Quote:
Originally Posted by ghlkal
I would love to see the schematic of the board. When I cut off the resistor (pins 4 and 10), why not put the pot for testing (and the ultimate resistor load) there, as opposed to across C4? No, nevermind, don’t tell me – I have enough to do and if this works, that’s great 
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Well, the "green" side of the board IS a schematic, acutally!
The points across C4 are just a whole lot easier to access than trying to solder a resistor across the pin locations. And, there's this:
Robert touched on the need for accurate resistor values; much more accurate than the typical 5%+ found in hobby stores, e.g., Radio Shack. However, when dealing with values in the 140,000 - 240,000 range, even 1% amounts to 1400 to 2400 Ω variance. Not a problem.
By using one or more small value resistors in series, it is possible to trim the ending accuracy well under 1% of the desired value while using readily available 5% resistors IF space is available and especially in DC circuits (as both are the case in this application).
You may already know this, but for other readers that might not, zeroing in on the precise resistance value can be helped with this very simple series/parallel formula and the help of an Excel spreadsheet:
=1/((1/A2)+(1/A3))+A4
Where:
A1 = the cell into which this formula is typed
A2 = one of two resistances connected in parallel
A3 = the other of two resistances connected in parallel
A4 = the total resistance of one or more resistors connected in series with the parallel resistors.
With this formula and a VOM and an assortment package of resistors (Radio Shack sells a packet of common values), one can measure the actual values and plug the measured values into the formula and determine what the actual resistance is going to be (in this application) before actually soldering them together. Once the resistors have been selected to produce the desired resistance, they then can be arranged and soldered together in a neat (flat) little pack and then attached across C4.
All this said, the ZR-1 tach is an analog (electrical and mechanical) device, and accuracy suffers especially when rpm is changing rapidly (e.g., WOT in low gears). For that reason, a (digital) shift light is far superior for indicating when a particular rpm threshold has been reached.
Hope this helps someone...
P.