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Beginner advice on voltage and current measuring/displaying circuits

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mknott

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I’m making a project for my 6 year old son. Basically it consists of 3 parts.
1) A part to generate electricity (at first, a solar panel, but maybe later a wind turbine too).
2) A part to store the energy; probably 4xAA rechargables.
3) A part to consume the energy. Probably a series of USB sockets with switches so he can plug in lights, MP3 player, speakers, etc.

Everything will be together in a console so he can monitor what’s going on. (The solar panel will be outside.)

I’m not an electrical engineer so I have some questions.

First, I want to provide visual feedback to show how much energy is being produced at a given point in time. The solar panel I like provides 6V at 200mA max, which I guess is adequate for charging the batteries. I want to have a kind of meter which lights a red LED if the current production is, say, 3-4V, a yellow LED for 4-5 and a green for 5-6. What kind of circuit would be suitable. (Having read the solar panel thread I might upgrade to a higher nominal voltage unit, but the same rule applies; I want my son to be able to position the panel, or just consider the weather, and note the effect on input.)

Also, I want to display the current charge of the battery pack, again similar to the above.

Also, I want to display how much current is being drawn from the battery by the consuming devices. For this I thought a small LED numeric display, 3-digit, since the load should never exceed 999mA. If you can get a multimeter for < 5 quid, can you just buy a circuit for approximately the same?

I’m flexible about either using several different coloured LEDs, or an array of similar colours, or even some prefabricated part if anyone knows of something that would do the trick. Any ideas would be welcome.

If anyone could point me in the right direction I’d be very obliged.
 
Displaying the instantaneous Battery Voltage, and/or Solar Panel or Load current is easy, either using analog meters (better for show/tell) or using inexpensive Digital Panel Meters (about $10 ea), or home-built bar graph displays is easy. Displaying the State-of-Charge (in AmpHours) in the battery is much harder. That gets into a micro-processor project.
 
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Glad to hear it's easy! But... can you point me int he direction of where to find out about such a device. Is there a generic name for such a circuit that I can Google and discover more about?

The second point perhaps wasn't made clear by me. I don't neccessarily want to display the energy level in the battery in mAh. I just want to display how many mA are being consumed by the devices currently running. Very much like an ammeter. Then my son can see that eg, a LED light uses much less than the mp3 player with speakers on full blast.

Again, a link for a suitable device will allow me to find out what terminolgy to search for.

Thanks for your reply.
 
Can you afford four of these?

I would use one to display the battery voltage (on the 20V scale). I would use the second to display the current being produced by the solar panel (20 or 200mA scale). A third to display the current being consumed by the IPod (~200mA scale). And the fourth to display the net charge/discharge current into/out of the battery (signed quantity).

This will demo nicely that there are times when the panel makes more current than is being consumed or vice versa. It also demos Kirchhoff's current law (sum of currents into an electrical node is zero). The voltmeter displays a crude state-of-charge estimate of the battery condition. Switching off the IPod will show that all the panel current is going into the battery. The volt meter will slowly increase. Switching on the IPod at night will show all of the IPod current coming out of the battery. Switching on the IPod on a cloudy day may show the a small net charge or discharge. Hopefully on a bright sun day, there will be enough current to both charge the battery and play the IPod.
 
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I think 4$ per meter is pretty reasonable. But this solution has two drawbacks. First it requires a 9 volt battery. My battery pack only supplies 4.8V. Second, I want to incorporate the meters/displays into a panel. Is it possible to buy multimeter parts separately somehow?
 
Sure, but you ain't going to like the new prices. Here and here and**broken link removed** are some surplus meters. **broken link removed** are some new meters.

The bar graph meters you talked about in your first post are battery powered, just like the multimeters. The 9V battery is inside the multimeter, and shouldn't have to be replaced unless your leave the multimeter turned on.

It is not "cheating" to use a powered meter to display volts/amps :D I would just bolt the little multimeters to the panel, and wire them into the demo.
 
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Thanks for the links. This is exactly the kind of thing I was after, especially for the ammeter showing how much juice the devices are using. At 10$ it's still within budget, especially for a pre built unit that I can just slot into the panel. Although theoretically it'd be cheaper to buy multimeters and cut them down. The nice thing about the meters in the links is that many are 5V supply, which fits nicely to my proposed system.

I'd still like to build some kind of led based feedback to indicate the voltage coming from the solar panel. It doesn't have to be super accurate; Just a green LED when solar voltage is bigger than battery voltage otherwise red. Is there some generic circuit type that lights leds based upon an input voltage?
 
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