FAQ
TL;DR: At 5 V, a 1 kΩ resistor gives ~5 mA; “All you have to change is the resistor value.” Choose ~250–500 Ω for 20 mA LEDs, using R = (V − Vf) / I. [Elektroda, Mike P OKeeffe, post #21664253]
Why it matters: This FAQ helps beginners convert 10 mA LED blinkers to 20 mA parts safely for Star Trek‑style panels.
Quick Facts
- Use R = (V − Vf) / I; design with the LED’s highest forward voltage (Vf). [Elektroda, Steve Lawson, post #21664255]
- Typical white 3 mm LED Vf: 3.2–3.4 V at 20 mA. [Elektroda, Zack MacKeigan, post #21664254]
- For 5 V and 20 mA, start near 250 Ω; for lower brightness, try 500 Ω. [Elektroda, Mike P OKeeffe, post #21664253]
- Cost reality: discrete “blinky” parts ~$4; an Arduino board ~$50. [Elektroda, Zack MacKeigan, post #21664258]
- Edge case: Vf shifts with color, current, age, lot, and temperature—leave margin. [Elektroda, Steve Lawson, post #21664255]
How do I convert a 10 mA LED schematic to use 20 mA LEDs at 5 V?
Change the series resistor. Use R = (V − Vf) / I. With 5 V supply and white LEDs, target ~250 Ω for 20 mA, or ~500 Ω for dimmer output. Stay at or below the LED’s continuous current rating. “All you have to change is the resistor value.” [Elektroda, Mike P OKeeffe, post #21664253]
How many 20 mA white LEDs can I run from one 5 V output?
With white LEDs at ~3.2–3.4 V Vf, 5 V usually supports one LED plus a resistor per output. For series strings, sum Vf and ensure the total stays below the supply; then compute R from the remaining voltage. Use the highest expected Vf for safety. [Elektroda, Steve Lawson, post #21664255]
What does “20 mA LED” actually mean?
It’s the LED’s maximum continuous current rating, not a built‑in draw. The actual current depends on how you drive it. Set the current with a series resistor or a current regulator per R = (V − Vf) / I. [Elektroda, Steve Lawson, post #21664255]
How do I pick a resistor when LED forward voltage varies?
Use the worst‑case (highest) Vf from the datasheet in R = (V − Vf) / I. That ensures current stays at or under your target across part and temperature variations. Add margin if no datasheet is available. [Elektroda, Steve Lawson, post #21664255]
Is it safer to run 20 mA LEDs below 20 mA for this blinker?
Yes. Driving below maximum reduces brightness, heat, and stress. For a 5 V CMOS blinker, try ~500 Ω for gentler current and compare brightness. You can experiment, but do not exceed the LED’s limit. [Elektroda, Mike P OKeeffe, post #21664253]
Can I share one resistor across two LEDs?
Only if they’re in series and the summed Vf is under the supply. Then compute R from the remaining voltage: R = (V − ΣVf) / I. Avoid parallel LEDs on one resistor; current will not split evenly. [Elektroda, Steve Lawson, post #21664255]
What is forward voltage (Vf) on an LED?
Vf is the voltage drop across the LED when it conducts. It depends on color, current, age, lot, and temperature. Use the highest expected Vf to size your resistor so current stays within spec. [Elektroda, Steve Lawson, post #21664255]
Should I use a microcontroller (Arduino) instead of a CMOS counter for random blinking?
A microcontroller gives richer, less repetitive patterns using a pseudo‑random number generator. You can even reseed on motion or sound for interactive effects. This avoids obvious repeating sequences. [Elektroda, Steve Lawson, post #21664256]
Are there cheaper beginner‑friendly alternatives to Arduino?
Yes. PICAXE chips program in BASIC and are very beginner‑friendly. You mainly need the USB programming cable (about £12), a breadboard, a few resistors, and the free software. [Elektroda, Frank Bushnell, post #21664270]
What hardware do I need to program a bare PIC microcontroller?
Get a programmer such as a PICkit 2 or 3. Expect around $40–$50, but it serves future PIC projects. You can also explore free IDE options from MikroElektronika or Microchip. [Elektroda, Steve Lawson, post #21664261]
How can I measure unknown LED Vf and set current without a datasheet?
Build a quick test: supply, ammeter, variable resistor, and LED socket. Adjust to your target current, read Vf, then compute a fixed resistor using R = (V − Vf) / I. Add margin for variation. [Elektroda, Steve Lawson, post #21664255]
What is a CMOS binary counter/divider doing in this circuit?
It steps outputs sequentially from a clock, presenting near‑supply voltage at each Q pin. Each active output drives its LED branch through a current‑limiting resistor, creating blinking channels. [Elektroda, Mike P OKeeffe, post #21664253]
How do I compute a resistor for a 3.2–3.4 V white LED on 5 V?
Use the highest Vf and your desired current. Example: for 10 mA, R ≈ (5 − 3.4) / 0.01 = 160 Ω. For 20 mA, R ≈ 80 Ω, but consider using higher resistance for longevity and brightness control. [Elektroda, Steve Lawson, post #21664255]
Quick how‑to: set LED current correctly in three steps
- Note supply V and LED’s highest Vf.
- Choose target current (≤ LED’s max).
- Compute R = (V − Vf) / I and pick the nearest standard value. [Elektroda, Steve Lawson, post #21664255]
Why did my build light but not blink randomly?
Randomness from simple counters can look repetitive. A microcontroller can produce varied patterns and sudden changes. “Use a pseudo random number generator…and reseed with sensors.” [Elektroda, Steve Lawson, post #21664256]
What’s a practical starting point if I’m nervous about capacitors and timing parts?
Prototype on a solderless breadboard, verify each channel lights, then tune resistors for brightness. If patterns feel monotone, migrate the logic to an Arduino and reuse the LEDs and resistors. [Elektroda, Zack MacKeigan, post #21664280]
