Trigger MOSFET or other switch with low voltage from DC Motor

Projects General Guidance
1

Good day everyone,

My 5th grader daughter and her teammates were given a task to model a source of renewable energy and she asked me for help. I'm actually kind of proud she was able to articulate a concept the way she did. She said "I'm going to model a farm, in a remote area. This farm has a windmill. It will have 2 street lights, 2 houses and a Barn - each building will have an outside light. Dad, How can I light up a few street lights and house lights when the windmill turns if we hand-crank it or blow on it...?

I told her we can use a toy DC motor. Poor me, I didn't know better. So, I'm creating a plan for her to present to her teammates and class teacher, and I told her that I'll help with connections, soldering etc.

I ran a few scenarios on this spreadsheet:

To be clear, I know very little about resistors, diodes, etc and this spreadsheet is just from very basic knowledge. Very limited for me to know that I need help. So, from the spreadsheet, I conclude that I can run 6 or 7 GREEN LEDs using 3 volts either in parallel with one single resistor or parallel using a resistor per light. I understand that using the former, I cannot use more than 7 LEDs and using the latter, I may be able to use more (but not sure how many more). So I decided to use the following circuit (refer to this diagram)
Google Photos

What I need help with:

  1. I found out the DC motor with a propeller and an actual fan blowing in front of it can only generate 0.6-0.8 volts (I'm using a 3-7 v dc motor click here for more details: https://photos.app.goo.gl/yD2MRukFySXBiPRA6). Not enough to illuminate a single LED. So, my research is telling me that perhaps a Mosfet can turn on another power supply (like in my diagram: a two AA battery pack supplying 3v). Now, how do I wire the mosfet to do exactly that? Will the supplied voltage from the DC motor is enough to trigger the mosfet? What type of Mosfet? Please also help to avoid the mosfet triggering the battery pack by just ambient voltage, feedback or someone touching exposed wires, etc.
  2. Each single LED is wired in parallel with its own resistor. But now she tells me that the Barn on the model will have two lights above the door. I don't want to use two separate sets of wire for each of the two lights going to the Barn, so I assume I connect each of those two lights in series in the parallel circuit. If I'm correct, do I need a higher power source or the 3v is sufficient to keep 4 individual LEDs in parallel and 2 in series, both scenarios with their individual resistor (refer to first photo above). And also, if I'm correct, what type of resistor do I need for the pair connected in series? If I'm all wrong, please provide me with another idea to accomplish what they have already envisioned.

Please know that I don't have the skills nor knowledge to create an arduino board, and this project will be put together by small kids with no experience in that field either; so a simple circuit is what they need.

Thank you all.

2

OK, as you already have learned small DC motors do not make very good DC generators, the output voltage as well as output current is really too low to do much with.

Next, she want to consider the use of a few small solar panels where a small battery is charged during light and powers things during darkness hours. A Google of "solar panel kit for school project" should get some results.

If she wants wind there are plenty of inexpensive wind turbine kits available. A Google of "wind turbine science projects set" will get you several inexpensive school project kits.

This is how I would likely approach things. I am sure others will chime in. Kudos to the daughter and a helpful dad. Brings back fond memories. :slight_smile:

Ron

3

Does this mean that electricity actually have to be generated? I'd think it's just a model, and PWM that DC motor to make the impression of a windmill. Everything from a battery or an wall adapter.

1 Like
4

When she eventually enters the workforce, will you also be doing her work?

5

Welcome to the forum!

Based on the datasheet, it looks like your motor should be able to generate 3 V if spinning at 4500 rpm; this may drop slightly due to the motor's internal resistance (about 1 Î©) and the current drawn by the load. Could you use a gearbox (and possibly a larger fan impeller) to get the required voltage output?

6

I'm not entirely clear about the expected behaviour of the model.

It will have six LEDs, yes? When the simulated wind (from whatever source) reaches a certain level, should the number alight increase as the 'wind' increases? Or must all six suddenly power up at full brightness? In either case, must there be some hysteresis, i.e. a period during which they stay on until the 'wind' speed drops a little below below the power up trigger point, to avoid flickering?

