Car Tachometer

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.

But what do I know/ I'm just a drummer :)
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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