Dim with a potentiometer
A potentiometer is a resistor with a third leg that slides along it. Wired in series with an LED and a fixed resistor, it is a brightness knob. Of the four in the box, the 1 kΩ is the one that uses the whole turn; the others give almost no change or all of it in the first tenth.
You need the first circuit, a 1 kΩ potentiometer and a screwdriver small enough for its screw. Everything else is the same.
Three legs
A potentiometer is a strip of resistive material with a leg at each end, and a third leg connected to a contact that slides along the strip. That contact is the wiper, and the middle leg is the wiper's.
The potentiometers in the box are small trimmers that you turn with a screwdriver. The value is on the body as three digits: two digits and the number of zeros, so 201 is 200 Ω, 102 is 1 kΩ, 103 is 10 kΩ and 105 is 1 MΩ. Confirm it with the meter: with the power off, the two outer legs read the full value, within about 20 %, and that reading does not change when you turn the screw.
A resistor you set by hand
Use the wiper and one end, and leave the other end alone. The resistance between them is the part of the track on that side, from nothing to the full value. In series with the LED and the 220 Ω resistor, the total is the fixed 220 Ω plus whatever the knob gives, and the current is the same arithmetic as in the last article.
Which one
There are four values in the box. All four work, and only one is useful here.
The 200 Ω one adds so little that the LED stays bright all the way round. The 10 kΩ one adds so much that only the first tenth of the turn lights it: the rest is dark, and all the control is crammed into a few degrees. The 1 MΩ one does it in the first hundredth. The 1 kΩ one changes the current from about 12.7 mA to about 2.3 mA, and does it smoothly over the whole turn, which is what a dimmer is.
Build it
The potentiometer's legs are in three neighbouring rows, one per leg. On the trimmers drawn here the three legs sit in a line. If yours has them in a triangle, put each leg in its own row anyway and keep track of which is the middle one.
Why the fixed resistor stays
You might think the 220 Ω resistor is redundant now that there is a potentiometer. It is not. At one end of the turn the potentiometer is 0 Ω, and without the fixed resistor nothing is left in the loop to limit the current.
Try it: set the fixed resistor to none and turn the knob to 0 %. The potentiometer is a variable resistor that can reach zero, so it must never be the only one.
Measure it
Do the first measurements with the power off and the potentiometer out of the board, so nothing else is in parallel with it. Set the meter to ohms, the symbol Ω, and touch the probes to the legs.
| Probes on | Expect |
|---|---|
| The two outer legs | The printed value, about 1 kΩ, within about 20 % |
| The middle leg and one outer leg, screw at one end | Near 0 Ω |
| The middle leg and the same outer leg, screw at the other end | The full value |
| The middle leg and each outer leg, any position | The two add up to the outer reading |
Then switch on and set the meter to DC volts. Put the probes across the fixed 220 Ω resistor, and turn the knob through the whole range, for a red LED on the 5 V side.
| Knob | Across the 220 Ω | Current |
|---|---|---|
| One end, 0 Ω | about 2.8 V | about 12.7 mA |
| Halfway, 500 Ω | about 0.9 V | about 3.9 mA |
| Other end, 1 kΩ | about 0.5 V | about 2.3 mA |
The reading across the LED hardly moves, about 2 V falling a little at the dim end, while the resistor's reading falls to a fifth. That is the control: the potentiometer is taking voltage, and the resistor and LED are left with less.
On the 3.3 V side
On 3.3 V the same circuit goes from about 5.9 mA to about 1.1 mA with a red LED, so the dim end is nearly dark. A blue or white LED goes from dim to dark: with the LED taking about 3 V, the module leaves only 0.3 V across the resistors, and the knob moves the current from about 1.4 mA to about 0.25 mA. For a dimmer, use the 5 V side.
When it does not work
The two outer legs were used and the middle one was not. Between the outer legs the resistance is always the full value, whatever the screw does. Move one wire to the middle leg, the wiper, and keep the other on one end. If it still does nothing and the potentiometer is the multi-turn type, it may have a clutch that clicks once you reach the end of its travel: turn it the other way.
That is a matter of which end you wired, not a fault. Moving the wire from one outer leg to the other swaps which direction of turn is brighter. The potentiometer has no direction of its own: the wiper is closer to one end or the other, and you chose the end that counts.
The wiper is skipping on the track, or one of the three legs is only half in its hole. Push the legs in and turn it slowly. On a worn or dirty trimmer the wiper can lose contact for a moment, and with only the wiper and one end wired that opens the circuit. A new trimmer from the box usually cures it.
The 200 Ω potentiometer was used. Its whole range adds only 200 Ω to the fixed 220 Ω, so the current moves from 12.7 to 6.7 mA and the LED stays bright. Swap it for the 1 kΩ one, which takes the same circuit from 12.7 mA down to 2.3 mA.
Replacing your hand on the wire with a button, and what a wire that goes nowhere really does.
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