crank flashlight from homemade motor

Submitted by amory on Fri, 11/29/2019 - 22:16

Hi Everybody,

I am a theatre and music teacher trying some new things out my comfort zone with electronics, and have

had some success building a coupkle small DC motors but am having trouble using the motor to generate a hand crank flash light.


it runs well on 9Volts, so I hoped that if I attached the right size light bulb it would light up.

The guy in micro center told me crypticallly that i needed to do the math, but I'm not sure what he meant.


Do I need a recharagable battery, or a rectifier or maybe just a light bulb of the right size, I'm not sure


thanks in advance for anybody with thoughts to share

I was hoping to attach a photo, but since i am not seeing that option, i will tell you that i also built a gear box to increase the power of the crank

(16 teeth to 30 teeth) x 3 if that makes sense


  Joined November 29, 2019      1
Friday at 09:58 PM

I am not given the rating of the motor or the bulb that you are trying to glow so I cant help you with calculaion. But here is a simple way to do it. All you need is a multimeter.

Power the motor with a 9V battery and make it run. Now use a multimeter to measure the voltage across the motor and the current drawn by it. Say for example the motor is consuming 9V with 600mA current running at 200rpm. This means if we are able to rotate the motor manually (with crank shaft) at 200rpm it will acts as a generator and can power a load (your light bulb) with 9V and 600mA current. So you can test you load with a power supply unit (or read spec sheet) to check how much current it will consume at which voltage level.

This is a crude way of doing it and numbers will not be accurate but should give you a rough idea. I would suggest you to connect a multimeter and to the motor terminals and measure the voltage as you roate it with crank shaft. This voltage value should give you a rough idea on how to proceed 

Hope this helps 

  Joined April 17, 2018      120
Tuesday at 07:57 AM