There are many types of oscillators used for many different applications. From motor speed control to light dimming, to switching power supplies, to RF broadcasting, to fiber optic data transmission, and on and on. The 555 timer IC
is a low cost circuit used to generate PWM (pulse width
modulation) clock pulses when used in its astable
oscillator mode. It is used for car turn signals, flashing
lights, and a myriad of other applications.
There is one problem with the 555 that to me renders
the part useless in PWM applications. It can never always
be 100% off or on. At best, its range is about 1% to 99%.
So, a motor or light can never be completely off or on.
That is terrible!
There is a much better solution in an eight-pin
package that provides maximum on and off control, PWM
modulation, FM modulation, and many more features.
What is this magical IC, you ask? It is a dual op-amp chip
such as the LM358 or MCP6022.
We’ll explore and analyze several circuits and their
applications in this article to show how effective and easy
it is to design and use this circuit topology. Let’s begin by
looking at the circuit in Figure 1.
The heart of the oscillator is U1A, R1-R5, and C2. R1
and R3 divide the V1 supply voltage in half, and their
combined resistance is R1*R3/(R1 + R3) = 50K. Since R2
= 100K, when the output of U1A switches from ground to
15 volts (+V1), the junction of U1A+ and R1-R2-R3
switches from five volts to 10 volts. R5 starts charging C2.
When C2 charges above 10 volts, the input of U1A- is
higher than U1A+. That causes the output of U1A to
switch to zero volts.
In turn, the junction of U1A+ and R1-R2-R3 switches
from 10 volts to five volts. R5 begins to discharge C2.
When C2 discharges below five volts, the input of U1A- is
lower than U1A+. The output of U1A switches from zero
volts to 15 volts (+V1), and the junction of U1A+ and R1-
R2-R3 switches from five volts to 10 volts. R5 begins to
charge C2 and the cycle keeps repeating. C2 is constantly
ramping up and down between +V1*1/3 (5V) and
The really cool thing is that this circuit oscillates at the
same frequency even if the supply voltage +V1 changes!
That is one of the big features of the 555. U1B acts as a
comparator to provide the 0% to 100% PWM output. R6,
R7, and R8 form a resistive divider. Notice that R6 and R8
are 10% lower resistance than potentiometer R7. Doing
the math, 15*(R7 + R8)/(R6 + R7 + R8) = 15* 19.1K/
( 28.2K) = 10. 16 volts at full clockwise (100% on), and
15*R8/(R6 + R7 + R8) = 15* 9.1/28.2 = 4.84 volts at full
counter-clockwise (0% off).
The U1B- input is connected to the junction of R5 and
C2 which ramps up and down between five and 10 volts.
U1B+ is connected to the wiper on potentiometer R7.
The secret to getting the most
bang for your buck and
performance is using the proper
circuit topology for your
By Ron Hoffman
48 November 2015
Topologies and Applications