Early VDO Tachometer Guts

Posted on Feb 5, 2014

The `crossed-flags` topology of a monostable multivibrator. Also, this particular board happened to be a 912 tach, so R5, which determines the discharge rate of C2,would be different for a 911. 912`s have a 50% duty cycle (four cylinders, 45 degrees of dwell or MARK time per ignition event- that`s 180 degrees total or 50% of 360 degrees of distributor rotation- another

Early VDO Tachometer Guts
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way to think about it is in 360 degrees of distributor rotation there are four ignition events or 90 degrees per event, of which the points are closed and distributor output is HIGH for 45 degrees or 50% of that) For a 911 you want a 63% MARK time, the points are closed 38 out of 60 degrees. With the car running +14V present on the voltage regulator circuit at the lower left. The Zener (value unknown at this point) has a breakdown voltage of a certain amount which limits the voltage on the top rail and the capacitor wired in parallel smooths out variations in voltage (ripple from the alternator that`s not damped out by the ship`s battery) Both the base of Q1 and Q2 are connected to the emitter of Q3 (I`m going to call this "ground" in the discussion below) through 2. 2K resistors but slight variances in the circuit mean one of them turns on first, it doesn`t matter. Suppose Q1 saturates first, its base is connected to ground through R5. Its collector gets pulled up above ground to positive voltage. This positive voltage is connected to the base of Q2, holding Q2 OFF. Next, a positive pulse arrives at the /1 terminal from the points. This goes through R8 and R6/R7 wired in parallel to the base of Q1, turning it OFF (thanks Loren for the clarification). Eventually C2 is fully charged and no more current flows, so the base of Q1 drops to ground through the 2. 2K resistor and C2 acts like a battery and discharges through the...

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