This circuit is from an Electronics World of 1967. The described calculations can be applied to equivalent circuits with modern transistors.
This circuit adds a beep to turn on with car flashers that do not have this feature. A 47 ohm to...
This 80's circuit uses a small light bulb as an indicator, but an LED with the appropriate resistor...
This circuito from the 1977 GE Semiconductor Manual circuit provides timings from 0 to 90 seconds...