Although most petrol-engined w cars and lorries have a counter as standard that is by no means the case in diesel-engined motor vehicles. The reason for this is that, since a diesel engine has no contact breaker, it is not so easy to derive pulses to drive a rev counter. There are, none the less. several possibilities of adding a rev counter if so desired.
Firstly, it would be possible to take the pulses from terminal W of the alternator. Unfortunately, that machine does not run at the same speed as the engine, so that some arithmetic unit would have to be added. Moreover, and more seriously. terminal W of modern alternators is no longer accessible externally.
A second way might be to attach a small magnet to each of the cranks on the crankshaft and so induce magnetic pulses in a fixed coil. The problem here is to attach these magnets securely.
A third method is an optical one proposed in this article. In this, the cranks are divided into sectors that are painted alternately white and black. A home-made light barrier is then used to evaluate the speed with which the sectors are rotating. If the cranks are divided into four sectors, pulses are generated that are suitable for driving commercially available rev counters, irrespective of whether the diesel engine has four, six or eight cylinders.
The circuit, in Fig. 1 may, therefore, be considered as an adaptor for the rev counter.
A small 12-V bulb lights the crankshaft, whereupon the light reflected by the white sectors falls on to phototransistor T1. This transistor is connected in a Darlington configuration with T2. The type of phototransistor is not important: it is thus not necessary to use the BP103 shown in the diagram. The output of T2 is applied via C2 to IC1, where it is chopped and amplified by about x50, to give a rectangular output signal of about 10 Vpp. That signal is perfect for driving a rev counter.
The circuit is best built on a small piece of prototyping (vero) board and then fitted in a tube, whose front is closed watertight by a circular piece of perspex. It may be necessary to separate the lamp and the phototransistor by a dark screen.
The circuit is connected to the rev counter by a three-core cable. The cable connections (+12 V, earth and pulse signal) to the circuit must be waterproof.
If the lamp is found to be too bright, it may be connected in series with a small resistor.
The circuit can be tested by measuring the voltage at the collector of T2, which should be 1-5 V when a white sector is being illuminated.
If the sensitivity is too high, replace R1by the circuit in fig,2, which keeps the voltage at the collector of T2 at around 5 V. The circuit draws a current of about 10 mA plus the current through the lamp.
