Re: Tach for 70 AMX

[Tom Jennings <tomj@xxxxxxx>]

OK so I cobbled up a tach design. I haven't built it haven't built it  
yet but I will start here. It should work with any ignition type  
within reason.

I left the charger for my laptop at work, so I have to sign off now.  
I'll build this circuit, and a calibrator for it, and give details.



Simple 555-based tachometer circuit
Tom Jennings
21 Nov 2005


Should work with any non-insane ignition, points, Duraspark, TFI,
HEI or MSD box. Might not work with true high-power CDIs. Will
work from any "tach output" signal. Should be fine 200 rpm to
10,000 rpm.  Accuracy will be as good as the quality (temperature
drift, mainly) of capacitor c3 (1% should be easy).




                            +--------+----------+---+------------ 
+---------------------------------------+-------+
                            |        |          |   |             
|                                       |       |
                            \        |          |   |             
\                                       |       |
                        r2  /    d2  _     +----+---+------+     /  
r3                                    |       \
                            \        ^     | reset  power   
|     /                                       |       / r5
  to                        /        |     |                
|     /                                       |       \
  "COIL -"                  |        |     |               |      
|                                       _       /
          r1                |        |     |               |      
|                                  c5   _       |
  <----/\/\/\/\---+----|<---+---||---+-----+ trig   thresh +----- 
+                                       |       |
                  |    d1   |   c2         |               |      
|                                       |       |
                  |         |              |        dischg +----- 
+                                       |       |
                  |         |              |               |      
|                                       |       |
                  _         |              |    NE555      |      
|                    +------------+     |       |
              c1  _         - d3           |               |      
|                    |            |     |       |
                  |         ^              |               |      
|      r4            |  meter     |     |       V
                  |         |              |           out  
+-----|----/\/\/\/----+----+  movement  +-----+       to
                  |         |              |               |      
|               |    |            |     |       switched
                  |         |              |               |      
_               |    |            |     |       battery
                  |         |              | timing  gnd   |     _  
c3            |    +------------+     |
                  |         |              +-----+---+-----+      
|               |                       |
                  |         |                    |   |            
|               |                       |
                  |         |                    _   |            
|               _                       |
                  |         |                c6  _   |            
|               _ c4                    |
                  |         |                    |   |            
|               |                       |
                  |         |                    |   |            
|               |                       |
                  +---------+--------------------+---+----------- 
+---------------+-----------------------+


COMPONENTS

         NE555 timer chip or equiv.
         r1 100 ohms
         r2 1000 ohms
         r3 tbd (47k?)
         r4 tbd (50k trimpot in series with 47K resistor, see text)
         r5 10 ohm, 1/2 watt (two 22 ohm 1/4 watt in parallel OK)

         c1 .01uF ceramic disk, 100V
         c2 .1uF ceramic disk, 16V or greater
         c3 tbd 0.1uF, good quality (not ceramic disc, tantulum or  
electrolytic), 16V or greater
         c4 1uF, 16V or greater


CIRCUIT OPERATION:

r1, r2, c1, d1 and d3 are the input network. The signal from "COIL
-" swings ideally from battery to ground as the "points" (actual
points, or electronic ignition). In reality, it may not go all the
way to ground, or to battery; there may be "jaggies" on the signal
from CDI or MSD ignitions, coil ringing, plus noise and ignition
HV hash. The input network smooths this crap out into a more
managable signal.

The ratio of points CLOSED (0 volts on "COIL -") to points OPEN
(battery voltage on "COIL -") on and off time is dwell time. The
amount of time the points are CLOSED (or OPEN) depends on engine
RPM; ideally the ratio of OPEN to CLOSE is constant; that's why
dwell specified in degrees (of distributor rotation).

Capacitor c2 (with diodes d2 and d3) "differentiate" the input
signal -- eg. separates out the leading edge of the signal. It
turns the now-slowly varying signal from the coil into a short
negative-going spike, that triggers the 555 timer.

The 555 timer is the heart of the tachometer. Every time the
points close, a sharp negative pulse triggers the 555 timer, which
generates an output pulse of fixed width into the tachometer meter
movement. r3 and c3 determine the width (in time) of this pulse.
Capacitor c4, and the inherent mass of the meter movement, smooth
the 555 pulses into a voltage that is proportional to engine rpm;
this is so because the current through the meter movement is the
average of the sum the fixed-width pulses over time; since they're
all the same width (because of the 555 business) if there are few
of them per second (low rpm) the average voltage will be low; if
there are more of them per second (high rpm) the average voltage
will be higher.

The actual values of the timing components, r3, r4, c3, need to be
determined experimentally, to match the meter movement you've got.
The values above are a good start.

CALIBRATING ON THE BENCH: