Logic Gate / MOSFIT / breaking circuit

Hi Guys,

i need a little help building a circuit, i need to turn on a LED for 4 secs then turn off, and stay off untill the power is switched off and on (reset)

or if theres any easyer way to do it ( cheapy )

i was thinking a transistor or 2 would do it would do it but i think im thinking to deep when its probs an easy circuit.

all help is welcome ! ;-)

This circuit is a monostable multivibrator, or one-shot, you may use a 555 timer chip.

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Seems the best possible solution.

i cant get the circuit to turn off after the 3 secs heres the drawing

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basic it just stays on is it somthing i have done ?

Need to conect pin 4 to Vcc,

If the RESET terminal of a 555 or 556 timer is not going to be used, it is normal practice to connect this input to the supply voltage. If the RESET terminal is left unconnected the operation of the timer will not be affected, however, the RESET of CMOS version of these timers should not be left unconnected as the inputs of these devices are more sensitive and this may cause problems.

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mister_rf said:

Need to conect pin 4 to Vcc,


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sorry to be a pain but this is what i have atm and it still will not turn off :-(

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I think in this situation maybe there’s a problem with the initial settings and the simulator doesn’t see the "power on" behavior.

You're not triggering it right to attend your purposes. 555 requires to go low ( less than 1/3 VCC ) and back to high for triggering ( at least LM555 ).
You have to put a capacitor between the trigger input and the ground, forming a RC network with R1. So when you switch the power on, the capacitor will set the trigger less than 1/3 VCC while charging, providing the right trigger.

Will be better if you try it out on a protoboard.

Iuri~


EDIT: I forgot to mention, you need to put a resistor in parallel with the new capacitor, so when you power off the capacitor will discharge on this resistor. Beware, when the capacitor is full charged, the voltage on the trigger will be the value defined by the voltage divider formed by R1 and this new resistor and this voltage need to be higher than 1/3VCC. Also, depending on how fast you'll switch on/off, the discharging time of the capacitor has to be treated.

If you misunderstood something just let me know, and I will try to be more clear.

Iuri~