FAQ
TL;DR: For a one‑hour LED timer in a small, battery‑powered build, use TI’s TPL5111 ("generate delays between 100 milliseconds and 7200 seconds") or a tiny 8‑pin micro; 555 RC is imprecise for 1 hour. [Elektroda, Rick, post #21680130]
Why it matters: This FAQ helps makers pick a reliable, compact 60‑minute LED‑on solution without trial‑and‑error.
Quick Facts
- Space target: Approx. 2.5 in diameter, ≤0.5 in tall; battery‑powered light‑pipe LED use case. [Elektroda, Anonymous, post #21680131]
- TPL5111 delay range: 0.1–7200 s on 1.8–5 V; add a transistor for higher LED current. [Elektroda, Rick, post #21680130]
- Pure RC approach estimate: ~72 F for 10 mA over ~0.5 V droop to reach 1 hour. [Elektroda, Anonymous, post #21680137]
- 555 for 1 hour needs ~22 MΩ and up to 1000 µF; leakage makes timing erratic. [Elektroda, Anonymous, post #21680138]
- Alternative counter timers to explore: CD4060 and CD4541 for long delays. [Elektroda, Anonymous, post #21680139]
What’s the simplest way to keep an LED on for about 1 hour?
Use a low‑power timer IC or an 8‑pin microcontroller. TPL5111 covers up to 7200 s on 1.8–5 V. Add a small NPN transistor if the LED needs more current than the timer pin can source. This approach fits tight spaces and keeps time well enough for a product claim. A pure RC fade is not practical for 60 minutes in your size. [Elektroda, Rick, post #21680130]
Will a single big capacitor keep an LED on for an hour?
Not realistically here. At 3 V supply and ~10 mA LED current, a rough calculation shows about 72 farads for a 0.5 V drop. That’s a supercapacitor class part and won’t fit a 0.5‑inch‑tall puck with other electronics. It also won’t hold constant brightness. [Elektroda, Anonymous, post #21680137]
Can a 555 timer do a precise 1‑hour delay?
It can, but not precisely or reliably in this use. Achieving ~3600 s needs ~22 MΩ and up to 1000 µF. Capacitor leakage and tolerance drift make the result erratic. As one expert notes, the 555 is “not very precisely” suited for long delays. [Elektroda, Anonymous, post #21680138]
What is the TPL5111 system timer and why use it?
TPL5111 is a low‑power system timer IC from TI that generates programmable delays from 100 ms to 7200 s on 1.8–5 V. It’s easy to use and ideal for battery builds. Use a transistor interface if your LED requires higher current or constant brightness. [Elektroda, Rick, post #21680130]
How do I build a 1‑hour LED timer with TPL5111?
- Power the TPL5111 from your battery (1.8–5 V) and set its delay for 3600 s.
- Drive a small NPN transistor from the TPL5111 output; size the LED resistor for your brightness.
- Wire the user switch to start the cycle and route LED power through the transistor. [Elektroda, Rick, post #21680130]
What about using a tiny 8‑pin microcontroller?
An 8‑pin Microchip PIC can time one hour using its internal oscillator and simple code. You add one decoupling cap, the LED, and a resistor. Consider a regulator if you want constant LED brightness as the battery sags. This keeps parts count and size very low. [Elektroda, Anonymous, post #21680135]
What is PICAXE, and can it do this easily?
PICAXE is a beginner‑friendly microcontroller platform programmed in BASIC. The 08M2 8‑pin device can time 1 hour and drive an LED with minimal parts. It’s cheap, easy to program, and suitable for quick proof‑of‑concept builds and small enclosures. [Elektroda, Anonymous, post #21680138]
Are CD4060 or CD4541 better for 1‑hour timing?
Yes, both are long‑delay timer/counter ICs. CD4060 provides an oscillator with divider stages, while CD4541 is a programmable timer. With the right RC and wiring, they can deliver a one‑shot around 1 hour in a compact board. “Nice call… doable.” [Elektroda, Anonymous, post #21680139]
I need battery power and a small enclosure; what should I watch?
Choose low‑power timing and avoid large RC values. The enclosure is about 2.5 inches in diameter and 0.5 inches tall. That rules out supercaps and bulky relays. Favor TPL5111 or an 8‑pin micro, plus a small transistor for LED drive. [Elektroda, Anonymous, post #21680131]
Do I need a transistor to drive a bright LED?
Often yes. Timer outputs source limited current. Use a single NPN transistor to switch the LED current set by a resistor. This keeps the timer happy and maintains brightness over the hour. It also preserves timing accuracy by reducing output loading. [Elektroda, Rick, post #21680130]
Should the timer retrigger if the switch is toggled again?
Decide behavior up front. Define whether a second toggle restarts the 1‑hour count or is ignored. Engineers recommend clarifying switch type, start conditions, and retrigger logic before choosing parts or code. This avoids surprises in user tests. [Elektroda, Anonymous, post #21680134]
Toggle vs. push‑button: when should the hour start?
Specify this in your design. With a toggle, start timing when turned ON, or when turned OFF after arming. With a push‑button, start at press or release. Document these rules so your circuit or code implements the correct user experience. [Elektroda, Anonymous, post #21680134]
Can I use mains frequency to keep one hour very accurate?
Yes, counting 50/60 Hz cycles gives accurate timing. However, this project is battery‑powered and space‑limited, so mains is impractical here. Keep this option in mind for mains‑powered products that demand tighter accuracy without crystals. [Elektroda, Anonymous, post #21680129]
Is a spring‑return mechanical timer switch viable?
It works for rough timing, but it’s bulky and inaccurate for a 0.5‑inch‑tall puck. Mechanical options suit low‑precision tasks. For product‑grade one‑hour timing, prefer IC‑based solutions with defined tolerances and compact footprints. [Elektroda, Anonymous, post #21680136]
Any gotchas that can break a 1‑hour design?
Watch electrolytic leakage and tolerance in large RC networks. They drift and shorten or lengthen the delay. Battery droop also dims LEDs unless you regulate or PWM. As one poster put it, large‑value 555 timing can become “erratic or unusable.” [Elektroda, Anonymous, post #21680138]
