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help with simple led circuit

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dimdiode

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Hi everyone

I'm a complete novice with electronics and i'm in need of your help (obsequious bowing)

I need to build a simple low power 12v dc led circuit such that when power is first applied the diode lights up for 3 to 4 seconds then switches off. It only comes on again when the power is disconnected then re-connected.

Hope someone can come the rescue :)
 
Thanks so much 4pyros but does this solution mean there is always a 9v connection? With my problem there is not always a current. I want the led to come on when there is a current but only for 3 to 4 seconds.
 
A 555 timer configured as monostable circuit as proposed by 4pyros will do what you need.
The on-time of the LED will be dependent on the value of R1 and C1.
Time on = 1.1 R1 * C1

I advice you read the brief discussion on the link 4pyros posted.
And when you get answers from your thread, try to pick up some terms like "555 monostable" and google. Try to research.
Then, ask clarifications on things you don't understand.
 
Yes it will work. You can hook up your load before or after the power switch.
Most 555 monostable circuits you will find will have a separate trigger input.
This was the best representation I could find within a few minutes looking on the web of one that would come on at power up.
You can all so try it with the trigger pin left open, most 555 will trigger once when first powered up anyway be may not be consistent and you may have to wait longer between power cycles.
 
Thanks again 4pyros and meowth08
I tried to build this circuit ( I am a beginner) on a breadboard. The led lit up but stayed on. Curiously, the wire to pin 2 disconnected without me noticing and the circuit appears to work as desired?? How strange! I'm assuming I have done something wrong :)

Ps. I couldn't log on yesterday as the text box for the password had vanished mysteriously.
 
That circuit linked to by 4pyros has an erroneously connected pin 7; this will cause problems (like the LED appearing on constantly). Disconnect pin 7 and ignore it.

The LED should be powered from the same side of the switch as the 555 timer (as you said you don't have power connected all the time). You can remove the transistor at some stage too, if you want, and connect the LED from pin 3, through a resistor to ground (0V).
 
Thanks dougy83
Please forgive me if im trying your patience :-( but i dont have any experience at all with electronics, quite fascinating though :)

I've tried your suggestion Dougy83. Disconnecting pin7 doesnt work. I removed the transistor and connected LED to pin3 then a transistor and then to ground, as suggested. Still, the LED stays on permanent with pin 2 connected (but i did notice it blink when i was hoping it would switch off) . Also in this configuration it sometimes does not light up at all and i have to wait a few seconds to try again (some sort of discharge occurring perhaps??). I've tried powering the breadboard from both sides.

Thanks again for your replies.
 
Please forgive me if im trying your patience :-(
Not at all.
I've tried your suggestion Dougy83. Disconnecting pin7 doesnt work. I removed the transistor and connected LED to pin3 then a transistor and then to ground, as suggested.
That should have fixed it, as the circuit is then correct.
Still, the LED stays on permanent with pin 2 connected (but i did notice it blink when i was hoping it would switch off) .
If it blinked, then there may be something connecting pin 3 or pin 7 to the input pins 2/6. I'm hoping you're using the same pin numbering as in the datasheet.
Also in this configuration it sometimes does not light up at all and i have to wait a few seconds to try again (some sort of discharge occurring perhaps??). I've tried powering the breadboard from both sides.
That is expected, and yes, you are correct; it is caused by the capacitor discharging through the timing resistor and the 555. It can be reduced by connecting a diode between the {capacitor +ve, pins 2 and 6} (diode anode) to the {positive supply in, pins 4 and 8} (diode cathode). This will effectively remove the timing resistor from the equation during discharge.

You could post a photo of your breadboard setup if you wanted someone to have a look at it.
 
Thanks again 4pyros and meowth08
I tried to build this circuit ( I am a beginner) on a breadboard. The led lit up but stayed on. Curiously, the wire to pin 2 disconnected without me noticing and the circuit appears to work as desired?? How strange! I'm assuming I have done something wrong :)

Ps. I couldn't log on yesterday as the text box for the password had vanished mysteriously.
Can you post a pic?
 
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