In the circuit description above the schematic diagram the meter is referred to as 50mA FSD, but in the paragraph below the diagram the meter is referred to as 0 - 1mA. This is a very important difference in trying to calibrate the circuit. Which is the correct meter 0-1 mA or 0-50mA ? Thanks.
I looked up a couple other ones on the net and found one that was 10V FSD meter and another that was 1mA FSD. I guess it would be best to check around and see if you can find another with the same pot hooked to it and go from there or just use a different design. Hope this helps a little.
A milli-ameter in series with a resistor is a voltmeter. So don't go looking for a voltmeter. This circuit requires a 1 ma meter. The 200k pot makes it a voltmeter with a 200v range (!!). I would suggest that 10K (or 20K) would be more suitable
50ma meter mentioned in the text must be a mistake. With the 200K pot set to it's mid position ( for example) it would require 5000v from the 555 to indicate FSD !
As the 555 can source (or sink) 200ma probably it would work with 50ma meter - if you could find one - in which case the adjuster pot might be 470 ohms. But not recommended !
The 4.7K resistor shown between the output and +v must be a mistake as well. Leave it out.
I've had mine working for 6 years now and re-visited because I'm about to make another. That circuit was probably cribbed from somewhere else on the net and the error introduced. I found the same circuit with a 50 uA (Micro-amp) meter NOT 50 Ma. With 200K that would work. But a 50uA meter is a very delicate instrument to put in a car. It's unnecessary, expensive and would be hard to find. 1 Ma meters + 10K is the way to go.
Incidentally a good way to trigger this tacho is with a small inductor wound on a circular ferrite core with the wire between CB and the ignition coil passing through the centre of the inductor. I got one from a scrap computer PSU. Connect inductor between Gnd. and pin 2