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