Posted on Oct 18, 2012 9833
Rl and R2 hold the inverting input at half supply voltage. R4 applies feedback to increase the input impedance at pin 3. Pin 3, the nonin-verting input, is connected to the junction of R3 and C. After the switch is opened, C charges via R3. When the capacitor has charged sufficiently for the potential at pin 3 to exceed that at pin 2 the output abruptly changes from 0 V to positive line potential. If reverse polarity operation is required, simply transpose R3 and C
R3 and C can be any values. Time delays from a fraction of a second to several hours can be obtained by judicious selection. The time delayâ€”independent of supply voltage â€”is 0CR seconds where C is in farads.