Are you ruling out the use of an Arduino?

Staying with 'wind power' as your approach, but recognising that a fast spinning motor hardly represents a real wind turbine generator, perhaps different ideas would be worth experiment? Depends on your deadline I suppose.

For example, I wonder if a quiet but relatively powerful home/office fan could be deployed to blow aginst a largish rectangular cardboard or light plastic/wood panel, causing a lightwight potentiometer to vary the voltage supply. Of course, you'd need to improvise a weighted or sprung counter balance to return the pot setting to its 'no wind today' point.

1 Like
7

Hi @Terrypin

Thank you for the insights. It's a fifth grader project, so it needed to be simple. I believe my girl may have been a little too ambitious with their project. They are not in an electronics class. They touched on the topic of renewables in her science class and the teacher assigned them into groups and have them come up with a model that showcases renewable energy. I also encouraged her to pursue it, when I knew clearly that her, her teammates, the one dad (me) in the entire group (unfortunately) didn't have the skills to properly design it the way they wanted it.

The idea as it was pitched to me was this:

  1. A windmill's propeller spins and creates electricity
  2. 6 LED's should illuminate when the windmill starts spinning
  3. All LEDs should turn off when the windmill stops

Unlike what @sonofcy believes, I'm just an adviser and proving support for what they are not trained to do - like handling a soldering gun. Yes, I could have taught them how to crimp connectors and just plug and play all wires, but they are using my materials (and tools) and I prefer to use the least as possible ( :grimacing: We don't have 2 day prime shipping in Belize; we have 2 months when you're lucky shipping speed).

My girl is her dad's daughter, she is the team lead and she procrastinated for her entire spring break and now she only has till the 17th to present their project. So, yes, now we have a deadline and no time to learn new methods or procure some sort of ready-made kit, which may be against the rules; the team is expected to fabricate everything, but the teacher acknowledges some soldering and use of resistors was required, so we asked and she allowed me to assist with that part. The girls are building the windmill, houses, lamp posts, roads, etc

So, after realizing the motor doesn't generate enough juice, the new idea is this, and I know it is cheating because the LEDs are now illuminating from a battery pack and not from the voltage generated by wind spinning the DC motor. But they already completed their concept essay, and the teacher accepted it, so now they have to complete it. I'm now pitching it to them that, like in the real world, energy generated by the windmill is stored on a battery and that is the reason the windmill has wires leading to the battery pack, and why the batteries are the direct source of power for the LED lights.

So, now, my girl doesn't know yet that what they wanted, the way they envisioned it, is not possible. The reason, why I haven't told her it's because it's too late. As far as they are concerned, the windmill will spin with the aid of an AC desk fan, and the LEDs will illuminate.

My objective, where I need ideas from smarter people than us on this project, is:

  1. This is my motor and propeller (microwave fan) https://photos.app.goo.gl/C1BLCXPcgtfy5Byb8
  2. Like explained in my first post, the highest millivoltage I registered was 840mv. user grb, I was unable to get anything over 1 volt. The minimum forward voltage of the green LED is 2.1v I believe.
  3. The millivoltage detected when the DC motor spins should trigger a relay or switch to close the LED circuit powered by 3 volts. I don't know how to do this.
  4. When the DC motor stops, it should trigger the switch/relay to open, cutting off the battery pack from illuminating the LEDs. I don't know how to do this.

I'm unable to get arduino boards, components, etc. So a simple switch, triggered by millivolts is all we have. If the above is not possible with I may be able to find around here, I'll have to tell the girls that their project will have to become an explainer, where they manually trigger a switch to turn on the LED. I have been trying to think of something mechanical that closes the LED battery-powered circuit the microwave spins (negating the use of a DC motor), and then closes it via some sort of sustained contact when the blades stop. But, I can't think of anything :confused:...

8

That depends. What do you have around there?

9

My suggestion was that if you connected a transmission (gear box) between the fan and the motor shaft, then the rpm of the fan would be amplified by the gear ratio, thereby producing a much larger rpm at the motor (and hence, a larger output voltage). However, I understand that your access to hardware is limited, so this may not be an option. Nonetheless, if you have time/inclination to experiment, a transmission could be something as simple as a belt & pulley setup (which you might be able to fabricate from available parts).

