Shop OBEX P1 Docs P2 Docs Learn Events
Missing pulse detector advice — Parallax Forums

Missing pulse detector advice

TCTC Posts: 1,019
edited 2014-04-30 03:30 in General Discussion
Hello all,

I have been spending quite a lot of time on my reflow oven project. Decide on something, find a better idea, re-do the code, hate myself because I am starting over, again :lol:

One thing I really want to have, is a back up in case the Prop locks up. The Prop controls the SSR's that are hooked to the heater elements. My luck would be that the prop would lock up when the outputs are HIGH (on), and leave the heaters on creating a fire. So I figured I would add a missing pulse detector, then AND its output with the Prop's output. Before every simple PWM (that is what I call it) cycle, the prop would pulse an output that would let the missing pulse detector know the prop is still in control.

But I am having a concern. I was going to use a 555 timer for the missing pulse. But I was wondering, what would happen if everything is working like it should. The Prop is pulsing a pin that is hooked to the 555, and the output of the 555 is HIGH, witch is going to one leg of an AND gate, and the other leg of the AND gate is coming from the heater control pin from the Prop. Now lets say the Prop gets stuck the exact moment it pules the 555 LOW, and never goes back to HIGH. ( I really hope that made sense ) What would happen to the output of the 555? would it go LOW? or would it stay HIGH since the cap is never going to charge?

I need a pules detector that no mater what, it will drive its output low anytime it does not see a pulse. HIGH to LOW, and LOW to HIGH

Thanks
TC

Comments

  • bill190bill190 Posts: 769
    edited 2014-04-28 05:39
    In the past, I have created test gizmos to create situations which will test things like that. For example, I might use a Stamp to send a test signal to a Prop or monitor what a pin on the Prop is doing.

    Also for critical things, it is good to have two totally separate safety devices. So maybe also have a separate temperature sensor which would shut everything off if the temperature got to be too high?

    Note the military uses "redundant systems" and once had one such system to launch a nuclear weapon. But the output of both systems went to ONE chip. Needless to say, that one chip failed and a nuclear weapon was almost launched! (Best to have TOTALLY separate systems.)
  • TCTC Posts: 1,019
    edited 2014-04-28 07:24
    bill190 wrote: »
    In the past, I have created test gizmos to create situations which will test things like that. For example, I might use a Stamp to send a test signal to a Prop or monitor what a pin on the Prop is doing.

    Also for critical things, it is good to have two totally separate safety devices. So maybe also have a separate temperature sensor which would shut everything off if the temperature got to be too high?

    Note the military uses "redundant systems" and once had one such system to launch a nuclear weapon. But the output of both systems went to ONE chip. Needless to say, that one chip failed and a nuclear weapon was almost launched! (Best to have TOTALLY separate systems.)

    I was thinking of maybe taking another thermocouple and making a separate system that would drive the RESET line low if the temperature got above a set point. But that would only cover if the prop locks up. Then I thought of using a thermal cut off to kill the power to the oven if the temperature got to high. But I am running in to two problems.
    1) the max temperature I could find for thermal cut offs is 240C, and some reflow profiles I am seeing go to 260C.
    2) I just spent a lot of time putting the extremely cheep toaster oven ($15USD) back together, and I really dont want to have to take it back apart, unless I really have to.
  • bill190bill190 Posts: 769
    edited 2014-04-28 08:27
    Google kiln heat probe

    or kiln temperature probe

    or kiln thermocouple

    Also try the main word...

    crematorium
  • TCTC Posts: 1,019
    edited 2014-04-28 09:41
    bill190 wrote: »
    Google kiln heat probe

    or kiln temperature probe

    or kiln thermocouple

    Also try the main word...

    crematorium

    Please forgive my lack of understanding, but the only thing I could take form your searches are for thermocouple probes. My oven currently is using 2 K-type thermocouples (one for above, and one below the board). They are made by Omega, and made for precision measuring.
  • JonnyMacJonnyMac Posts: 9,107
    edited 2014-04-28 09:58
    A buddy of mine in England uses the 555 as a watchdog in many of his projects. Sprinkled through is code are blips to the 555 that keep its output from causing a reset. If the 555 times out, it creates a pulse on the processor reset input.
  • TCTC Posts: 1,019
    edited 2014-04-28 10:38
    JonnyMac wrote: »
    A buddy of mine in England uses the 555 as a watchdog in many of his projects. Sprinkled through is code are blips to the 555 that keep its output from causing a reset. If the 555 times out, it creates a pulse on the processor reset input.

