Microcontroller-based Ta chometer


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Atachometer is nothing but a
simple electronic digital transducer.
Normally, it is used for
measuring the speed of a rotating shaft.
The number of revolutions per minute
(rpm) is valuable information for understanding
any rotational system. For
example, there is an optimum speed
for drilling a particular-size hole in a
particular metal piece; there is an ideal
sanding disk speed that depends on
the material being finished. You may
also want to measure the speed of fans
you use.

How it works?

Just point the light-sensitive probe tip
atop the spinning shaft towards the
spinning blade, disk or chuck and read
the rpm. The only requirement is that
phototranyou
first place a contrasting colour
mask. A strip of white adhesive tape is
ideal on the spinning object. Position it
such that the intensity of light reflected
rotates.
Each time the tape spins past
the probe, the momentary increase
in reflected light is detected by the
phototransistor. The signal processor
and microcontroller circuit counts
the increase in the number of such light
reflections sensed by it and thereby
evaluates the rpm, which is displayed
on the 4-digit, 7-segment display.

Circuit description

Fig. 1 shows the circuit of the microcontroller-
based tachometer. The
tachometer comprises AT89C2051
microcontroller, ULN2003 high-current
Darlington transistor array, CA3140
operational amplifier, common-anode
7-segment (4-digit multiplexed) display
and its four anode-driving transistors.

Software

The software is written in Assembly
language and assembled using 8051
cross-assembler. It is well commented
and easy to understand. It uses
AT89C2051’s internal timer for measuring
the period of one cycle of the
rotation in units of 100 microseconds.
Thus if the speed is 1500 rpm, it is 25
rps, and the time taken for one cycle
is 40 ms.