To start, I have no experience with electronics except what I've been googling the past week. What I'm having issues with is finding a way to make a voltmeter that functions like an led vu-meter would.

What I'm making is a gear driven dynamo and I want to make a visual representation to show the user that a faster spin produces more voltage by using LED's as user feedback. It is not important that it is accurate just that a faster spin makes more lights come on.

How can I make a volt-meter show that a dynamo is producing more voltage?

additional note: The intention is to make the dynamo the only source of power as well as what is being metered.

  • \$\begingroup\$ An LED VU meter is just a voltmeter calibrated in dB. Have you looked for LED VU meter designs? \$\endgroup\$ – Peter Bennett Dec 21 '16 at 21:01
  • \$\begingroup\$ I've looked at VU meters, and I'm looking to order one. It seems like it would work with some modification, I just don't know how I would do that. \$\endgroup\$ – 4LPH4NUM3R1C Dec 21 '16 at 21:51
  • \$\begingroup\$ This is called an led tachometer. To measure RPMS. The right Google term will help. \$\endgroup\$ – Passerby Dec 21 '16 at 22:00
  • \$\begingroup\$ To my understanding a tachometer needs a feedback signal from a photo sensor or an ignition coil/switch. This would be more complicated than modifying a VU meter I think. \$\endgroup\$ – 4LPH4NUM3R1C Dec 21 '16 at 22:21

Check out the LM3914/LM3915. There are a few tutorials on how to use them. You'll have to reduce the output voltage to between 0V and 5V -- you can probably just use a voltage divider to achieve this.




A VU meter makes no sense here. VU meters show human-perceived volume, but with a formal definition. Humans perceive volume logarithmically, but I don't see a logarithmic relationship from generator speed to anything meaningful.

Instead, I think you want to show power produced by the generator. In the general case, this is the smoothed out instanenous voltage times the instantaneous current. That's not too hard to do, but measuring just the voltage is easier. If you know the load, like a fixed resistance, then you can calculate power from just the voltage.

One way or another, once you have power, you use it to decide how many LEDs in the bar graph to turn on. That should be linearly proportional to power, so is just a single divide, easily done in the same microcontroller that is taking the measurements, computing instantaneous power, and low pass filtering the result. The actual driving of the LEDs is trivial in comparison, and shouldn't be done more than 4 times a second or so anyway.

  • \$\begingroup\$ What do you mean by a single divide? And what microcontroller - would that be the lm3914/lm3915? The low pas filter would be to adjust the sensitivity of the response? What would I use for that? Sorry if this is too many questions, I am an electronics n00b. \$\endgroup\$ – 4LPH4NUM3R1C Jan 2 '17 at 15:02
  • \$\begingroup\$ @4LP: You could do this with stone knives and bearskins, but nowadays microcontrollers are cheap and available. Lots of analog comparators or a diode ladder or something could be used here, but again, this is 2016. \$\endgroup\$ – Olin Lathrop Jan 2 '17 at 15:27

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