# dc motor as generator to power 1 led

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#### bigtex526

##### New Member
hello,

I am making a hand cranked model. I wish to add one bright, wide angle LED to the model. My plan is to use a DC motor as a generator with a capacitor. What I do not know is the what type of DC motor to order (12v, 24v, 90v...). The system will be at low RPM and I do not know how that affects it. It has to be cheap and small too. Thanks

#### Triode

##### Well-Known Member
If you want it to be simple use a brushed dc motor. A small one that's geared down works well for hand cranking because a hand can generate more force but at a lower speed, and the motor needs to spin fast but without much force.

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#### birdman0_o

##### Active Member
Get a 5V dc motor if it exists, definitely above 3 and don't go too far above 5, you would simply be wasting energy.

You will need gears. How much do you know about led circuits?

#### bigtex526

##### New Member
i have minimal experience with building circuits. my worry with a low voltage dc motor is that it would have to be spun very quickly to sustain the LED. I was told that a motor of 12 or even 24v would be too low because of the low rpm. the space inside my model is also limited, so i cannot fit any huge gears to drastically increase the revs. I was thinking a 1:10 ratio is about as big as i can get.

#### Chippie

##### Member
A bit of lateral thinking is required here...

Try getting hold of a battery screwdriver...they are geared dc motors designed to run at around 3v...You should be able to cobble summat out of that...

After all we are supposed to be innovative

#### audioguru

##### Well-Known Member
A red LED needs only 1.8V to 2.2V. A blue LED needs about 3.5V.
If you use a 12V or 24V motor then it does not need to be spinned very fast to produce the low voltage for an LED.
But if the motor is spinned too fast then the LED might burn out without a current-limiting circuit.

#### colin55

##### Well-Known Member
You will definitely need a 6v motor as it will only produce 3-4v when operated by a hand cranked generator.
The best is to get a super cap 0.22F and connect it via a bridge.
One minute of charging will produce 5 minutes of illumination.

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#### Hero999

##### Banned
I would recommend a Schottky diode rather than a bridge because it will conduct at a lower voltage.

#### bigtex526

##### New Member
the 6v motor, schottky diode, capacitor and led would then all be wired in series? in that order?

I appreciate the advice, as my budget does not allow for too much experimentation at this time.

#### Hero999

##### Banned
No the capacitor needs to be in parallel with the LED.

Think about it, if the capacitor were in series it would charge up but how would it discharge?

The LED should also have a series resistor to limit the current.

#### audioguru

##### Well-Known Member
An LED lights with DC current.
A capacitor blocks DC current but charges with a momentary DC current.
So if an LED was in series with a current-limiting resistor and a capacitor then the LED will blink for a moment when power is applied to charge the capacitor.

I think the bridge rectifier was recommended in case you connect the battery backwards.

#### bigtex526

##### New Member
I had not intended on using a battery because I only want it to function when cranked by hand. Do i still need a rectifier bridge to deal with the motor turning in either direction? or does that not affect the polarity?
thanks

#### colin55

##### Well-Known Member
If you look at my very informative article, (link above) it will explain the answers to all the questions you are asking.
You can buy one of these torches for $3.00 in the$2.00 shop.

#### audioguru

##### Well-Known Member
A DC motor used as a generator changes its polarity when its direction is changed.

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