Power-On Triggering problem in 555

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kgram007

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Hello,

I had designed circuit using 555 in its monostable mode. the circuit works fine. but the problem now is when i power on the system, sometimes the output gets triggered with out giving input to the triggering pin. i have attached my circuit diagram with the post. This is a serious problem, because i use a 12v transformer with 7812 and when ever there is fluctuation in power supply, there is false triggering. looking forward for a solution. Thanks in advance.

 

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In your schematic I do not see any blocking capacitor from 12 V to GND. Also, no values of pull-up resistors to TRIG and RESET are shown, or, input details.
I would recommend to set the pullup resistors to not more than ~20 kOhms. Triggering is AC-coupled, I assume. Use a smallest capacitor value needed, and treat the input trigger line as low-level audio input. Screened wires, avoiding coupling to power-carrying wires, etc.
 


Sorry for not mentioning the values... the pullup resistors used are R2=R3= 10 K Ohms , other resistors are used to limit voltage for LEDs = 1 K Ohms...

The capacitor c1= 1000 uF and its the blocking capacitor used... and if i am correct u mentioned about the capacitor after the 7812??... i would try adding that....

Then reg. the inputs, the input for triggering and resetting are through Switches... the switches are connected between input and ground....

think the details are enough !!... kindly help me....
 

All seem good but there is no mention of circuits connected to TRIG input. Use an oscilloscope to see any interference there.
555 is a good guy, it usually does not trigger by itself. How about interference coming from the AC circuit? Try connecting 1 to 10 nF capacitors in parallel to rectifier diodes. Enclose your circuit in a metal case.
I used a 555 monostable to be triggered by 0.6 us TTL pulses, and it works fine. No problems but no interference allowed.
 

I just connect a switch between trigger input and GND. so thats the only input !! i guess its the noise from AC .... i dono what to do to solve this...
 

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