■ FIGURE 2
nal will be down in the noise of lightning and automotive ignition. A better
solution is to generate an accurate 60
Hz from a crystal as shown in Figure 2.
The crystal can be pulled a little so
if the frequency is off, you can increase
C1 and C2 to lower the frequency or
decrease them to increase the frequency. The crystal tolerance is typically
50 ppm. I divided the crystal frequency
down to 120 Hz then divided by two
because the 4060 does not have a Q11
output. The added advantage is that the
output is a symmetrical square wave.
The procedure for finding the
divider outputs to decode the frequency is this: Divide the input frequency
by the output frequency to find the
division factor (N). This must be a
whole number. Find the highest power
of 2 that can be subtracted from N. Do
the subtraction; the result being a new
value for N. Repeat until the result is
zero. The powers of 2 that you use are
the Q outputs needed to decode.
QI enjoyed T.J. Byers’ column
for years and am delighted
that you are carrying on that
quality with your column.
I have a question that could have
applications in many circuits and so may
be of interest to a variety of readers.
I am controlling a motor’s speed
based on a reference voltage: where
five volts equals no speed, eight volts
equals full speed forward, and two
volts equals full speed in reverse. I
need a simple circuit to provide a
linear acceleration/deceleration ramp
where an instant voltage change at the
input to the circuit (from say five volts
to eight volts) would produce that
same change at the output of the circuit, in the range of five to 10 seconds.
It is important to me that this
ramping up (or down) be linear.
Simply charging a capacitor through a
resistor won’t do. I would prefer to
use simple gates or op-amps and/or
discrete components, as I don’t get
along well with microprocessors or
any of their relatives.
Any help or suggestions would be
— Clark W. Kuhl
AThe circuit in Figure 3
provides the linear ramp that
you want. The B section of
the op-amp provides a five
volt reference so the motor can be
reversed. You could use a five volt power supply instead. A five volt change in
the output of the A op-amp, caused by
a three volt input, takes 5. 5 seconds.
You can increase the time by increasing R4 or using a larger capacitor (C1).
This circuit cannot maintain zero
speed for very long because it is not
possible to match the reference
voltage perfectly. The speed will drift
higher over time in one direction or
the other. Some kind of tachometer
feedback would be needed to maintain zero speed.
QI am looking for a small size
fly zapper, but I can’t find
one. It is either a big
commercial unit, or it uses
some scented refills.
How hard would it be to make
■ FIGURE 3
September 2007 29