PWM Modulator

If you ever thought of
experimenting with pulse-width modulation, this circuit should get you
started nicely. We’ve kept simplicity in mind and used a dual 555 timer,
making the circuit a piece of cake. We have even designed a small PCB
for this, so building it shouldn’t be a problem at all. This certainly
isn’t an original circuit, and is here mainly as an addition to the
‘Dimmer with MOSFET’ article elsewhere in this website. The design has
therefore been tailored to this use. A frequency of 500 Hz was chosen,
splitting each half-period of the dimmer into five (a low frequency
generates less interference).

The finished project

The finished project

The first timer is configured as a standard astable frequency
generator. There is no need to explain its operation here, since this
can easily be found on the Internet in the datasheet and application
notes. All we need to mention is that the frequency equals 1.49 /
((R1+2R2) × C1) [Hz] R2 has been kept small so that the frequency can be
varied easily by adjusting the values of R1 and/or C1. The second timer
works as a monostable multivibrator and is triggered by the
differentiator constructed using R3 and C3.

PCB layout

PCB layout

The trigger input reacts to a rising edge. A low level at the
trigger input forces the output of the timer low. R3 and C3 have
therefore been added, to make the control range as large as possible.
The pulse-width of the monostable timer is given by 1.1xR4xC4 and in
this case equals just over a millisecond. This is roughly half the
period of IC1a. The pulse-width is varied using P1 to change the voltage
on the CNTR input. This changes the voltage to the internal comparators of the timer and hence varies the time required to charge up C4.


Circuit diagram

The control range is also affected by the supply voltage; hence
we’ve chosen 15V for this. The voltage range of P1 is limited by R6, R7
and R5. In this design the control voltage varies between 3.32 V and
12.55 V (the supply voltage of the prototype was 14.8 V). Only when the
voltage reaches 3.51 V does the output become active, with a duty-cycle
of 13.5 %. The advantage of this initial ‘quiet’ range is that the lamp
will be off. R8 protects the output against short circuits. With the
opto-coupler of the dimmer as load, the maximum current consumption of
the circuit is about 30 mA.

Power supply

Power supply


R1 = 270k
R2,R3 = 10k
R4 = 100k
R5,R8 = 1k
R6,R7 = 220R
P1 = 2k2, linear, mono

C1,C4 = 10nF
C2,C5,C6 = 100nF
C3 = 1nF
C7 = 2µF2 63V radial
C8 = 100µF 25V radial

D1 = 1N4002
IC1 = NE556
IC2 = 78L15

P1 = 3-way pinheader
K1 = 2-way pinheader

Author: Ton Giesberts
Copyright: Elektor Electronics

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