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Reducing DC motor speed w/min components
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| My new cordless screwdriver runs on 7.2vdc Li-Ion batteries (pm brush motor).
It's not variable but runs at one speed (2000rpm) which is nice when you need
it, but for many tasks it's way too fast.
I like everything about this tool except the speed. I'd like to add a second
speed, half or maybe slightly slower than half.
Obviously space is at a premium in cordless tools like this, so minimum
component count for this modification is paramount.
I know I can just hang a large resistor off of the battery and a large switch
for routing current through it, but I was hoping for a solid-state solution.
Is there any advantage to using a bipolar or MOSFET or such over the
bog-simple resistor method? (ie, it requires a smaller switch; a transistor
might be easier to mount externally than a resistor to cool it, etc.).
Ideas?
Those who think a PIC can be adapted to this requirement, please be prepared
to show your work. (c:
Thanks,
--
DaveC
me@bogusdomain.net
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| |
| Andrew Gabriel 2007-03-10, 9:25 am |
| In article <0001HW.C217BFD200ABEE20F01826C8@news.sf.sbcglobal.net>,
DaveC <me@privacy.net> writes:
> My new cordless screwdriver runs on 7.2vdc Li-Ion batteries (pm brush motor).
> It's not variable but runs at one speed (2000rpm) which is nice when you need
> it, but for many tasks it's way too fast.
>
> I like everything about this tool except the speed. I'd like to add a second
> speed, half or maybe slightly slower than half.
>
> Obviously space is at a premium in cordless tools like this, so minimum
> component count for this modification is paramount.
>
> I know I can just hang a large resistor off of the battery and a large switch
> for routing current through it, but I was hoping for a solid-state solution.
> Is there any advantage to using a bipolar or MOSFET or such over the
> bog-simple resistor method? (ie, it requires a smaller switch; a transistor
> might be easier to mount externally than a resistor to cool it, etc.).
>
> Ideas?
A resistor will result in reduced torque as you apply load, as it will
waste more power just when you need it. Something which drops a more
constant voltage would be better, such as a zener diode, or just 4 or so
regular diodes in series, and on a heat sink. Another way would be to
use just one of the two Li-Ion cells, which will give half voltage.
I'm not sure what effect it would have on the charging if one was
significantly more discharged than the other.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
| |
| Lionel 2007-03-10, 9:25 am |
| On Sat, 10 Mar 2007 09:49:29 GMT, DaveC <me@privacy.net> wrote:
>My new cordless screwdriver runs on 7.2vdc Li-Ion batteries (pm brush motor).
>It's not variable but runs at one speed (2000rpm) which is nice when you need
>it, but for many tasks it's way too fast.
>
>I like everything about this tool except the speed. I'd like to add a second
>speed, half or maybe slightly slower than half.
>
>Obviously space is at a premium in cordless tools like this, so minimum
>component count for this modification is paramount.
>
>I know I can just hang a large resistor off of the battery and a large switch
>for routing current through it, but I was hoping for a solid-state solution.
>Is there any advantage to using a bipolar or MOSFET or such over the
>bog-simple resistor method? (ie, it requires a smaller switch; a transistor
>might be easier to mount externally than a resistor to cool it, etc.).
>
>Ideas?
>
>Those who think a PIC can be adapted to this requirement, please be prepared
>to show your work. (c:
*EASY!*
(1) Solder all the I/O pins together & attach a wire. (2) Solder Vcc &
Vdd together & attach a wire. (3) Wire as many such chips in series
with the battery as required to get the motor speed to where you want
it. ;^)
--
W "Some people are alive only because it is illegal to kill them."
. | ,. w ,
\|/ \|/ Perna condita delenda est
---^----^---------------------------------------------------------------
| |
|
| DaveC wrote:
> My new cordless screwdriver runs on 7.2vdc Li-Ion batteries (pm brush motor).
> It's not variable but runs at one speed (2000rpm) which is nice when you need
> it, but for many tasks it's way too fast.
>
> I like everything about this tool except the speed. I'd like to add a second
> speed, half or maybe slightly slower than half.
>
> Obviously space is at a premium in cordless tools like this, so minimum
> component count for this modification is paramount.
