ATo tell direction, two
detectors will be needed.
The slotted type of detector
will be the easiest to work
with; I selected Digi-Key part number
425-1970-5-ND. The Mouser part
doesn’t include a rise or fall time
spec, so I can't use it. The aperture
for the sensor is 0.5 mm and the rise
and fall times are 15 and 20 µs.
That determines the hole size and
maximum speed of the rim of the disc.
Rim Speed = RPM Circumference =
RPM 2πR
where R is the radius to the hole
center see (Figure 2).
The sensor aperture is a slot.
That, plus the hole diameter and rim
speed, will determine the pulse width:
Pw = (D + S)/SPEED. Since the
minimum pulse width is determined
by the rise and fall time of the sensor,
then Tr + Tf = (Dmin + S)/SPEED and
Dmin = RPM 2πR (Tr + Tf) -S.
If I make the distance between
the holes equal to the hole diameter,
■ FIGURE 2
■ FIGURE 3
then the circumference
=N*2*D whereNis
the number of holes. I
want to work with
seconds instead of minutes, so RPS
= RPM/60. Now: Dmin = RPM
(Tr + Tf) N Dmin/30 - S. Solving for
Dmin:Dmin = 30 S/(RPM N
(Tr + Tf)). In this problem, S = 0.5 mm,
RPM = 10,000, N = 36, Tr = 15 µs, Tf
= 20 µs Dmin is, therefore = 0.6 mm.
I wouldn't go that small; make D = 2
mm then the circumference becomes:
36 2 D = 144 mm. The disc
diameter is C/π= 45. 9 mm.
I recently designed an analog
speedometer for my truck. That circuit
will be ideal to measure the RPM of
the motor (see Figure 3). The
LM2907 (IC1), is a frequency to DC
converter IC; Digi-Key part number
LM2907N-ND. A square wave signal
at the input produces a DC voltage at
the output that is a linear function of
frequency. The LM2907 datasheet
will explain the theory.
R1 and R2 set the input (pin 11)
at 1.1 volts; R4 and R5 set the other
input (pin 1) at two volts. When the
transistor is turned on, the collector
goes close to ground so the input is
close to square. Depending on your
need for accuracy, you might want to
divide the input by two to get a better
square wave.
The circuit to tell direction (Figure
4), uses a D type flip-flop. If the data
signal (D) comes before the clock,
the Q output will be high. If the data
signal comes after the clock, the Q
■ FIGURE 4
output will be low. The capacitor, C1,
stretches the pulse to insure that
it is concurrent with the clock when
required.
STABLE CONSTANT
CURRENT
QI'd like to be able to use a
Fluke 87 meter to read
down to 300 micro ohms
or so by injecting a stable
■ FIGURE 5
January 2009 27
