FM- 23 is a power FET lamp dimmer (using R5,
Q2, and DS1) with R1 = 1K or 5K and C1 = .047
µF. Here again, the output low time (Q2 off) is
fixed; this time, at about 32 µs. This
implementation also has a fixed resistor of 390
ohms in series with the potentiometer allowing
the pot to be adjusted to zero ohms (without
damaging the 555 or causing the circuit to
malfunction). This will yield close to a 50% duty
The frequency is high enough so that the
lamp appears to be on continuously at varying
levels of intensity. However, since the duty cycle
is limited to a minimum of 50%, you will not be
able to dim it to completely off. Strictly speaking,
R5 is not necessary, but I prefer to not have the
gate of an FET floating while the circuit is being
In general, a PWM circuit operates with a
constant period while both the on-time and off-time
changes. With this circuit, the off-time remains constant
while the on-time and period are changing so that longer
periods produce higher duty cycles. In both cases, it’s the
duty cycle that is important. The higher the duty cycle
(longer output high time), the brighter the bulb.
The circuit of Schematic 2 can be used as the basis
for all three of the above circuits using either mode 4 or 5.
It has the advantage of allowing independent modification
of both the on-time and off-time (or period), and its duty
cycle can span 0.1% to 99.9%. The table in the schematic
shows the resistor values required for different ranges and
modes of operation.
Just a note for the lamp dimmer: Frequencies above
60 Hz are usually not noticeable by most people.
A recent project I did used PWM to drive a DC fan
motor. Schematic 7 shows the driver circuit I
I used the circuit of Schematic 2 and this driver to get
a very smooth speed control. I set the PIC 555
replacement to mode 5 with a period of about 30 µs. The
reason for the relatively high frequency was due to the
type of motor being used. Lower frequencies tended to
make the motor chatter.
The IXDD609 is a FET gate driver and is required to
overcome the gate capacitance of the FET. Without this
driver, the gate input rise and fall times were slow enough
that the FET turned on and off slowly and became quite
hot. With the driver, the FET doesn’t even get warm when
driving a five amp motor at full speed.
There is one caution: The supply voltage to the IXDD
— or whatever FET driver you use — must not exceed the
maximum VGS of the FET (or the max voltage of the
driver IC), while still being high enough to ensure full turn-on of the FET.
I used a Schottky diode because they are faster than a
normal power diode and they have a lower forward
voltage drop yielding lower power dissipation. In this
application, the diode gets hotter than the FET. I put both
the diode and the FET on heatsinks.
Mims Circuit 24
The light/dark detector shown in Schematic 8 uses a
photoresistor connected to the Reset input as a gate for
the 555 wired as an astable multivibrator. The Reset input
has to be high in order for the 555 to oscillate. The
position of the switch determines whether the 555 is
enabled when light hits the photoresistor (lower resistance
when exposed to light) or does not hit it.
The threshold of the Reset input is about 1V but the
input resistance will be different depending on the
July/August 2018 61
SCHEMATIC 8. 555
FET gate driver.