TD-1089 Toilet Flush Alarm

[Terry Perdue]

Seeing the recent query about the TD-1089 got me to thinking that some of you might enjoy hearing a little story about it.

Ever wonder what the 'TD' stands for? Toilet Detector! Read on...

I designed the original prototype for the Programmable Doorbell as a home project after a Heath technician named Joe Huber introduced me to digital logic. (I may be wrong - Joe might have been an engineer; if he wasn't, he should have been. In several months of spare time he had been designing circuitry to decode Morse code into text on a CRT, and the circuitry filled a picnic table in the condo he was renting on Lake Michigan. It worked quite well, and it didn't even use a microprocessor - just descrete logic, aside from the character generator!)

Anyway, after I built my prototype doorbell, I took it in to Heath to see if they might be interested in kitting it. They were, but Ray Freridge, then Director of Engineering, wouldn't let me turn it into a kit because I was in the Amateur Radio group, and it wasn't a ham product. So instead, he gave it to Bjorn Heyning in the General group to finish up.

Bjorn modified my design a little to save money: Where I had used posts and snap-on clips for ease of programming the tune, Bjorn used rows of cheap wire sockets. Where I used button inputs with enough hysteresis to avoid falsing, as I remember Bjorn used transistor inputs with minimal hysteresis.

Since I had done the original design, I was given one of the proofbuilds - (basically a pre-production kit to check the manual for errors). I built the kit, and it worked fine. It was a little tedious to program, but it looked good, and did what it was supposed to.

After the proofbuild had passed through the Evaluation department, I got it back, and was encouraged to install it in place of my old doorbell, which I did. It installed easily, and when I pressed the doorbell, it played the tune I'd programmed.

But I soon noticed that pressing the doorbell button wasn't the only thing that triggered the TD-1089. Two other things did, as well. Whenever I used my ham station on certain bands, either the doorbell would ring, or I'd hear a raspy version of my voice coming from the TD-1089's speaker. Worse than that, whenever the toilet was flushed, the doorbell would ring!

The problem with RF getting into the doorbell wasn't too surprising, but flushing the toilet?

We got our water from a well, and it turned out that the volume of air in the tank was so small that the pump turned on whenever the toilet was flushed, causing a voltage glitch that coupled into the doorbell's button inputs. Putting a bypass capacitor on each of those cured the problem.

(No - The TD in TD-1089 didn't really come from that.)

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