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Heathkit rain sensor problem

Discussion in 'General Electronics' started by George Rachor, Jun 8, 2005.

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  1. I've got an old heathkit weather computer where the rain sensor
    has become very erratic. I've disassembled the rain collector and don't see anything
    wrong with the mechanics.

    The sensor seems to be recording much more rain than is possible during the
    measured period. I noticed that the heart of this is a reed relay.

    Can these relay's fail in such a way to be very bouncy?

    George Rachor
  2. Robert Baer

    Robert Baer Guest

    *What* belongs to the relay?
    DO NOT use the apostrophe, unless it is needed and required.
    The *relays* either work or do not work.
    If the drive to the relay has decreased a lot, then excess chattering
    at pull-in is possible; but the relays are not at fault.
    Use some logic.
    What is used for a detector?
    Is a float used, and a magnet is on the float, and the magnet
    operates one or more relays?
    Something else?
  3. OK... Bad terms....

    It isn't a reed relay but a magnet driven switch inside a glass bulb.

    The switch simply closes a circuit generating a pulse that the weather computer reads everytime
    enough rain enters the collector. The rain gathers in a cup that when it gets heavy enough it tips
    the collector cup. This action causes a magnet to slide by the magnet driven switch.

    The weather computer is seeing 4-20 inches of rain in a 24 hour period and I know that is just plain wrong.

    George Rachor
  4. Robert Baer

    Robert Baer Guest

    Magnets do not go bad, and the reed relays that closes the pulse
    generator(s) either work or do not work.
    If the spacing between the magnet and the sensing reeds have not
    changed, then there will not be excessive bounce from minimal magnetic
    Check to see that all other associated moving parts are clean, free
    of sticky oils, gummy deposits, etc.
    Observe its operation carefully during a simulated rainfall, to see
    if the cup fills properly before tipping.
    Maybe it tips when only half full, which would double the reading.
  5. Magnets can and do lose their strength. I have also witnessed (first
    hand) the failure of a magnetic reed switch after only a hundred cycles
    or so.
    Perhaps, but if the current thru the reed switch is excessive then
    damage to the contacts is possible over time.

    Since contact bounce is a way of life with any mechanical switch, there
    must be some kind of debounce circuitry in the sensor or (more likely
    IMO) inside the base unit. It is possible that it was barely sufficient
    when the reed was new and is now unable to suppress the glitches. Or
    even some component in the debounce circuitry has failed due to age or
    lightning induced voltage surges.
    The readings the OP are getting are far higher than if the cup was
    tipping when only half full.
  6. Robert Baer

    Robert Baer Guest

    1) Only the old iron magnets get weak over time. Alnico and ferrite
    magnets DO NOT. However, if you put those magnets in a high temperature
    environment, then the magnetism can be removed (look in the Handbook of
    Chemistry and Physics about the Curie Point).
    2) The "half full" statement was for illustration purposes only, and was
    not to be taken literally or even exactly. I could have easily said "one
    third" or some other fraction. Maybe your cat got curious every time
    and tipped his cup...
  7. Karl Uppiano

    Karl Uppiano Guest

    The weather computer is seeing 4-20 inches of rain in a 24 hour period and
    Is it over-reporting rainfall, or is it registering rain on sunny days?

    It is quite common for magnetic reed switches to bounce, yielding multiple
    pulses per pass of the magnet. High speed counter logic will count each
    bounce as a separate pass. The traditional way to debounce mechanical
    switches is to use a Schmitt trigger with a small resistor/capacitor network
    on the switch input. This works pretty well, especially for slow events like
    rain sensor buckets tipping back and forth. Software debouncing is also
    sometimes used. So you have some things you can look at, in order of

    - The power supply could be getting noisy. All sensor inputs might have much
    lower noise margins than they used to. If it gets bad enough the entire
    system could become flaky.
    - The reed switch might be going bad. If the internal contacts have
    corroded, the debouncing strategy might not be adequate anymore.
    - The Schmitt trigger IC (if there is one) might be going bad, but I think
    that is somewhat unlikely.
    - If the debouncing capacitor is electrolytic (unlikely) it might have dried
    out. But usually they use small-valued caps, and those tend to be ceramic,
    and not prone to failure.
    - Circuitry exposed to the elements is much more likely to fail. You might
    need to clean connectors and reflow solder joints, and possibly replace some
    - Has anybody installed a new radio transmitter in your vicinity? RF
    interference from anything bigger than a cell phone or wireless network
    could be causing all sorts of problems. Shielding and bypassing RF is beyond
    the scope of this post. :)

    Given its age, I would check the power supply for ripple. Use a good DMM
    that blocks DC on the AC ranges. Most "good" power supplies have less than 3
    millivolts of AC. YMMV, but if you see anything approaching 1 volt of AC,
    it's a pretty good bet that some of the electrolytic filter and bypass
    capacitors have dried out, and it's probably time to replace them all.

    If you have the Heathkit assembly manual, it probably has schematics,
    voltage charts, circuit descriptions and layout diagrams which should help
    tremendously in debugging this problem.

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