3. Potentiometer: a dimmer
Step 3 of the automatic light: VR1 (10 k) between the switch and R1. Knob at 0: about 15 mA. Knob at the far end: 10.47 k in total, under 1 mA, a dim glow. Component 3 of 14: the potentiometer. A potentiometer is a resistor with a sliding contact (the wiper, the middle pin). Used with one end and the wiper it is a variable resistor (a rheostat); used with both ends it is an adjustable voltage divider. Here it is a rheostat in series with R1.
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Parts list
Test procedure
Each test has an expected value and a pass range, like a factory test sheet:
- Dimmer at minimum resistance
- Dimmer at maximum
How it works
The potentiometer. A potentiometer is a resistor with a sliding contact (the wiper, the middle pin). Used with one end and the wiper it is a variable resistor (a rheostat); used with both ends it is an adjustable voltage divider. Here it is a rheostat in series with R1.
Reading it. Printed code: B10K = 10 kΩ linear (A = logarithmic, for audio volume). The value is the resistance end to end. R1 stays in series so that at zero ohms the LED is still protected.
Testing it. Ω between the two outer pins: the full value, whatever the knob. Outer pin to wiper: smoothly from 0 to the full value as you turn it; any jump or open spot is a worn track.
In the build. VR1 (10 k) between the switch and R1. Knob at 0: about 15 mA. Knob at the far end: 10.47 k in total, under 1 mA, a dim glow.
Builds on lesson 2: everything from before is still there.
Next: lesson 4, diode.
Things to try
- Drag the VR1 slider and watch the brightness and the current.
- At the far end the LED is still faintly lit: that is the 0.7 mA through 10.47 k.
Background
One circuit, built up one part at a time: by lesson 14 it is a solar-powered, dusk-sensing, motion-triggered timed lamp with an alarm.