I am showing a method of
using a CdS photoresistor to control a standard triac dimmer circuit that is
wired to an electric drill motor (a universal motor). The CdS photoresistor is driven by an LED
that is controlled by a PWM pin
on an arduino microcontroller. This system provides a very easy way to
control 120V AC devices with a single PWM pin, and not use any timing code or
method to detect the zero-crossing of the AC power since this is done
intrinsically in the circuit. It also
offers true electrical isolation between the AC line voltage and
microcontroller.
okay in this video I'm
going to talk
about speed controlling a
drill motor
with an Arduino I
originally started
this and thought the best
way to do it
would be to rectify the AC
into plain DC
and then use an opto
isolator and raw
PWM just to chop up that DC
and send it
to the motor but this
turned out not to
work very well because my
circuit wasn't
very robust and I had a lot
of failures
in here I'm still not exactly
sure what
the problem was but
apparently
controlling a drill motor
which is a
very high power motor it's
a universal
motor so it's sort of like
a series DC
motor that can also run on
AC because it
has modifications so the
eddy current
losses aren't so bad but
anyway there
were a lot of problems with
that and I
kept blowing out MOSFETs
and protection
diodes so I eventually
abandoned that
idea
I searched the net and
found that a lot
of people do this by
controlling a triac
an opto triac with the
Arduino but then
the problem is you need a
zero cross
detector so if you use a
triac to
control AC voltage you need
some way of
knowing when the AC cycle
starts and
then you can start a timer
in the
Arduino and turn on the
triac at some
point after the AC signal
crosses zero
but this gets to be
complicated because
you need to program with
interrupts and
then you need to off to
couplers one to
send the zero cross signal
and the other
one to send the triac
firing signal back
out of the microprocessor
and so I was
kind of trying to avoid all
this
complexity and I think I
came upon a
pretty good solution so
what I did was I
took the trigger switch out
of this
drill motor and hardwired
it so that it
just spins one direction
I'm not going
to worry about Direction
control yet
this is just speed control
and inside
the trigger switch was this
little
circuit board here and on
it there's
just a triac
a variable resistor in a
capacitor so
when you pulled the trigger
the variable
resistor would change value
and the
capacitor would charge at a
different
rate so this was all set up
so that when
the AC voltage cross 0 it
would take
some time for the capacitor
to charge up
and then it would fire the
triac and
send the rest of the AC
signal to the
motor and then when the AC
signal wins
into the other half of its
you know
below zero the same thing
would happen
and it would fire again
sometime later
so as I said doing this
with a
microcontroller is a
problem because you
have to know the zero
across and to get
around this I came up with this
solution
here see if I can zoom in
so you can get
a look at this
I replaced the physical
potentiometer
with a CD s photo resistor
and hot-glued
an LED to it and the LED is
controlled
directly by the Arduino
just one one pin
pwm pin and I didn't even bother
using a
filtering capacitor so the
switching is
it's running at about
thirty kilohertz
or something like that and
if that value
of the resistor changes at
30 kilohertz
it it ends up not even
mattering because
all we're doing is changing
how fast
that capacitor charges up
so even if the
value of the resistance
fluctuates a lot
it really doesn't matter
once the
capacitor charges to the
correct value
the triac will fire and
send the rest of
the power through the drill
motor so I
can zoom in here you can see
this LED
working
so I just have brightness
control over
that with a PWM signal so
in order to
protect this thing from the
room light
so I'm just going to put
this piece of
tape on it eventually once
this project
gets to be a little more
finished
they'll have a better you
know light
tight case and I'm going to
plug the
drill motor in now when I
vary the
brightness of that LED
[Applause]
very very fine control of
the speed of
the motor there so this is
great this is
a really low component
solution you
don't really need to code
anything in
the yard do you know it's
just straight
pwm out no time and no
interrupts and
the only components that
you need
outside of the
microcontroller or this
triac dimmers circuit
you could also hijack a
standard dimmer
for like a house you know
house lighting
it's the same circuit and
see DSL and
LED so this is great you
can control
high power loads with this
and I'm
really happy with this
solution so let
me know what you think
s
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