Specifications
or visual warning of speeds in excess of that level. Many
anti-pollution devices included on several recent automobile
models have included a speed switch to disable the vacuum
advance function until a certain speed is attained
2
. A circuit
which will perform these kind of functions is shown in
Figure
9.
A typical magnetic pickup for automotive applications will
provide a thousand pulses per mile so that at 60 mph the
incoming frequency will be 16.6 Hz. If the reference level on
the comparator is set by two equal resistors R1 and R2 then
the desired value of C1 and R1 can be determined from the
simple relationship:
V
CC
2
e
V
CC
#
C1
#
R1
#
f.
or C1R1f
e
0.5
and hence C1R1
e
0.03
From the RC selection chart in
Figure 10
we can choose
suitable values for R1 and C1. Examples are 100 kX and
0.3 mF. The circuit will then switch at approximately 60 mph
with the stated input frequency relationship to speed. To
determine the ripple voltage refer back to the equation for
ripple voltage (under ‘‘Choosing R1, C1 and C2’’). From this
we can determine that there will be about 10 mV of ripple at
the switching level. To prevent this from causing chattering
of the load a certain amount of hysteresis is added by in-
cluding R3. This will provide typically 1% of supply as a
hysteresis or 1.2 mph in the example. Note that since the
reference to the comparator is a function of supply voltage
as is the output from the charge pump there is no need to
regulate the power supply. The frequency at which switch-
ing occurs is independent of supply voltage.
In some industrial applications it is useful to have an indica-
tion of past speed excesses, for example in notifying the
need for checking of bearings. The LM2907 can be made to
latch until the power supply is turned ‘‘OFF’’ in the case
where the frequency exceeds a certain limit, by simply con-
necting the output transistor emitter back to the non-invert-
ing input of the comparator as shown in
Figure 11.
It can
also serve to shut off a tape recorder or editing machine at
the end of a rewind cycle. When the speed suddenly in-
creases, the device will sense the condition and shut down
the motor.
Analog Displays
The LM2907, LM2917 series devices are particularly useful
for analog display of frequency inputs. In situations where
the display device is a moving coil instrument the advan-
tages of the uncommitted output transistor can be realized
by providing a current drive to the meter. This avoids tem-
perature tracking problems with the varying meter resist-
ance and enables high resistance instruments to be driven
accurately with relatively large voltages as illustrated in
Fig-
ure 12.
The LM2917 version is employed here to provide a
regulated current to the instrument. The onboard 7.6V zener
is compatible with car and boat batteries and enables the
moving coil instrument to employ the full battery voltage for
its deflection. This enables high torque meters to be used.
This is particularly useful in high vibration environments
such as boats and motorcycles. In the case of boats, the
most common speed pickup for the knot meter employs a
rotating propeller driving a magnetic pickup device. Meteo-
rologists employ a large number of anemometers for mea-
suring wind velocities and these are frequently coupled by a
magnetic pickup. In examples like these, where there is fre-
quently a large distance between the display device and the
sensor, the configuration of
Figure 13
can be usefully em-
ployed to cut down on the number of wires needed. Here
the output current is conducted along the supply line so that
a local current sensing device in the supply line can be used
to get a direct reading of the frequency at the remote loca-
tion where the electronics may also be situated. The small
zero speed offset due to the device quiescent current may
be compensated by offsetting the zero on the display de-
vice. This also permits one display device to be shared be-
tween several inputs.
TL/H/7451–26
TL/H/7451–27
V
O
e
F
IN
V
a
R1 C1
SETPOINT
e
V
a
RB
RD
a
RA
(
Latchup occurs when
F
IN
e
RB
RA
a
RB
1
R1 C1
Independent of V
a
!
FIGURE 11. Overspeed Latch
TL/H/7451–28
FIGURE 12. Analog Display of Frequency
9