Great results on a low budget
By Julian Edgar
Quick and easy construction
Electronic Building Blocks
Multi-pulser
The item discussed this month is a
multi-purpose variable duty cycle and
frequency driver with 5A capability
that can perform lots of different
functions. Examples? It can be used
to help teach electronics; test pulse-
width modulated (PWM) solenoids
and car fuel injectors; act as a DC motor
speed control or light dimmer; or even
a fully-confi gurable DC light fl asher.
We’ll look at each of those uses in a
moment, but fi rst, what do we actual-
ly have here? In fact, it’s two modules:
a frequency generator module, and a
MOSFET power board – together, they
form the ‘Multi-pulser’!
Frequency generator module
The frequency generator module is a
self-contained device that outputs a
variable duty cycle, variable frequen-
cy, square wave signal. Control of
frequency (1Hz – 150kHz) and duty
cycle (0-100%) is via front-mounted
up/down pushbuttons, and the select-
ed frequency and duty cycle are shown
on a back-lit LCD. Power supply can be
anything from 3.3 – 30V. The selected
frequency and duty cycle settings are
and varying the duty cycle, and then
holding a fi xed duty cycle (eg, 20%)
and varying the frequency. To make
such a teaching tool, mount the dis-
play and a bright LED in a plastic box,
powered by a 9V battery and equipped
with an on/off switch.
Note that when changing the frequen-
cy (but not the duty cycle), the output
can momentarily go to 100% duty cy-
cle, irrespective of the actual duty cycle
output that has been set. This could be
important in some applications.MOSFET power module
So, how to run higher current loads
than the 30mA the frequency genera-
tor module can deliver?
I initially connected the module to a
standard DC:DC Solid State Relay (SSR)
but found that the SSRs I tried didn’t
like being operated at high frequencies.
(And plus, SSRs are still relative-
ly expensive.) I then bought a small
MOSFET-based PWM control module- dubbed on eBay ‘3-20V MOSFET
MOS Transistor Trigger Switch Driver
Board PWM Control Module BDAU’, at
the time of writing item 163330158628.
This costs under £2 delivered.
Two inputs on this board are for the
PWM control signal – provided in this
case by the frequency generator mod-
ule. There are also inputs for power and
ground, and the output to the device
being driven. The MOSFET is an FQD-
60N03L, whose data sheet shows that
it can handle quite high current (over
40A); however, that’s obviously with
plenty of heatsinking. In this applica-
tion, the board’s maker claims 10A,
but I was happier with 5A continuous - this resulted in a 47°C running tem-
perature in 20°C ambient conditions.
(Note that the board can therefore drive
saved, even with power switched off.
Maximum output power of the module
is 30mA, and the PWM output swing
refl ects the supply voltage. The module
is designed to be fl ush-mounted on a
panel, requiring a cut-out of 72 x 39mm.
To fi nd the module, search eBay un-
der ‘Signal Generator Test Equipment
PWM Pulse Frequency Adjustable
Module’, at the time of writing item303100197720. The cost is incredibly
low – from around £4.15 delivered.
Connections are straightforward – two
for power and two for the output.
If you connect an LED to the output
(remember to equip it with a dropping
resistor to suit the supply voltage),
you immediately have an excellent
teaching tool for demonstrating what
‘variable duty cycle’ and ‘variable fre-
quency’ mean. Students just watch the
LED and alter the settings! Nothing
brings these concepts more to life than
holding a fi xed frequency of (say) 1Hz
The frequency generator module has
an in-built LCD that shows selected
frequency and duty cycle. The up/down
keys each side of the display allow you to
select frequency (1Hz – 150kHz) and duty
cycle (0-100%). Cost of the module is just
over £4 delivered!The backlit LCD is clear and bright. Here
the output has been set to a frequency of
350Hz and a duty cycle of 40%.The four connections to the frequency
generator are well-labelled – power (–) and
(+), and the PWM output – also polarised.