# Total noob question on selecting potentiomenter value

Discussion in 'The Projects Forum' started by Sean Elkins, Sep 11, 2014.

1. ### Sean Elkins Thread Starter New Member

Sep 11, 2014
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So please forgive my absolute ignorance here, and also please forgive my trying to take a shortcut of asking rather than just experimentally determining it for myself. I'm hoping to avoid having to buy stuff I don't need.

I'm trying to teach middle school kids the difference between analog and digital means of transmitting a signal. One way I thought I might let them experience the difference is by stringing together a simple 'telegraph' using a mini bulb clipped from a string of Christmas lights and a couple of 1.5 volt batteries. I'm envisioning giving them a copy of the ASCII code and having one student think up a random number and then transmit it through the on/off flashes of the bulb as an example of digital communication.

To illustrate analog (and how digital has certain advantages) I wanted to use the same setup but insert a potentiometer into the circuit with the direction to adjust the brightness from 1-10 for each number and have the receiving student try to figure out the value. The idea being that the same transmission system can be used for both, but the analog version has a much greater chance of being 'misheard' on the receiving end because it's not just an on/off signal.

So here's the question: what value of pot do I need to order to make it work? I'm afraid that if I get high value (like 10K) pots the bulb will go dark in the first tiny movement of the knob, or that if I get low (100 ohm) pots the bulb won't go completely dark. I know what Ohm's law is but I don't know the thresholds for a bulb might be. Like I said, it would be great if I could just try some out but I'm going to have to order them to get enough for a class set. I don't want to drop \$40 at Radio Shack out of my own pocket just to get stuff for one part of one lesson and I'm already having to buy all the other stuff.

Any help appreciated.

Sean

2. ### wayneh Expert

Sep 9, 2010
12,405
3,256
A better solution than a pot would be a PWM dimmer as often used for LED dimmers. This would give smooth and easy control over a full range. These things are very inexpensive on e-bay for instance and you might find other uses for it. (Motor speed control, the aforementioned LED dimming, and so forth).

Pots are not so cheap when they must withstand even small amounts of power. You would need one with a very low range to have a decent sensitivity and then you'd find that you might not be able to dim below, say, 10% brightness. I'll guess that any pot above 100Ω would just be like a switch, with the light only coming on in the last percent of rotation.

You might consider a bank of several fixed resistors to choose amongst. That's getting farther away from the concept, though, as it blurs between digital and analog.

3. ### MrChips Moderator

Oct 2, 2009
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You need to protect the LED from excessive current.
With a 3V source (two 1.5V batteries) use a 100-ohm fixed resistor coming off the battery first.

Now for a variable resistor in series with the LED, 2000 to 5000-ohm would work.

I would go with a 500 to 1000-ohm variable resistor to adjust the higher brightness range.
In order to adjust the lower brightness range, i.e. to turn the LED off completely, connect the unused arm of the pot to the return to the battery.
This creates a variable voltage source (voltage divider circuit) as opposed to a simple series resistor. (In this case your 100-ohm pot will be suitable, but it draws more current from the battery, 15mA).

(I know a circuit diagram would be helpful but that will have to wait for when I have the time.)

Last edited: Sep 11, 2014
4. ### wayneh Expert

Sep 9, 2010
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I believe he is using an incandescent mini bulb and not an LED.

5. ### alfacliff Well-Known Member

Dec 13, 2013
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you might try a "fader" used in car radios for front - rear volume adjustment, available at radio shack, that should have a high enough current rating to handle the bulbs.

6. ### Sean Elkins Thread Starter New Member

Sep 11, 2014
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1
Would a little mini Christmas tree bulb really draw enough current to damage a pot? I'm talking about the little tiny guys that come on 100 light strings.

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7. ### wayneh Expert

Sep 9, 2010
12,405
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Depends on the pot. It would need to be rated to 1W or more. I think those bulbs are ~1/2W 2.5V and thus ~200mA. You can think of it as 12.5Ω for your Ohm's law calculations. Note that the resistance drops with brightness (heat).

Last edited: Sep 11, 2014
8. ### Sensacell Well-Known Member

Jun 19, 2012
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Use an analog panel meter to demonstrate sending an "analog" signal, so much simpler and the kids will get the idea much faster.

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9. ### wayneh Expert

Sep 9, 2010
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Good idea. More expensive than a lightbulb but it would work very well with the pot arrangement in #3.

10. ### Sean Elkins Thread Starter New Member

Sep 11, 2014
3
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The analog meter is a great idea for a demonstration, but for a class working in groups I would need 8-10 of them in order for them to actually interact with the stuff.

11. ### wayneh Expert

Sep 9, 2010
12,405
3,256
You can get digital voltmeters on e-bay for quite cheap. Harbor Freight gives away entire multimeters when they're not selling them for \$4.99 or whatever. Of course any digital readout would stray away from the point you are teaching about analog.