You do have the ability to source MOSFETs, though (as you suggested in your OP), or do you just happen to have a specific MOSFET on hand (and if so, which one)?

10

I think the OP has proven how little energy there is in wind and why it takes massive construction to get any usable energy from wind. That should be a very interesting alternative to all the hype about renewable energy and why so much real estate is needed for those projects.

11

It's difficult to advise without knowing in more detail what components you have or can obtain and your skill level. I'd be inclined to go with a simple delay circuit. A 10K resistor and capacitor (I'm guessing say 220/470 uF) would receive low DC power at the same time as you switch on the mains powered fan. A 1k resistor from their junction to the base of an NPN transistor would quickly fade up your LEDs a few seconds after the fan starts. Tried it on a breadboard and works, but further trial/error needed on the component values.

Have to go out now but can respond in the morning to queries, and draw a corresponding circuit if you need it.

12

I had the same thought. I tend to go simple though: two bottle lids, e.g., soda bottle sized and coffee-can sized with a rubber band for a belt should produce enough speed multiplication. Assembly might be tricky (use lots of hot glue) but in theory it should work. You only need to get up to about 1.2V to turn on an LED.

I'm also wondering if just going to a larger propeller might not be enough to speed it up.

13

Since this is meant to demonstrate the concept rather than operate a real system, you can simply turn the LEDs on and off in sync with the motor. If you're using an Arduino, you can simulate the effect by fading the LEDs up and down to match the windmill’s changing speed. For added realism, you can also PWM-control the windmill motor. A potentiometer can be used to adjust the speed dynamically.

You could probably find most if not all of the code in a book called the Arduino Cookbook. It is well written and your daughter should be able to understand it without much help. It may be in the school library or your public library.

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14

That's why I had proposed a belt & pulley as an approach that might be within the realm of the possible.

Or a leaf blower to create the "wind"...

15

Thank you all.

@Terrypin thank you for your input in regards to components and I understand the frustration we newbies burden you guys with our lack of experience on these type of forums. If you have the time to draw the circuit and detail the parts I need, I can tell you within hours, if the components are available before my deadline. Like I said on my first post, this is pretty much the set up. How to trigger the power source by the spin of the fan, because direct voltage from motor is not possible, is what I don't know:
Google Photos

In the meantime I'll try @grb and other's suggestion in multiplying the fan RMPs (the leaf blower would blow the entire model built with popsicle sticks and foam away). I should be able mock up something for testing and see if I can attain at least 2 volts. The LEDs I have have a forward voltage of 2.1v. Do I still need resistors in parallel if the motor outputs a matching LED voltage or slightly less?

Thank you

16

You probably wouldn't need a resistor. A simple way to increase the voltage is to place a standard AA battery (or similar) in series, which would add about 1.5V. However, I'm not sure how well that would work in practice, as the motor’s internal resistance would act as the current limiter. You could also use the motor’s output voltage to feed an analog input on an Arduino, which could then control PWM to the LEDs. I would suggest RED leds they will give you the most bang for the watt.

17

If the motor voltage is lower than the forward bias voltage, no current will flow.

If the motor voltage is larger than the forward bias voltage, then (assuming the LED max. current is 20 mA, per your spreadsheet), you would need 50 Î© of resistance for every 1 V that your motor voltage is above the LED forward bias. For example, if the motor outputs 2.2  (0.1 V above the LED forward voltage), then you would need a current-limiting resistor of 5 Î©; for a 2.5-V motor voltage, you would need 15-Ω resistors, etc.

18

I got a +1 there :wink:

19

Except that OP wrote:

20

  • Suggest the output of the fan motor be connected to a transistor switch.

  • The small motor current/voltage turns on the transistor, which turns on a light.
    There is a hidden battery that supplies the voltage to the transistor circuit (no audience member needs to know about the battery).

  • We really need to know what kind of electonics component stores are available where you are.