Parts List:
   R1 = 1 Meg, Preset Pot
   R2 = 10K    
R3,R4 = 1K

   C1 = 10uF, 16V, electrolytic
   C2 = 0.01uF, ceramic
   Q1 = BC547 (Gen Purp NPN) or 2N3904
   Q2 = 2N2222 (Hi Current NPN, up to 800mA)
   D1 = 1N4001 (Gen Purp Si)
  IC1 = 555 (Lo-Power version)
 RLA1 = Relay, 9V (amps of your choice)
Circuit Operation:
IC1 is wired as a monostable multivibrator. Input pulses are applied to the base of transistor Q1. This causes Q1 to conduct, taking pin 2 of IC1 'low'. IC1 then enters a timing cycle, the duration of which is set by R1*C1, causing pin 3 of IC1 to go 'high'. This causes Q2 to conduct. At the end of the timing cycle Q2 switches off, and the circuit waits for the next pulse to Q1.

Tony's comments:
Q1 replacements: 2N3904, 2N4400, 2N4401, PN100, NTE123AP (for NTE: check lead configuration!).
Q2 replacements: NTE123A. Not a critical device. Almost anything will work including another 2N3904, depending on the current draw of the relay. Circuit will work with 12V.

Paul's Note:
A question I see cropping up quite often on the forum is "how do I switch on a relay for 'x' seconds with a 'y' volts pulse". You can use this circuit as a building block.

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Project Copyright © 2004 - by Paul Hancock