>
> I know I can just hang a large resistor off of the battery and a large switch
> for routing current through it, but I was hoping for a solid-state solution.
> Is there any advantage to using a bipolar or MOSFET or such over the
> bog-simple resistor method? (ie, it requires a smaller switch; a transistor
> might be easier to mount externally than a resistor to cool it, etc.).
>
> Ideas?
>
> Those who think a PIC can be adapted to this requirement, please be prepared
> to show your work. (c:
>
> Thanks,
use a 555 timer, a logic level high current Fet and pulse width drive
the motor, this is commonly done in a lot of variable speed drills.
the electronics is built in the trigger.
50% duty cycle gives you half speed.
the idea of using PWM is that it's either on or off, analog control
creates heat build up which is energy wasted and less regulation.
You can get fancy enough with the controller and monitor the voltage
output between the on pulses to use that has a regulator. The Motor will
regenerate back voltage and is a good way to test actual speed. THey
call this Armature feed back in most cases.
--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
| |
|
| > use a 555 timer, a logic level high current Fet and pulse width drive
> the motor, this is commonly done in a lot of variable speed drills.
> the electronics is built in the trigger.
What makes a FET "logic level"? Is 2N7224 (a.k.a. IRFM150) a logic level
device? I've got a few of them...
<http://www.allcomponents.ru/irf/jantx2n7224.htm>
Vds is 10v, so does this rule it out for use with 7.2v?
> 50% duty cycle gives you half speed.
> the idea of using PWM is that it's either on or off, analog control
> creates heat build up which is energy wasted and less regulation.
Yeah, I was wondering about heat. Sounds like pwm is the way to go. Now, how
to fit it all in...
> You can get fancy enough with the controller and monitor the voltage
> output between the on pulses to use that has a regulator. The Motor will
> regenerate back voltage and is a good way to test actual speed. THey
> call this Armature feed back in most cases.
Don't want to get too fancy... just slow it down. Accurate speed regulation
isn't too important.
Thanks,
--
DaveC
me@bogusdomain.net
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| |
|
| DaveC wrote:
>
>
> What makes a FET "logic level"? Is 2N7224 (a.k.a. IRFM150) a logic level
> device? I've got a few of them...
>
> <http://www.allcomponents.ru/irf/jantx2n7224.htm>
>
> Vds is 10v, so does this rule it out for use with 7.2v?
>
>
>
>
> Yeah, I was wondering about heat. Sounds like pwm is the way to go. Now, how
> to fit it all in...
>
>
>
>
> Don't want to get too fancy... just slow it down. Accurate speed regulation
> isn't too important.
>
> Thanks,
Logic level Fets come in different ranges, but you should get one that
turns on at about 3 volts. the logic fets normally turn on fully at that
point.
the Source of the Fet should be on the common side of the supply and
assuming that you are using - volts for the common side, you use an N
channel type.
the Drain of the fet will connect to one of the armature leads while
the other motor lead connects to the + side of the battery.
use a diode across the armature leads to remove kick back high voltage.
the cathode would connect to the + side of the armature.
--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
| |
| Salmon Egg 2007-03-10, 8:25 pm |
| On 3/10/07 1:49 AM, in article
0001HW.C217BFD200ABEE20F01826C8@news.sf.sbcglobal.net, "DaveC"
<me@privacy.net> wrote:
> My new cordless screwdriver runs on 7.2vdc Li-Ion batteries (pm brush motor).
> It's not variable but runs at one speed (2000rpm) which is nice when you need
> it, but for many tasks it's way too fast.
>
> I like everything about this tool except the speed. I'd like to add a second
> speed, half or maybe slightly slower than half.
>
> Obviously space is at a premium in cordless tools like this, so minimum
> component count for this modification is paramount.
>
> I know I can just hang a large resistor off of the battery and a large switch
> for routing current through it, but I was hoping for a solid-state solution.
> Is there any advantage to using a bipolar or MOSFET or such over the
> bog-simple resistor method? (ie, it requires a smaller switch; a transistor
> might be easier to mount externally than a resistor to cool it, etc.).
>
> Ideas?
