#714 Slow Glow 2
A simple slowly-glowing LED effect using a 555 timer. This provides a better fade-in/fade-out than the Slow Glow circuit.
Here’s a quick demo..
Notes
This project is based on the UP/DOWN FADING LED Circuit. I’ve modified it for 5V operation.
It is an unusual circuit in a few respects:
- an unconventional 555 configuration
- using a high-side NPN transistor
The end result, however, is quite pleasing. It achieves a steady fade-in and fade-out of the LED. Certainly a better result than my previous Slow Glow circuit: LEAP#559.
Circuit Design
The original circuit was designed for 9V. To use with 9V, recommended to change R2 to something higher to avoid over-driving the LED e.g. 470Ω.
With a 5V supply, it has just enough forward voltage to drive an LED
How it Works
The following scope trace shows the circuit in operation:
- CH1 (Yellow) - timer output
- CH2 (Blue) - transistor base/capacitor anode/timer threshold & trigger
- CH3 (Red) - transistor emitter/LED anode
- Note: all inputs scaled to 1v/div but offset by -1v
Basic operating principle and notes:
- with the 555 output high, the capacitor C1 is charged via R1 with time constant (𝛕) of about 3.3s.
- the 555 timer threshold/trigger pins are tied and will limit the oscillation between 1/3 and 2/3 VCC
- when capacitor C1 reaches 2/3 VCC, the 555 threshold pin resets the output low, starting discharge.
- The 555 discharge pin (7) is unused in this configuration. Discharge is via the transistor B-E connection, hence the slower discharge.
- when C1 reaches 1/3 VCC, the 555 trigger pin sets the output high, restarting the charge cycle.
