Engine RPM (Part 5)

Part 1, 2, 3, 4, 5

The last two posts explained how to check whether the speed of a rotating object agreed with a predefined value or not. To the cost of an additional optical sensor and a display, we may now measure any rotating speed still without contacts between the moving object and the handled measuring device.

We will firstly “stick” to principle of the adhesive piece of white paper attached to the moving part. Then we will send a light beam towards the location of this piece of paper using a Reflective Optical Sensor. This is a compact electronic component featuring a light emitting diode (aka LED) and a photo-transistor. I selected the popular TRCT5OOO from Vishay, because it is small, it has a pretty fast response time, it is sensitive and, last but not least, it is cheap.

TCRT5000

The principle of operation is simple and intuitive.

Reflective_Opto_Sensor

Case 1: The LED (on the right hand side of the component) emits a light beam at a certain angle. If the light beam does not hit any surface, it is lost in blue sky and nothing happens.

Case 2: The LED emits a light beam at a certain angle. When the light beam hits a surface, it bounces at a symmetric angle (versus the hit surface) and a part of the emitted light is directed on the photo-transistor which lets current flow across its collector/emitter junction.

In a photo-transistor, Ice (Current flowing from collector to emitter) does not depend on the base current but on the light hitting the base which is optically exposed.

Biasing the Reflective Optical Sensor is easy and requires only a pair of resistors. A 100 Ohm resistor for the led and a 10 kOhm resistor between +5V and the collector of the photo-transistor. Then take the signal at the collector and send it to any digital or analog pin.

Let’s have a look at we can see on the scope when the probe is tied down to the collector side of the optical sensor.

Here is an example of the optical sensor located 3 cm above the target:

too_high

The reflected beam is weak and produces little changes on the photo-transistor. As the signal does not cross the trigger line (half Vcc, in other words 2.5 V), no counts will be registered.

Next is an example of the optical sensor located 3 mm above the target:

too_low

We have now an excellent optical feed-back, so good that we know see some signal (higher frequencies) which relates to the light remitted by the blades of the fan. Also, the rest signal value is too low and too close from the trigger line, so that extra counts will be registered.

And now, talaaaaa ! Here comes the plot taken at the optical sensor collector side when this sensor is located at an appropriate distance:

correct

Next posts will explain how to interpret the pulses and convert them into a digital rpm value

rpm_seven_segments_display

Arduino digital pins will have no problems in detecting edges and count pulses.

More on optical sensors.

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