The explained idea of a simple tachometer circuit can be used as a revolution counter tester for determining or setting the correct optimal engine speed which may ensure maximum engine efficiency from the vehicle. The pulses emanating from the contact breaker are shaped and limited by Fl2/R5/ D2. Transistor T2 conducts only when the contact breaker is open, because the full battery voltage is then present at the input (PL2) of the rev counter (see figure 48). Capacitor C2 is then discharged rapidly through R6 and T2. When T2 is cut off when the contact breaker closes, C2 charges only slowly via R4.
This ensures that the trailing edge caused by the closing of the contact breaker is applied to the next stage as a steep pulse, while the noise spikes are suppressed by the slow charging of C2. It follows that C2 must be carefully matched to the particular car engine. Diodes D4 and D5 protect the trigger input of IC1 against too high positive and negative voltages. The real heart of the circuit is C1: this versatile counter is connected as a monostable multivibrator (MMV). After every trigger pulse at pin 2 (negative voltage jump applied via C4) it generates an output pulse of constant period which may be preset with P1. Unfortunately, the counter auto-triggers if pin 2 is still logic low after the output pulse period has lapsed, and because of that R8 ensures that C4 can only pass very narrow trigger pulses.
The resistor and capacitor form a so called differentiating network (see figure 49). It would now be possible to connect a suitable moving-coil meter to the output (pin 3) of IC1. Because of the inertia of this instrument, the individual output pulses (pulse width = O.7C7 (P1+R11) sec) are averaged (integrated). The meter reading corresponds exactly to the average value of the pulse height (see figure 50) and is therefore provided Pl has been adjusted correctly proportional to the instantaneous engine speed.
The comparator, A3, following IC1 has no inertia and the output pulses from IC1 must therefore be smoothed’: this is effected by two integrating networks, R7/C5 and R9/C8 (see figure 51). Across C8 there exists therefore a reasonably constant direct voltage (the remaining ripple caused by the original pulses may be ignored) which is directly proportional to the engine speed (see figure 52). This direct voltage is set to 5 V at maximum engine speed during the calibration which follows later.
The following image shows the simple tachometer circuit design
The waveform images can be verified from the folowing images.
This ensures that the trailing edge caused by the closing of the contact breaker is applied to the next stage as a steep pulse, while the noise spikes are suppressed by the slow charging of C2. It follows that C2 must be carefully matched to the particular car engine. Diodes D4 and D5 protect the trigger input of IC1 against too high positive and negative voltages. The real heart of the circuit is C1: this versatile counter is connected as a monostable multivibrator (MMV). After every trigger pulse at pin 2 (negative voltage jump applied via C4) it generates an output pulse of constant period which may be preset with P1. Unfortunately, the counter auto-triggers if pin 2 is still logic low after the output pulse period has lapsed, and because of that R8 ensures that C4 can only pass very narrow trigger pulses.
The resistor and capacitor form a so called differentiating network (see figure 49). It would now be possible to connect a suitable moving-coil meter to the output (pin 3) of IC1. Because of the inertia of this instrument, the individual output pulses (pulse width = O.7C7 (P1+R11) sec) are averaged (integrated). The meter reading corresponds exactly to the average value of the pulse height (see figure 50) and is therefore provided Pl has been adjusted correctly proportional to the instantaneous engine speed.
The comparator, A3, following IC1 has no inertia and the output pulses from IC1 must therefore be smoothed’: this is effected by two integrating networks, R7/C5 and R9/C8 (see figure 51). Across C8 there exists therefore a reasonably constant direct voltage (the remaining ripple caused by the original pulses may be ignored) which is directly proportional to the engine speed (see figure 52). This direct voltage is set to 5 V at maximum engine speed during the calibration which follows later.
The following image shows the simple tachometer circuit design
The waveform images can be verified from the folowing images.
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