>
> Those who think a PIC can be adapted to this requirement, please be prepared
> to show your work. (c:
>
> Thanks,
It takes two lithium cells to get 7.2V. Use a single cell and add the means
to have the single cell connect properly. That will give you half speed.
Bill
-- Fermez le Bush--about two years to go.
| |
|
| > It takes two lithium cells to get 7.2V. Use a single cell and add the means
> to have the single cell connect properly. That will give you half speed.
A really simple solution. But the battery pack is modular (encapsulated) and
would require modification of the pack. Also, would result in imbalance of
charge in the cells. And from what I understand about Li-Ion chemistry. they
don't balance very happily during charge.
I think a 555 PWM solution is in my future.
Thanks,
--
DaveC
me@bogusdomain.net
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| |
|
| Jamie sez:
> use a 555 timer, a logic level high current Fet and pulse width drive
> the motor, this is commonly done in a lot of variable speed drills.
> the electronics is built in the trigger.
> 50% duty cycle gives you half speed.
This is now my plan. The only question remaining is re. reversing direction.
How is motor reversing accomplished in commercial cordless drills? I see 2
wires connecting the battery to the speed circuit/trigger, and 2 going to the
motor. How can I do it simply?
Ideas?
Thanks,
--
DaveC
me@bogusdomain.net
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| |
| Lionel 2007-03-11, 3:25 am |
| On Sun, 11 Mar 2007 07:45:14 GMT, DaveC <me@privacy.net> wrote:
>Jamie sez:
>
>
>This is now my plan. The only question remaining is re. reversing direction.
>
>How is motor reversing accomplished in commercial cordless drills? I see 2
>wires connecting the battery to the speed circuit/trigger, and 2 going to the
>motor. How can I do it simply?
The simplest possible way is to add a DPDT switch immediately before
the motor, wired to reverse the polarity.
--
W "Some people are alive only because it is illegal to kill them."
. | ,. w ,
\|/ \|/ Perna condita delenda est
---^----^---------------------------------------------------------------
| |
| Tony Williams 2007-03-11, 9:25 am |
| In article <0001HW.C217BFD200ABEE20F01826C8@news.sf.sbcglobal.net>,
DaveC <me@privacy.net> wrote:
> My new cordless screwdriver runs on 7.2vdc Li-Ion batteries (pm
> brush motor). It's not variable but runs at one speed (2000rpm)
> which is nice when you need it, but for many tasks it's way too
> fast.
ISTM that the right solution here is a plug-on
reduction gearbox. Doing it mechanically would
also do the required increase in torque. If there
is not one on the market already then someone is
missing a commercial opportunity.
Say a 2.5 or 3:1 ratio, input/output shafts inline,
make both shafts as hex sockets so that it can be
used to increase or decrease speed. Supply a hex
bar to couple the screwdriver/drill and gearbox
hex sockets.
--
Tony XXXXXXXX.
| |
| Palindrome 2007-03-11, 9:25 am |
| DaveC wrote:
>
>
> A really simple solution. But the battery pack is modular (encapsulated) and
> would require modification of the pack. Also, would result in imbalance of
> charge in the cells. And from what I understand about Li-Ion chemistry. they
> don't balance very happily during charge.
>
> I think a 555 PWM solution is in my future.
>
A two pole c/o series/parallel (slide) switch... and the modification is
usually pretty trivial. The slide switch can be very small and won't
require anything like the space that an electronic unit (including its
own switch/control) will need.
--
Sue
| |
|
| > A two pole c/o series/parallel (slide) switch... and the modification is
> usually pretty trivial. The slide switch can be very small and won't
> require anything like the space that an electronic unit (including its
> own switch/control) will need.
Thanks, Sue.
Modifying the battery does look like the obvious route. As does the
series/parallel switch (what's "c/o"?). I'll have to find one to handle max
current, of course, in such a small package...
The main objection I have to applying this mod is that it means I'll have to
rely on my memory to place the switch in series before placing the pack in
the charger. Charging two 3.6v lithium-ion cells in a 7.2v charger is Not A
Good Thing (sm). I'll have to balance the pro's and con's. Unless you have an
idea to help protect the cells & charger from that possibility... (c:
FYI, it's this tool:
<http://www.makitauk.com/index.php?s...&pid=1664&catid
=81>
--
DaveC
me@bogusdomain.net
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| |
| John Gilmer 2007-03-11, 9:25 pm |
|
>
> A resistor will result in reduced torque as you apply load, as it will
> waste more power just when you need it. Something which drops a more
> constant voltage would be better, such as a zener diode, or just 4 or so
> regular diodes in series, and on a heat sink. Another way would be to
> use just one of the two Li-Ion cells, which will give half voltage.
