Problem the 555 get triggered when i power up circuit

[meliyka]

I am connecting a Not and And gate to an activation switch (S1 or S2) that trigger a 555 to turn on a motor using a relay , i am using a main power up toogle switch that power the 555(vcc of 555 is sensor1) , so when it is off the 555 can't be triggered ,my problem is that everytime i turn on the toogle switch ( connecting 5 V to Sensor1) , the 555 get triggered alone and one cycle is run , attached is the circuit , and gate and not gate are connected to 5 V before the toogle switch is in position of sensor1...

Any ideas of how to fix it ?

Thanks

 

[vk6kro]

Are you using the 556 as a monostable? If so, I think I know what causes this.

Try removing the capacitor C3 on pin 3. This is not necessary and it causes a delay on the lower two 5K resistors and you get a false triggering, just on turn-on.
On a 555 this is pin 5 and you do the same thing.

 

[meliyka]

Yes i am using the 556 as monostable, i will try what you said , i let you know , thank you for your quick response...

abril

 

[meliyka]

unfortunatelly i just tried that and it did not work , i took off the C3 and keep auto triggering when i put the toogle switch in position of sensor 1

any other idea ?

Thank you

 

[ShadowPho1]

Pull down resistor on TRIGGER_555

edit: currently you have a pull up on trigger_555, thus during power up it will turn HIGH first.

 

[dlgoff]

Looks to me like you are triggering the 555 when applying the 5volt Vcc. (which is what ShadowPho1 suggested) Ususally one would have a power-on reset circuit of some sort. In this case, I whould hold the trigger low for a time while the power supply comes up. Not quite sure why you are using the switch for test/sensor.

 

[vk6kro]

Your monostable circuit is fairly standard.
see:
http://www.eleinmec.com/article.asp?4

You could try removing the trigger input to the 556 (temporarily) to see if it still triggers on startup. It may be getting a pulse from earlier in the circuit when you turn on.

Leave C3 disconnected, though. It does cause this effect.
I had a camera being used for aerial photography which was being driven by a 555 monostable and it was taking a very expensive photo of the runway every time it was turned on. Removing this capacitor, which is always shown in circuit diagrams, fixed the problem.