As you've probably already guessed, what we're building here is a rev-counter. Please don't expect this model to show any professionalism. The purpose is to demonstrate through a model, just which possibilities are open to relative beginners in the field of digital electronics. This my own design and I swear, it did work, even if it was only an electrically driven model of an ignition distributor with a Hall-sensor.
The circuit has been left deliberately incomplete, because I would like to see a replica built, by those who have understood this set-up. However, take care, senseless testing will destroy, not only parts of the circuit. The best thing to do is to work from the two seven-segment-displays backwards. Why only two? Because in our digital rev-counter only the hundreds and the thousands change.
Then, we have the 74LS47 as a BCD-seven-segment-coder. Each of the two ICs receives its four bits from one respective 74LS13 component. There is then also the Schmitt-trigger 74LS13. I have used this to reduce a signal, which presumably is way above TTL-level. In a way, this IC protects the circuit a little. As soon as the input-signal exceeds a certain value, the output is switched.
Now, how does a counter become a RPM-display? In addition, one must also know, that a counter on the R-input can also be reset. If then, we receive the impulse through the 74LS13 from the low-voltage part of the ignition, in our case from the Hall-IC, then all it needs is a (e.g., shown in part 2) timer, which, in exactly defined intervals, passes the switch-position on, and resets the counter to zero.
Indeed, only owners of (not too expensive) veteran cars, will perhaps install a circuit like this, because two (or four, with the two zeros) seven-segment-displays can easily be positioned somewhere in the car, without having to make any major alterations to the interior. I would advise anyone, to buy a circuit like this, in kit form, or to buy ICs which can be well combined.
I hope that these six pages benefit the better understanding of digital-electric processes. At least that would be a great deal better, than the moans and groans which can be heard, when once again the subject of 'bits and bytes' comes up. 08/11