Rev Counter Installation in Petrol-Engined Vehicles
In most petrol-engined W cars and lorries, a standard feature is the presence of a rev counter. However, the situation is quite different for diesel-engine motor vehicles. The absence of a contact breaker in diesel engines makes it challenging to derive pulses for driving a rev counter. Despite this challenge, there are various methods available for adding a rev counter to diesel engines if desired.
Option 1: Utilizing Alternator Terminal W
One approach involves obtaining pulses from terminal W of the alternator. However, this method presents its own set of complications. The alternator operates at a different speed than the engine, necessitating the incorporation of arithmetic units to adjust the readings. Additionally, accessing terminal W of modern alternators externally has become increasingly difficult, posing a significant obstacle to this solution.
Option 2: Magnetic Pulse Induction from Crankshaft
Another potential method entails affixing small magnets to each crank on the crankshaft, thereby generating magnetic pulses in a fixed coil. While this approach offers a solution, a major challenge lies in securely attaching these magnets to the crankshaft. Careful consideration and expertise are required to implement this technique effectively.
A Novel Optical Approach for Speed Evaluation
In this article, a unique optical method for measuring speed is introduced. The cranks under consideration are divided into alternating white and black sectors. A makeshift light barrier is employed to gauge the rotation speed of these sectors. When the cranks are segmented into four sectors, pulses are generated. These pulses can effectively drive commonly available rev counters, regardless of the engine type, be it four, six, or eight cylinders. Consequently, the circuit depicted in Fig. 1 can be viewed as an adapter specifically designed for the rev counter.
Utilizing Light to Determine Crankshaft Speed
The process involves illuminating the crankshaft with a small 12-volt bulb. Subsequently, the light reflected by the white sectors falls onto phototransistor T1. T1 is configured in a Darlington arrangement with T2. The specific type of phototransistor used, such as the one denoted as BP103 in the diagram, is not critical. The output from T2 is then directed through C2 to IC1, where it undergoes chopping and amplification by a factor of approximately x50. This results in a rectangular output signal of around 10 Vpp, which proves ideal for driving a rev counter.
Building and Waterproofing the Optical Speed Measurement Circuit
To implement this optical speed measurement circuit effectively, it is recommended to construct it on a small prototyping (vero) board and enclose it within a tube. The tube’s front should be sealed watertight using a circular piece of perspex. To prevent interference, it might be necessary to introduce a dark screen between the lamp and the phototransistor. Connection to the rev counter is established through a three-core cable, and it is crucial to ensure that the cable connections (+12 V, ground, and pulse signal) are waterproof. If the lamp’s brightness proves excessive, it can be regulated by integrating a small resistor in series. A white sector’s illumination should yield a voltage reading between 1 and 5 V at the collector of T2, verifying the circuit’s functionality.
Adjusting Sensitivity and Current Consumption
In cases where the sensitivity needs adjustment, R1 can be replaced with the circuit illustrated in Fig. 2. This modification maintains the voltage at the collector of T2 around 5 V, ensuring optimal sensitivity levels. It’s important to note that the circuit draws a current of approximately 10 mA in addition to the current flowing through the lamp. Careful adjustments and considerations must be made to balance sensitivity while managing the overall current consumption of the circuit.