LED trigger circuit?

Several models of cordless drills have a white LED "headlight" that lights the drill's target. I'd like to add one to my drill.
Battery voltage is 14.4.
It seems simple enough: some device senses a small voltage going to the motor and closes a circuit from battery to LED.
What device would you use to sense and control these voltages?
Ideas?
Thanks,
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DaveC
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A resistor. For added value, maybe a capacitor too. A 10uF capacitor across the LED, and a 500 ohm resistor. Actually, thinking about it, you'd want a bridge rectifier connected to the motor terminals, a 420 ohm resistor between that and the capacitor/LED.
Hmm. If it does PWM, as it probably will, if it's not the very cheapest, you'd want to swap that around so that the bridge is first, feeding the capacitor (upgraded to 100uF@25V), then the resistor and LED in series.
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Buy a led with trigged included...
Regards!
then the resistor and LED in series.

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On Tue, 14 Mar 2006 10:50:25 -0800, DaveC wrote:

I'd just duct-tape an LED flashlight to it. :-)
Good Luck! Rich
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As the subject suggests, strobe the LED to make the drill stop. I'm not sure what good this would do, but would be interesting.
greg
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GregS wrote:

you could connect the led across the motor with a current regulator in series to control the led current. you can use an lm317 3 terminal regulator as a current regulator with just a single resistor. set the current reg to put out the current your led needs and it will power it appropriately any time the motor is running at an average voltage of 5v or so which should be any time it is turning.
you could also use an npn transistor like a 2n2222 with the base connected to the switched motor lead through a 1k or so resistor. you would then hook the led up to the 14.4v through an appropriate current limit resistor on the high side and the transistor making the led to ground connection.
-bob
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Thus spake bob mcree:

Thanks, Bob. I think that is the sort of design I was looking for.
What about the PWM output of the motor controller turning the transistor on and off? Is there a way to ensure that this doesn't pulse the LED?
Will a simple cap at the base keep the trans turned on? Would that interfere with the motor control circuit (ie, the SCR, or whatever it is that controls the motor current)?
Thanks,
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DaveC wrote:

This reply assumes you intend to modify the drill and connect the LED to the drill's battery.
You *MUST* use Ian's idea of a bridge rectifier or equivalent. The motor can go forward or reverse, so you have to "steer" the voltage. You can use an LM317 and a resistor so that the LED always glows the same brightness, but you'll need to add capacitors, too, because the supply will be electrically noisy. Doubtful you'll have the room inside for all of that.
Instead: wire the - of the led to the - lead from the battery. Wire a resistor in series with the other lead from the LED. Connect the other side of the resistor to the banded ends of two 1N4148 diodes. The other ends of the diodes go to the trigger switch wires that go to the motor. You *might* be able to squeeze all of that into the drill. So much the better if you also have room to add an electrolytic cap wired across the LED.
Since it is doubtful you will be able to mount the LED in the drill in the first place, go with a mofification of Rich's idea. Glue a steel plate on the drill in an appropriate place. Use a strong magnet or two to attach the LED flashlight to the drill. You can get some really strong magnets that are about the size of a small "button" battery, like catalog # MAG-80 from http://www.allelectronics.com/
Ed
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Thus spake ehsjr:

Thanks, Ed. An elegant solution, as always.
There is plenty of room in the handle (above the battery) for components, but you're right, there's no room for the LED in the body below the chuck. I plan on adding a little JB Weld on the external of the drill body to mount an LED holder.
Thanks again,
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On Wed, 15 Mar 2006 09:02:45 -0800, DaveC wrote:

http://www.copquest.com/10-4650.htm Fifth hit at: http://www.google.com/search?hl=en&q=%22tiny+LED+flashlight%22
Cheers! Rich
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Thus spake ehsjr:

Ed, what size cap would you suggest?
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Don't wire it across the LED. Wire it across the LED and resistor - this way it'll smooth the PWM much better.
10uF@25V should be fine.
Whatever you can easily fit @25V, is what I'd use.
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On Wed, 15 Mar 2006 19:24:04 +0000, Ian Stirling wrote:

If you're using the actual PWM, I wouldn't put a cap right at its output, because it will affect the PWM waveform, and might affect the operation of the drill motor. And I wouldn't put the cap right across the LED, because it won't charge over Vf of the LED, and won't discharge lower than Vf. (Well, it won't discharge below Vf anyway...)
I'd use a resistor on _both_ sides of the cap. One to isolate it from the PWM, and the other to give it a chance to hold some charge before the LED discharges it.
IOW, if this is an exercise in drill modification and circuit- building, then it sounds like an "interesting" project [ ;-) ], but, if all I wanted to do is have some light to drill holes by in dark nooks, I'd _still_ duct-tape an LED flashlight to the drill. :-) (or glue it, or maybe Sculpey.)
Cheers! Rich
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On Wed, 15 Mar 2006 07:56:27 +0000, ehsjr wrote:

<snip>
I haven't been following this discussion, but one could also use two LEDs anti-parallel. ...or one could take the trigger/polarity switchs (don't have the wiring diagram) and wire it to a pair of complementary transistors to drive the LED.

Who cares about noise? Any such noise is certainly going to be above the flicker fusion threshold. ...or a simple capacitor (smaller than a garbage can) isn't going to help.

Again, what's the cap do?

A good idea, but doesn't answer the question asked.
--
Keith

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Keith wrote:

The LED will, if/when it burns out. :-) The LM317 behaves badly when there's noise & no caps. It won't deliver constant current to the LED, so the brightness will vary, negating the reason for using the LM317 in the first place.
Any such noise is certainly going to be above the

Of course it will. Look at the LM317 datasheet.

Poorly stated. Intended was across the series R & LED. Reduces flicker.

The question asked was already answered. Is there a problem with providing alternative ideas ?
Ed
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This is the best idea, because you probably want the light on before you pull the trigger. Actually, I remember seeing one of these drill headlamps that had its own little watch battery and pushbutton switch.
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Thus spake gnoge:

It's an exercise in drill modification and circuit design. I don't want to just "tag something on" as it will interfere with the drill's fit in its vacuum-molded case, and it'll be one more thing I'll have to think about: shutting off and dismounting the light before packing the drill.
I'll probably try Ed's idea with 2 diodes running from the motor leads to the LED. Some combination of resistor(s) and cap will be tried also, as needed.
As far as needing light before beginning drilling, I note that power runs to the motor at the slightest pull of the trigger, which rotates the motor imperceptibly, if at all. So with this design I will have light before I spin up the motor for drilling.
Other approaches of circuit design are still welcome. I haven't unholstered the soldering iron yet!
Thanks,
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DaveC wrote:

Here's a thought: Install two contacts on the face of the trigger switch and make a touch switch. Using a Mosfet and a couple of resistors, the conduction through your finger will be enough to turn on the LED by touching the trigger. I'd post a ckt, but you can Google a bunch of them faster than I can hack ASCII line art.
Of course, this won't work if you wear gloves while operating the drill.
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While you're at it, get two of those LED lasers that have the optics to put out a line instead of a spot. Mount them at right angles so that they put out an alignment cross just beyond the drill tip.
The mounting method will have to incorporate some means of lateral adjustment to 'bore site' the beams.
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