> I'm not sure what effect it would have on the charging if one was
> significantly more discharged than the other.
Using only one of the two cells can cause problems as you end up with one
cell completely discharged in series with a partly charged cell.
You don't need a zenner diode. You just need one or two or three "regular"
diodes. Each will drop from .5 to .9 volts depending upon diode size and
load.
>
| |
|
| On 2007-03-10, DaveC <me@privacy.net> wrote:
> My new cordless screwdriver runs on 7.2vdc Li-Ion batteries (pm brush motor).
> It's not variable but runs at one speed (2000rpm) which is nice when you need
> it, but for many tasks it's way too fast.
2000 rpm 7.2V cordless screwdriver? I don't beleive it.
Bye.
Jasen
| |
|
|
| Tony Williams 2007-03-14, 9:25 am |
| In article <0001HW.C21A5B1F012A3C8EF01826C8@news.sf.sbcglobal.net>,
DaveC <me@privacy.net> wrote:
> Actually 2300 rpm:
> <http://www.makitauk.com/index.php?s...&pid=1664&catid
> =81>
That's an impact driver and I suspect that it needs the
rpm to get the inertia to ride through the impacts. Its
the impacts that torque the screw, not the direct motor
speed/torque. Reducing the speed may not have the effect
you desire.
Notice the next driver down, which is not an impact, and
which does the nearly the same torque by means of a larger
step down in the gearbox.
--
Tony XXXXXXXX.
| |
|
| Thanks for your comments, Tony.
> That's an impact driver and I suspect that it needs the
> rpm to get the inertia to ride through the impacts. Its
> the impacts that torque the screw, not the direct motor
> speed/torque. Reducing the speed may not have the effect
> you desire.
I want to double the use of this tool by providing lower speeds. Yes, the
impact is useful for driving/undoing tight screws, but it's useless (way too
fast) when, for example, installing small screws in covers on residential
light switches or receptacles.
By providing low-torque low speeds, I make this tool useful for small screws,
too. Yes, I could buy another tool to cover that purpose, but for a few
bucks, (and having a bit of fun in process of design and implementation) I
can do this.
(And I already own it...)
Thanks,
--
DaveC
me@bogusdomain.net
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Please reply in the news group
| |
|
| >>> use a 555 timer, a logic level high current Fet and pulse width drive
[color=darkred]
[color=darkred]
> The simplest possible way is to add a DPDT switch immediately before
> the motor, wired to reverse the polarity.
But this means that another switch must be put between the battery and the
PWM circuit to turn it off. This spoils my "plug n play" solution. I was
hoping to simply insert the circuit between the battery and the existing
reversing switch but now must either add an on-off switch or...?
I thought over the ideas suggested by some re. a rotary switch and dropping
diodes. But I can't find a small rotary switch to handle the current (3A
constant, 5A peak on start) -- there's precious little room for a HD rotary
in there...
I like the MOSFET w/speed pot idea best, but
1) it gives only 90 percent max, so I thought to use a switch on the back of
the pot to short across the MOSFET to give 100 percent
2) the circuit needs a switch on the battery supply conductor to turn it off.
Would be nice to use the switch on the pot, but I'd like to use that for max
speed...
More ideas?
Thanks,
--
DaveC
me@bogusdomain.net
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| |
|
| > You don't need a zenner diode. You just need one or two or three "regular"
> diodes. Each will drop from .5 to .9 volts depending upon diode size and
> load.
I've thought about this method (without zeners). If each diode drops ~0.7v,
to give 4 speeds (full speed plus 3 speeds obtained with diodes) with 7.2v
battery would mean 3 diodes / speed, so:
low speed 1 = 3 diodes (2.1v)
low speed 2 = 3 more diodes (4.2v)
medium speed = 3 more diodes (6.5v)
That's 9 diodes in a pretty small space.