    Never thought of doing that. Thank you
  • kwinnkwinn Posts: 8,697
    edited 2014-04-28 11:01
    If you are concerned with a low (or high) level from the prop pin holding the 555 in a specific state you can always couple the signal from the prop through a capacitor.
  • TCTC Posts: 1,019
    edited 2014-04-28 12:32
    kwinn wrote: »
    If you are concerned with a low (or high) level from the prop pin holding the 555 in a specific state you can always couple the signal from the prop through a capacitor.

    Would you be able to explain a little more? I don't quite understand.
  • jmgjmg Posts: 15,173
    edited 2014-04-28 12:42
    TC wrote: »
    I need a pules detector that no mater what, it will drive its output low anytime it does not see a pulse. HIGH to LOW, and LOW to HIGH

    You need an EDGE driven WatchDog. Proper monostables (the 555 is not one of those) like HEF4528 or HC123 are edge retriggerable.

    You still have to worry about the SSR failing, so an alternative is a simple Mains Timer, so you cannot have the oven on for hours..
    Also, thermal cutouts do not have to be inside the oven.
  • kwinnkwinn Posts: 8,697
    edited 2014-04-28 18:21
    TC wrote: »
    Would you be able to explain a little more? I don't quite understand.

    Putting a capacitor between the propeller pin that sends "heartbeat" pulses to the 555 missing pulse detector circuit will block the dc levels but allow the pulses through. If the prop stops toggling that pin it can be high, low, or in the high impedance input state, but because the capacitor is blocking the dc level it will not affect the 555 state.
  • TCTC Posts: 1,019
    edited 2014-04-29 04:18
    jmg wrote: »
    You need an EDGE driven WatchDog. Proper monostables (the 555 is not one of those) like HEF4528 or HC123 are edge retriggerable.

    You still have to worry about the SSR failing, so an alternative is a simple Mains Timer, so you cannot have the oven on for hours..
    Also, thermal cutouts do not have to be inside the oven.

    I could not spend a lot of time looking(am at work), but I could not quickly find an example of a missing pulse detector using those parts. Would you have any suggestions on where I should look?

    Also, if the thermal cutout does not have to be inside the oven, i would assume you are talking about me putting in the area next to the closed oven area. Would I just put a thermocouple in that area, and ramp the oven to 260C say for 30min, to find the temperature thermal cutout I should use? I have the oven insulated, what side effects could that create?
    kwinn wrote: »
    Putting a capacitor between the propeller pin that sends "heartbeat" pulses to the 555 missing pulse detector circuit will block the dc levels but allow the pulses through. If the prop stops toggling that pin it can be high, low, or in the high impedance input state, but because the capacitor is blocking the dc level it will not affect the 555 state.

    Ok, I think I understand what you are talking about.


    (prop pin)
    (capacitor)
    (555 timer trigger)

    Is there a way I can figure out what capacitor value I should use?
  • kwinnkwinn Posts: 8,697
    edited 2014-04-29 12:12
    TC wrote: »
    Ok, I think I understand what you are talking about.


    (prop pin)
    (capacitor)
    (555 timer trigger)

    Is there a way I can figure out what capacitor value I should use?

    It depends a bit on the type of missing pulse detector circuit you are using and the component values for the timing, but a 0.1uF is a good general starting point. Post a schematic and I will take a look at it.

    BTW, for over temp switches and such take a look at what is available at a furnace/boiler parts supplier.
  • TCTC Posts: 1,019
    edited 2014-04-30 03:30
    kwinn wrote: »
    It depends a bit on the type of missing pulse detector circuit you are using and the component values for the timing, but a 0.1uF is a good general starting point. Post a schematic and I will take a look at it.

    BTW, for over temp switches and such take a look at what is available at a furnace/boiler parts supplier.

    Currently this is in the idea phase. Before bill190 suggested a "redundant" system, I was only looking at the 555. now I am looking at other options. I do not have a schematic yet, since I have not fully settled on what I want to do. I am working with your suggestion of furnace/boiler supplier. Sadly, there are no places close by to me where I could go talk to someone, and ask questions. So I have to check whats online.
Sign In or Register to comment.