My project is...I would have a circuit which will be power by 12 v DC only when car engine is running. Then, in time t =5 secs, the capacitor will fully be charged while engine still running or maybe still running for 5 or 6 hrs....still then capacitor is full. Now, the main point..as soon as engine is OFF, the capacitor will be discharged which will activate a relay to turn on the load bulb connected to this relay as long as the charged is in the capacitor. Could any kindly sketch a diagram of this project. Thanks.enter image description hereenter image description hereenter image description here

  • Please do correct the given diagram.
    – Thang Tons
    Aug 30, 2019 at 6:13
  • So diagram is fine, now how do you work out the time the relay stays energized using the information I gave you in the other answer. Have a look at learningaboutelectronics.com/Articles/…
    – Solar Mike
    Aug 30, 2019 at 6:17
  • But sir...when engine is OFF, capacitor discharge and relays is energised. However, still confused with when engine is on..... current flows to capacitor as well as the relay which gets activated. I don't want this relay to activate during engine ON. It should be active only as soon as engine is OFF. Thanks.
    – Thang Tons
    Aug 30, 2019 at 6:19
  • Relay should not be active when engine is running or on. It should be active or energise only when engine is OFF.
    – Thang Tons
    Aug 30, 2019 at 6:21
  • Add a second relay controlled by the ignition, so when the key is on, that second relay is off...
    – Solar Mike
    Aug 30, 2019 at 6:21

1 Answer 1


Diagram in black will do what you want, if you explained it correctly, without any capacitor...

The other diagram shows what you need to do to your diagram, you do know the difference between the normally open and normally closed contacts ... enter image description here

  • I didn't understand your first diagram??
    – Thang Tons
    Aug 30, 2019 at 6:47
  • Not much different to yours...
    – Solar Mike
    Aug 30, 2019 at 6:48
  • Confusing is that how to block the relay being activated when engine is on??? This part I can't make out the diagram.
    – Thang Tons
    Aug 30, 2019 at 6:54
  • When the ign os off, that relay is normally closed (as stated in Red), when the ign is on, the relay contacts are opened...
    – Solar Mike
    Aug 30, 2019 at 7:01
  • Latest diagram which I supposed to be all correct is now uploaded. please do correct me if I'm wrong again. Thanks.
    – Thang Tons
    Aug 30, 2019 at 8:00

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