Digital Tachometer

Digital Tachometer
ENGR 4803
Electromechanical Systems &
Digital Tachometer
Basic stamp : Measurement of
Encoder Pulses
Use the COUNT function to determine the speed of the motor. The syntax of COUNT
COUNT pin, period, variable
Count the number of cycles (0-1-0 or 1-0-1) on the specified pin during period
number of milliseconds and store that number in variable.
PBASIC Program
‘Tachometer Program
counts VAR word
RPM VAR word
counts = 0
count 0, 1000, counts
RPM = counts * 60
debug dec ? RPM, 13
goto loop
‘counts is a 16 bit variable
‘RPM is a 16 bit variable
‘initialize counts
Earlier we said that a pn junction is sensitive to light. Special diodes, called photodiodes, are
designed to detect photons and can be used in circuits to sense light as shown in Figure 3.14 .
Note that it is the reverse current that flows through the diode when sensing light. It takes a
considerable number of photons to provide detectable voltages with these devices. The
phototransistor ( see Section 3.4.6) can be a more sensitive device, although it is slower to
respond. The photodiode is based on quantum effects. If photons excite carriers in a reversebiased pn junction, a very small current proportional to the light intensity flows. The sensitivity
depends on the wavelength of the light.
LED Switch
Our objective is to turn a dashboard LED on or off with a digital device having an output voltage
of either 0 V or 5 V and a maximum output current of 5 mA. The LED requires 20– 40 mA to
provide a bright display and has a 2 V voltage drop when forward biased. We use a transistor
switch circuit employing a small- signal transistor ( e. g., 2N3904 npn) to provide sufficient
current to the LED. The required circuit follows.
A special class of transistor is the phototransistor, whose junction between the base and
emitter acts as a photodiode ( see Section 3.3.3). LEDs and phototransistors are often found in
pairs, where the LED is used to create the light, and this light in turn biases the phototransistor.
The pair can be used to detect the presence of an object that may partially or completely
interrupt the light beam between the LED and transistor ( see Lab Exercise 5). An optoisolator is
composed of an LED and a phototransistor separated by a small gap as illustrated in Figure 3.27 .
The light emitted by the LED causes current to flow in the phototransistor circuit. This output
circuit can have a different ground reference, and the supply voltage Vs can be chosen to
establish a desired output volt-age range. With no common ground, the optoisolator creates a
state of electrical isolation between the input and output circuits by transmitting the signal
optically rather than through an electrical connection. A benefit of this isolation is that the
output is protected from any excessive input voltages that could damage components in the
output circuit. Also, because the supplies and grounds are separate, any fluctuations or
disturbances that might occur in the output circuit have no effect on the control signals on the
input side.
Angular Position of a Robotic Scanner
A circuit to count and display the pulses over a given interval of time T.
The following figure illustrates all the required components.
555 Timer
555 Timer
555 Timer
555 Timer
William Kleitz
Digital Electronics: A Practical Approach, 7e
Copyright ©2005 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
555 Timer
The 7490 decade counter counts the
pulses and is reset by a negative edge on
signal R after the time period T.
The period T is set by a resistorcapacitor combination using a 555
oscillator circuit. If the count
can exceed nine during the period
T, additional 7490s must be
cascaded to provide the full
Just prior to counter reset, the
output count is stored by 7475
data latches that are enabled by
a brief pulse on signal L. The
latches are necessary to hold the
previous count for display while
the counter begins a new count
One of the two 74123 one- shots is
positive edge- triggered by the clock
signal CK to generate a latch pulse L of
length ∆t. Note that the latch and reset
pulse widths must be small ( t ≪T ) to
maintain count accuracy ( see Class
Discussion Item 6.15 ). The trailing edge
of the latch pulse triggers the second
one- shot, which is negative edgetriggered, to produce a delayed reset
pulse R for the counter.
The 7447 LED decoder and driver
converts the latched BCD count into the
seven signals required to drive the LED
display. The display reports the number
of pulses that have occurred during the
counting period T.

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