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Re: [GPSL] PICAXE for cutdown

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  • PAUL VERHAGE
    I have most of the cutdown designed and because we have finals today, I think I can finish during class. It uses a PICAXE-04 and has a temperature and
    Message 1 of 8 , Jun 5, 2007
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      I have most of the cutdown designed and because we have finals today, I think I can finish during class.  It uses a PICAXE-04 and has a temperature and pressure sensor as inputs.  The PICAXE can be programmed on the cutdown PCB, so the code can be tweaked without removing the chip.  I'd recommend using the PICAXE-08M so it can do some data logging during the flight.  That feature should allow us to adjust the code for best performance.  I want to program it to cut away the balloon based on time, change in temperature, or on pressure (and change in pressure).  Adding a datalogging feature measures cramming a lot of code into 256 bytes.  Does anyone have a micro shoe horn? 
       
      The pressure sensor is a SM5812 which I've had pretty good luck with.  The temperature sensor is a thermistor and 47k resistor.  A 47k resistor, according to my calculations, gives you the greatest sensitivity in the voltage dividing temperature sensor.  The thermistor is the tiny Radio Shack one and should be mounted outside the cutdown enclosure so it senses temperature changes.  The PCB will be small and use two batteries, one for logic power and the other for the burner. 
      I should have several boards and will bring them to GPSL.
       
      Paul
      >>> <mconner1@...> 6/4/2007 8:27 AM >>>
      PICAXE website:
      http://www.rev-ed.co.uk/picaxe/

      The 8-pin versions are $4.  Nice alternative to PICs if you don't want to bother with getting/making a PIC programmer.  About 80 lines of BASIC code available for use (shared with data memory).  The resonators are internal so all you have to do is hook up +5V, ground, serial in/out for programming, and whatever sensor you want.

      Looks extremely simple and powerful for non-complex tasks like timing circuits, making the barometric cutdown Paul mentioned, etc.  You could make a combination time/baro cutdown - have it cut down at a predetermined pressure, or after X amount of time (in case your ascent rate is way too low and you want to avoid an unintentional long-duration flight).  The most expensive part of the whole thing is probably the pressure sensor (about $30). 

      Total parts are probably the PICAXE, MOSFET to switch the nichrome on, pressure sensor, and batteries.  You might include a small beeper to go off before the cutdown is engaged so that if it was turned on way before launch you'd have a chance to reset the cutdown rather than have it release your balloon while you're still holding the payloads.  The beeper would also be a nice feature to verify correct operation (e.g., beep every 10 sec in normal mode, every 1 sec as a cutdown warning).

      Might also want power to the PICAXE/sensors be isolated from the cutdown power (separate battery) so that you don't have a brownout when tripping the nichrome.

      Sure wish I had the time to mess with this.

      73 de Mark N9XTN



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    • D
      ... today, I think I can finish during class. It uses a PICAXE-04 and has a temperature and pressure sensor as inputs. The PICAXE can be programmed on the
      Message 2 of 8 , Jun 6, 2007
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        --- In GPSL@yahoogroups.com, "PAUL VERHAGE" <paul.verhage@...> wrote:
        >
        > I have most of the cutdown designed and because we have finals
        today, I think I can finish during class. It uses a PICAXE-04 and has
        a temperature and pressure sensor as inputs. The PICAXE can be
        programmed on the cutdown PCB, so the code can be tweaked without
        removing the chip. I'd recommend using the PICAXE-08M so it can do
        some data logging during the flight. That feature should allow us to
        adjust the code for best performance. I want to program it to cut
        away the balloon based on time, change in temperature, or on pressure
        (and change in pressure). Adding a datalogging feature measures
        cramming a lot of code into 256 bytes. Does anyone have a micro shoe
        horn?

        Been there, done that before. Good luck squeezing the code.

        >
        > The pressure sensor is a SM5812 which I've had pretty good luck
        with. The temperature sensor is a thermistor and 47k resistor. A 47k
        resistor, according to my calculations, gives you the greatest
        sensitivity in the voltage dividing temperature sensor. The
        thermistor is the tiny Radio Shack one and should be mounted outside
        the cutdown enclosure so it senses temperature changes. The PCB will
        be small and use two batteries, one for logic power and the other for
        the burner.

        Would you have better luck, rather than using a poorly calibrated
        thermistor, using something like a National Semiconductor LM19
        temperature sensor IC (in a TO-92 package)? 2.4V to 5.5V supply
        voltage at 10 microAmps of current. +130C to -55C temperature
        range.

        http://www.national.com/ds/LM/LM19.pdf

        Digikey has them for US$1.11 (which may even be cheaper than the
        Radio Shack thermistors!).

        Yeah, yeah, I know. Don't you just hate engineering changes late
        in the design cycle. ;-)

        > I should have several boards and will bring them to GPSL.
        >
        > Paul

        Dave
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