Since you brought up Tiny Tachs - I have messed with Tiny Tachs with mixed results. I have found a newer electronic tach, called TTO, which works perfectly.
Also, I have found a couple ways to put hours on the tachs if they lose accumulated hours memory and/or get reset to zero.
Following is a summary I wrote for another newsgroup, EaglersNest; maybe of some value:
In the email attached below this one is a description of the Tiny Tachs that we have and tried.
Update on the Tiny Tach TT4RX, "Tiny-Tach 4C reset w/ filter" - This is an older version, calibrated for one spark per 2 revolutions, and not adjustable. After it locked up, I was able to reset it to operation by peeling off the front cover, digging out some rubber, and shorting the terminals. I think these units lock up if there is too much signal. Our DE has dual ignition, primary is distributor ignition, secondary is magneto. We had run our Tiny Tach wire alongside the magneto on/off switch wiring, and I think it may have been overcome with the signal noise from magneto. As part of the effort, I found someone on the web that ran his TT wire through the grounding braid of a piece of coax that he had removed the inner conductor and its insulator from. A lot of coax is too tightly constructed to allow this, but I had a piece that I was able to do it. I worked the TT wire through it. The TT wire is itself a coax; its inner wire is wrapped around the spark plug wire, and the shield is connected to a ground at the engine. On the added, outer coax I insulated the engine end, but grounded the panel end of the coax. Also, I routed the coax away from the magneto wiring.
This all seemed to work, and work well, too. Problem is, this TT model accumulates total time in whole hours. Not hours/minutes, or hours/tenths of hour. We wanted to see better accumulated time resolution than whole hours.
A couple of extra tidbits of this older model. There is a reset button, and it will reset the accumulated time back to zero. I had made a plastic cover to protect the button to prevent accidental time reset. I also found that a run signal can be faked into these OLDER models. Connect the ground wire and the signal wire of the TT to the secondary of a 12 volt, AC transformer (not a DC power supply, has to be AC). I used this technique to build the run time back up to match our engine run time.
Kiblinger and Spencer came up with a TTO tachometer. I ordered one and tried it out. It worked great. No wandering, steady updates, no lock-up. It shows accumulated time in hours/tenths of hour, non-resettable. Immediately after shutting engine down, it shows the max rpm for maybe 10 seconds, then shows the accumulated hours. It is smaller than the Tiny Tachs, but the display is about the same size. It has a setting button on the back surface, and can be set for # pulses/crank rotation, fast/slow updates and for high/low sensitivity. The wire is not a coax, and it does not require a ground connection at the engine, just some wraps around the spark plug wire. I did take care to route the wire away from the mag wiring, because it could possibly be confused with all that electrical noise. I like it. It is a "Trail Tech TTO Tach/Hour Meter 72-A00", ordered from PowerSportSuperStore.com, and it cost $29.65 total. There are sources a few cents cheaper, but I had purchased from this place previously, so went with them.
Only problem, it had zero time on it, and I wanted it to match the engine time. During my testing of Tiny Tachs, I found that the newer, settable versions could not be fooled to operate with the 12 VAC transformer trick. I figured that these newer tachs sense a signal that has a faster transition time than 60 hz. I had a switching power supply from an old TV digital converter, and thought that there may be some fast transition signal available in there, so I opened it up and tested it with one of the newer model TTs. Just to be sure not to get any higher DC voltage in the TT, I put a 100 pf capacitor in series with the signal lead, and began touching points of the power supply circuit board. I found a spot that tricked the TT to operate at about 15,000 rpm and it started accumulating time (Didn't have to connect the ground wire to anything to make the tach work, so I simply connected the ground wire of the TT to the metal work bench (yeah, I know, electricity on a metal bench). I connected a house light timer to it and set in a dozen hours and let it run. Next day the TT had the right amount of hours on it. Problem solved.
I tried the same trick on the TTO and it worked, although there is no ground wire to connect to the bench. So, I was able to set the TTO with the correct engine hours. We put it in the airplane and have a couple add'l hours on it.
The switching power supply is a wall wart, 120vac input, 5 vdc output, 1.72 amps. I can't guarantee that any other PS will work; may have gotten lucky with this one. I think what is happening here is that the switching power supply converts 60 hz to maybe a couple thousand hz in the process of making the DC in the output. I suppose the tachs sense this higher frequency as a spark signal.
A previous method I used was to connect a neon transformer (120vac input, approx 5,000 vac output) to a spark plug with spark plug wire, and wrap the TT signal wire around the spark plug wire. This worked great, but I did not like running the sparking plug unattended for a dozen hours, and it was somewhat cumbersome.
The newer TTs, even with the wire run away from the mag wiring, did not operate as smoothly as the TTO. I still think that the newer TTs can work well in some applications, but may be more sensitive to extraneous electrical noise, causing the display to constantly fluctuate. Maybe the added outer coax would work. But, I probably won't be messing any more with them soon. Gonna try to do some flying.
Tom H
