555 verses 4098

Discussion in 'The Projects Forum' started by BReeves, Nov 24, 2012.

  1. BReeves

    Thread Starter Member

    Nov 24, 2012
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    Hi Folks,

    New guy on the block with a whole bunch of questions but will post one at a time. I'm in the process of adapting several circuits that a buddy of mine designed some time ago. Sadly he passed away from cancer a while back so I can't ask him why he did some of the things he did. I spent most of my working life as a technician and although I can look at a schematic and understand what it does I am not a design engineer.

    First question is... For some reason that escapes me he simply didn't like using 555 timers and most of the circuits he designed uses the CD4098 for a simple timer. I happen to like the 555 and am comfortable making it do what I want. The component count is less with a 555 and we don't care about how much power the IC draws.

    So any ides on why he might chose the 4098 over a 555 and just make things more complicated?
    Thanks
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,087
    3,026
    It looks like it's a bit more flexible in some ways, such as triggering on up or down versus downward for the 555. You also have more/easier control over the output pulse widths, on versus off. Probably other goodies too, but those are a couple of things that jump out at me from the datasheet.

    On the negative side, the 555 wins for current sinking and sourcing. The 4098 can do very little.
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,353
    Once again, the devil is in the details. The specific application and requirements might dictate a preference for one chip over another.

    You really need to indicate the application and show a circuit schematic.
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,220
    The component count is not necessarily less for a 555. A one-shot only requires one resistor and one capacitor with a CD4098.
     
  5. BReeves

    Thread Starter Member

    Nov 24, 2012
    412
    64
    Thanks for the response..

    The circuit is a pretty simple debounce timer for a proximity sensor, the sensor is coupled through a transistor and we drive a transistor with the output. Nothing special, I can run the 555 off 12 volts and eliminate a 7805 he used to power the 4098.

    Just wondering if I might be missing something like maybe the 555 being more susceptible to falsing or some other abstract trait he might have been aware of I'm not.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    As I said, you have to get down to the details:

    1. You want to operate at 12V and eliminate the 5V regulator.
    2. You want to debounce a proximity sensor.

    What is the proximity sensor?
    What are you bringing in proximity to what?
    What is the proximity sensor driving? That is, are you detecting proximity or are you counting objects?

    You can debounce a noisy signal using a resistor and a capacitor.
     
  7. BReeves

    Thread Starter Member

    Nov 24, 2012
    412
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    Yes and Yes...

    The sensor is on the track of a roller coaster telling a photo system when to take the photo. Sometimes we need to trigger on each car of a train and sometimes we just need to trigger on a single car. All the timer has to do is prevent a double trigger on one car. We trigger the system on the rising edge of the timer's output and adjust the pulse time depending on the ride.

    As you can imagine around a roller coaster we have motors starting up and all kinds of static noise going on. All inputs and outputs are low impedance mostly switching 12 volts through optoisolators. We really haven't had a big issue with falsing but the potential is there.

    Apreciate your opinion,
    Bob
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,415
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    Hah! We're getting somewhere.

    You still have not said what you are using as the proximity sensor?
    Is it a magnetic reed switch that closes on a passing magnet?
    Or a Hall Effect sensor?
    Or a photo sensor, photo coupler, IR detector, etc.?

    Going back to your original query, I can imagine that a 555 timer circuit should work for you. All you now need to do is debounce or filter the trigger input, possibly with a simple resistor and capacitor circuit.

    It would help if you provided the circuit that you have now.
     
  9. BReeves

    Thread Starter Member

    Nov 24, 2012
    412
    64
    Thanks for the help on this.. I'm working on the schematic as we need it to also alternatly fire two strobes and trying to get everything on one board. When I get it finished (as I mentioned in the first post) I'll probably have a few more questions.

    The sensor might be just about anything that will give us a high to trigger the opto. We have used all the above depending on the requirements of the ride. We even did one Carnival ride that used a micro switch.

    I am more or less Retired but still enjoy writing software and decided to see if I couldn't develop a ride photo system that was aforadable for small parks and carnivals. Have the software finished and now working on the interface electronics. Spent 20 years running around the country working out of motels, that part is over for me :D
     
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