R1,R3 = 10K Q1 = 2N3904, or equivalent
R2 = 680K (see text) IC1 = 4001, or equivalent
R4,R5 = 6K8 D1,D2,D3 = 1N4001, or equivalent
C1 = see text Ry = Relay, 12V
C2 = 0.1uF, ceramic
The above circuit is similar to the "Delayed Turn-on" type but now configured as an automatic turn-off type. The diagram
shows how the circuit function can be reversed so that the relay turns on when power is applied but turns off again
automatically after a preset delay. This response is obtained by modifying the relay-driving stage for an NPN
transistor like the 2N3904.
It is worth noting again that the diagram provides a time delay of about 0.5 seconds for every microfarad in the
value of capacitor C1. This permits delays of up to several minutes. If desired, the delay periods can be made
variable by replacing resistor R2 with a fixed and variable resistor in series whose nominal values are
approximately equal of the total value of R2 (680K).
Copyright © 2003 - Tony van Roon
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