This circuit was obtained in a Hands-On Electronics from the 90's. The circuit turns on one LED at the end of the timer, erasing the other. The timing can be increased by increasing C1.
This circuit of very high input resistance was found in a Radio Shack documentation from 1977. The...
Combining an Ultra-Low Power MCU with an 802.15.4 Radio, the SAM R30 Enables Long-Lasting Battery...
An interesting circuit suggested to indicate when a device is turned off. In the figure we have...