switch. The timer is an oscillator, the 4017 is the counter that records the states. The basic circuit is sound.
     
  7. whatsthatsmell

    Active Member

    Oct 9, 2009
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  8. retched

    AAC Fanatic!

    Dec 5, 2009
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    Whats wrong with a latching relay?

    Every button press would switch the poles.
     
  9. drteeth

    Thread Starter New Member

    Jun 25, 2005
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    After searching around a bit after reading your post, I found this: http://www.bowdenshobbycircuits.info/page9.htm#bistable.gif

    Would that work? I tried it in my simulator but the button just lit up both LEDs full brightness. I understand that simulators can't do everything so does this bistable flip flop seem sound? How can I modify it to only use 3V? What purpose do the capacitors play in this circuit?

    Thanks again for all the answers and suggestions. I thought of a relay, but aren't relays like that expensive? I guess I'm not sure exactly what I'd be looking for.
     
  10. Wendy

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  11. djsfantasi

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    Apr 11, 2010
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  12. GetDeviceInfo

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