Doesn't using a 1.8v zener (or one close to this v) for each 3 diodes perform
the same purpose (with the added advantage of more effectively giving me 3
evenly-spaced speeds)?
Configure as a "diode ladder" and pick off voltages to switch to the motor? I
presume I'll have to use a high-current zeners.
Thanks,
--
DaveC
me@bogusdomain.net
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| |
|
| > Configure as a "diode ladder" and pick off voltages to switch to the motor? I
> presume I'll have to use a high-current zeners.
Better yet is to use two 2.4v zeners. This gives steps of 2.4v (one zener),
4.8v (two) and 7.2v (none; full battery voltage).
How best to use these? Is just switching them in series with the motor +
terminal all that's needed? The motor can pull 3A; are there zeners that can
handle this current?
Thanks,
--
DaveC
me@bogusdomain.net
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| |
| Lionel 2007-03-15, 8:25 pm |
| On Thu, 15 Mar 2007 12:06:40 -0700, DaveC <me@privacy.net> wrote:
>
>
>But this means that another switch must be put between the battery and the
>PWM circuit to turn it off. This spoils my "plug n play" solution. I was
>hoping to simply insert the circuit between the battery and the existing
>reversing switch but now must either add an on-off switch or...?
<shrug> Use a 3 position switch instead, centre-off.
--
W "Some people are alive only because it is illegal to kill them."
. | ,. w ,
\|/ \|/ Perna condita delenda est
---^----^---------------------------------------------------------------
| |
|
| > <shrug> Use a 3 position switch instead, centre-off.
So, let's see... the existing fwd/rev rocker switch; a new speed pot; and a
new on/off rocker switch. Getting pretty crowded on this little dude.
My goal is to consolidate functions of the pot and one switch. It looks like
I can do without a power switch if I use a CMOS 7555 timer. It draws less
than 100uA and at this rate the battery (1Ah) will last about a year if left
in its box on idle, w/o charging. Longer if I remember to pull the battery
when putting it away.
I think I'm going to use this approach.
--
DaveC
me@bogusdomain.net
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| |
| Lionel 2007-03-16, 3:25 am |
| On Thu, 15 Mar 2007 20:08:03 -0700, DaveC <me@privacy.net> wrote:
>
>So, let's see... the existing fwd/rev rocker switch; a new speed pot; and a
>new on/off rocker switch. Getting pretty crowded on this little dude.
Um. I though you were intending to replace the orignal control system.
Why would you add a new F/R switch instead of using (or replacing) the
existing one?
--
W "Some people are alive only because it is illegal to kill them."
. | ,. w ,
\|/ \|/ Perna condita delenda est
---^----^---------------------------------------------------------------
| |
|
| > Um. I though you were intending to replace the orignal control system.
> Why would you add a new F/R switch instead of using (or replacing) the
> existing one?
The single existing on/direction switch simply connects the battery to the
motor in forward polarity or reverse polarity. I want/need to keep that for
direction control (and being quite custom fit, it requires a bit of a hack to
replace it with anything). If I want a PWM solution, adding a (small) pot
w/switch solves most of the problem: I get speed variation and with the
switch I can short across the MOSFET for full speed (MOSFET only gives 90
percent, or so). But that requires the PWM circuit to remain running (it
needs to be powered in order to fire the MOSFET, doesn't it). So another
switch to turn it on/off? Pretty soon this screwdriver starts to look like
some transport ship on an early Star Wars movie (the later ones look much
sleeker, for some reason...).
So I've decided that since the CMOS 7555 uses only 100uA it will run for
years from this 1Ah battery without turning it off. So that's the route I'm
currently taking.
Now if I can only find a mini-size 500K pot with push/pull or
push-on/push-off switch. Every parts house seems to have huge minimum orders
for a Bourns PTR91 series w/switch or stopped carrying it.
