series parallel circuit for kids - first KiCad sanity check and suggestions

[williamhamilton]

Hi all, this is my first attempt at KiCad and first design to be made PCB. The design is for local school kids to solder up and be taught about series, parallel, polarity, resistance etc. Something simple but able to cover some basic concepts. 

I have attached a schematic file for feedback if anyone is willing - be gentle   (KiCad file not allowed so just png file now)

My next step is to check footprints against parts I can source and make up the PCB layout. Off to watch more Chris G videos

cheers

W

[rstofer]

Ignore most of this...

I wouldn't spend the time or money to build a PCB for this.  What you can learn from the circuit will be accomplished in a few minutes.  That is just an opinion, choose to ignore.  I would just breadboard it and maybe play around measuring volltages and currents with my DMM.  Then move on...

Using a pot to feed the LED is a recipe for disaster.  When you turn the knob toward the top of the drawing, the resistance declines and the LED current begins to increase rapidly.  Unfortunately, an LED is not a linear load and once you start to flow current, it increases rapidly with increasing voltage.  You can overcome this by putting a fixed 100 Ohm resistor in series with each pot.  The value is too high, 50 Ohms is probably more realistic (given a 3V source), if you had but one potentiometer.  Since you can have two pots in parallel both turned to 0 Ohms, you can count on the parallel 100 Ohm resistors to protect the LED.

If you really want to simplify the project, calculate the resistor required to get 10 mA through the LED (dim) and use 2 of them in parallel to get 'bright' at 20 mA.

Assuming an LED with a V_f of 2V at 20 mA and a source voltage of 3V, you calculate the resistor parallel resistance as (3V - 2V) / 0.02A = 50 Ohms.  This is the parallel combination so each resistor would be 100 Ohms.

This would give you dim (10 mA) when the first switch is closed and bright (20 mA) when both switches are closed.

https://www.alliedelec.com/m/d/6355b8aba0b01578df0bb7b871ceefd7.pdf

[Audioguru]

Doubling or halving the current in an LED does not change the brightness much. You need an exponential change in current to see bright and dim.

[james_s]

You're gonna need at least one more resistor in series with the LED to limit the maximum current. I think I would just build a circuit like this on a solderless breadboard though.

[williamhamilton]

Thanks for the feedback everyone.  I think I have been convinced to breadboard this one and have the kids solder some other board.

I originally had a bulb instead of the LED but the local electronics store said they would probably not be stocking them much longer. 

I will sit down  and have a bit more of a play.

[vealmike]

When I was of that age, we etched PCBs at school by drawing the circuit on with nail varnish.

I don't think we were allowed near the acid itself, but the process was explained and demonstrated. We prepped the boards and handed them over.
Certainly got me engaged, but then I was already interested in electronics I guess.

[james_s]

Miniature incandescent bulbs are not nearly as common as they once were but they are still readily available. Around the holidays you can still (here at least) get cheap miniature christmas lights that are incandescent, 2.5 and 3.5V bulbs are the most common.

As far as etching PCBs, something like a 2 transistor multivibrator is a classic that is still quite easy to build and does something more interesting.

[rstofer]

Doubling or halving the current in an LED does not change the brightness much. You need an exponential change in current to see bright and dim.

Which is a fact overlooked by many when they design an LED as a general purpose indicator buried on a PCB.  20 mA is often overkill and 10 mA is more like it.  Even lower will work.

Even though there won't be a substantial difference in luminance, I suspect it will be visible.  I think the real subject should be Ohm's Law and making a few measurements of LED current (or battery current) should be entertaining.  The voltage across the LED itself may not change much (non-linear device) but it is still possible to chase the voltage gain/drops around a closed circuit.  Lots of good stuff to learn.  Kiirchoff's Laws come to mind.

I would use very low value pots - probably around 100 Ohms with that 100 Ohm series resistor I mentioned above.  Not so much to vary intensity, that probably won't be visible, but to play with Ohm's Law.

My grandson came in first place in a science fair back in elementary school for a jazzed up demonstration of Ohm's Law.  We mounted a circuit and 3 cheap DMMs to a display board and provided a little write-up.  It's all about the knobs and dials when it comes to science fairs.

[james_s]

I've observed that many if not most board mounted LEDs are *way* too bright, especially small sizes like 0603 where the light is not diffused at all. I've had a few boards where I had to stick a piece of tape over the LED or change the resistor because a chip LED was blinding.