Sun PSI-1 contact t...
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Forum 141

Test Equipment Sun PSI-1 contact tachometer

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The General
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Here's an unusual little piece of test kit - this was an ebay purchase & I thought it may come in useful. It came in a nice steel carrying case along with two rubber faced contact probes & a rubber tyred contact wheel for measuring surface speed. It has two ranges & measures RPM & surface speed in MPH.

On opening it up, there's hardly anything in it - it works by alternately charging a capacitor from a constant voltage supply & then discharging it through the meter. There are two (possibly reed) switches in the transmitter head which switch alternately at rotational speed to accomplish this. The power supply is a 2.7V mercury battery which provides a constant voltage. However the battery was missing in this unit. Powering it from a bench supply proved that the unit worked & seemed accurate as far as I could tell.

Well of course, someone has decided we can't have mercury batteries any more 🤬 so a solution is needed.

I ended up making up a little regulator board with a LDO regulator powered from three AA cells. I fitted a three position switch in place of the original battery test switch so that I could turn the unit off when not in use but retain a battery test feature.

All seems to work quite well now & it was an interesting little project to while away an afternoon...

IMG 20231116 205811 465
IMG 20231105 172818 109
IMG 20231105 170854 926
PSI 1 schematic


Posted : 02/03/2024 8:08 pm
Lloyd reacted