<http://www.bourns.com/pdfs/PTR90.pdf >
Maplin in UK has another brand that might work. But patriotism, y'know... (c;
--
DaveC
me@bogusdomain.net
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| |
| Lionel 2007-03-16, 3:25 am |
| On Thu, 15 Mar 2007 21:43:13 -0700, DaveC <me@privacy.net> wrote:
>
>The single existing on/direction switch simply connects the battery to the
>motor in forward polarity or reverse polarity. I want/need to keep that for
>direction control (and being quite custom fit, it requires a bit of a hack to
>replace it with anything). If I want a PWM solution, adding a (small) pot
>w/switch solves most of the problem: I get speed variation and with the
>switch I can short across the MOSFET for full speed (MOSFET only gives 90
>percent, or so). But that requires the PWM circuit to remain running (it
>needs to be powered in order to fire the MOSFET, doesn't it).
Why? - Why not just have the switch bypass the MOSFET completely?
>Now if I can only find a mini-size 500K pot with push/pull or
>push-on/push-off switch. Every parts house seems to have huge minimum orders
>for a Bourns PTR91 series w/switch or stopped carrying it.
For a single unit, it might be easier to salvage one from the junk
box.
--
W "Some people are alive only because it is illegal to kill them."
. | ,. w ,
\|/ \|/ Perna condita delenda est
---^----^---------------------------------------------------------------
| |
|
| > Why? - Why not just have the switch bypass the MOSFET completely?
Which switch bypass the MOSFET? (By my count, we're talkin' about 3 switches.
I lost track of which you mean...)
1. The original fwd-off-rev rocker, that's gotta stay. That is the current
means to connect polar voltage (+ or -) to the motor.
2. Pot and switch (plan to locate and install): pot controls duty cycle for
the PWM circuit. The switch bypasses the MOSFET when full-speed is
desired....
3. No plans for a 3rd switch, but I mentioned that if I could figure out how
to turn off the 555 (or 7555) easily, I'd do so. Unless I add a 3rd switch
(which doesn't seem possible with the limited real estate -- both internal
and external -- on this tool) I'm going to use a CMOS 7555 and let it run all
the time.
> For a single unit, it might be easier to salvage one from the junk
> box.
Your junk box must be fuller than mine. Care to donate to the cause? (c: I
was hopin' to find something useable from a dimmer switch at the hardware
store (a mini push-on/push-off switch and pot combo). But the ones that
aren't sliders (which most of them are) are full-size pots. Dang!
--
DaveC
me@bogusdomain.net
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| |
| Lionel 2007-03-16, 9:25 pm |
| On Fri, 16 Mar 2007 01:18:50 -0700, DaveC <me@privacy.net> wrote:
>
>Which switch bypass the MOSFET? (By my count, we're talkin' about 3 switches.
>I lost track of which you mean...)
>
>1. The original fwd-off-rev rocker, that's gotta stay. That is the current
>means to connect polar voltage (+ or -) to the motor.
>2. Pot and switch (plan to locate and install): pot controls duty cycle for
>the PWM circuit. The switch bypasses the MOSFET when full-speed is
>desired....
>3. No plans for a 3rd switch, but I mentioned that if I could figure out how
>to turn off the 555 (or 7555) easily, I'd do so. Unless I add a 3rd switch
>(which doesn't seem possible with the limited real estate -- both internal
>and external -- on this tool) I'm going to use a CMOS 7555 and let it run all
>the time.
This whole thing is getting insanely complex. All you really need is a
dial to set the PWM ratio (including down to zero), & a switch to set
the direction. Using a switching pot to cut off all power when the
tool isn't it in use is a good idea. A cute way to change direction
would be to use an integrated H-bridge to drive the motor, & switch an
inverter in or out of the PWM output with a simple SPST switch, or a
single set of contacts on the existing switch. Do all that, & all you
need to do is add the pot, & rewire the existing switch.
>
>Your junk box must be fuller than mine. Care to donate to the cause?
If you live in Melbourne, Australia, no problem. ;^)
> (c: I
>was hopin' to find something useable from a dimmer switch at the hardware
>store (a mini push-on/push-off switch and pot combo). But the ones that
>aren't sliders (which most of them are) are full-size pots.
Find or buy one of those cheap little portable radios. The volume
control is typically a tiny 5K or 10K log pot with an integral switch.
With careful design, one should be a good match for an RC time delay.
--
W "Some people are alive only because it is illegal to kill them."
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\|/ \|/ Perna condita delenda est
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