Ah, that age-old old-timer, the 555 chip. It can be run in either astable (think repeating clock signal) or monostable (single pulse, then done) mode. I'm going to outline a simple monostable design, and some of the tricks that I needed to use to get it working great. Design of a 555 Monostable 1 to 10 second Timer Monostable timer with 24V trigger Two Component Timing This particular design I needed to turn on a relay for about 5 seconds when the 24V rail came on. The basic timing of the 555 depends on R1+R6 and C1, in my circuit. The equation is as follows: τ = 1.1 R C This is also displayed on the datasheet on page 10 as a graph of common resistor values. I chose 4.7uF and 1MΩ, which equates to τ = 5.17. I wanted my timing adjustable by at least 1 order of magnitude, centered on my 5 seconds. Thus, a minimum R of 200KΩ and a maximum of 2MΩ. I went with a split series resistance: a fixed 100KΩ and a 25-turn 5MΩ potentiometer. Eh, a little more than 10 sec