problem with 555 monostable

Thread Starter

pheonix992

Joined Jun 23, 2013
21
i making a 555 monostable circuit which its output get high when trigger pin gets low put the problem is that sometimes the output get high without push the button here 's the circuit and the 400 ohm resistor is relay




any help
 

#12

Joined Nov 30, 2010
18,224
The schematic looks correct, except for a local capacitor or 2 on the 555 chip, but that won't cause a false trigger. I can only guess you have an intermittent connection. Have you tried building this with solder?
 

#12

Joined Nov 30, 2010
18,224
a 555 chip can throw glitches in the power supply to itself if it doesn't have a 10 uf and a .1 uf capacitor attached very close to the chip.

Perhaps try an improved trigger?
 

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tracecom

Joined Apr 16, 2010
3,944
The schematic looks pretty standard, but the PCB layout is not good.

T1 on the schematic appears to be transformer, but T1 on the PCB appears to be a TO92 transistor. I don't get it. Are you really bringing 220 mains voltage onto the same PCB as your 555 circuit? Where is the mains transformer?
 

#12

Joined Nov 30, 2010
18,224
This is not a trigger problem at all. First is a start up problem. You can fix that by adding a 10K resistor from the positive supply to pin 4 (reset) and add a 10 uf capacitor from Pin 4 (reset) to ground. That will keep the chip from responding for a tenth of a second when the power starts up.

If you don't want it to trigger when you stick your finger in the circuit, don't stick your finger in the circuit!
 

bertus

Joined Apr 5, 2008
22,270
Hello,

I do not see the decoupling capacitors on the power regulator on your PCB:



They should be placed real close to the regulator.

Also there are no decoupling capacitors near the NE555.
A 10 μF and a 0.1 μF parallel on the power points (pin 4 and 8) of the NE555 will give a lot more stability.

Bertus
 

Thread Starter

pheonix992

Joined Jun 23, 2013
21
he schematic looks pretty standard, but the PCB layout is not good.

T1 on the schematic appears to be transformer, but T1 on the PCB appears to be a TO92 transistor. I don't get it. Are you really bringing 220 mains voltage onto the same PCB as your 555 circuit? Where is the mains transformer?

t1 is transistor and 220out and 12v these are the transformes wire

I do not see the decoupling capacitors on the power regulator on your PCB:



They should be placed real close to the regulator.

Also there are no decoupling capacitors near the NE555.
A 10 μF and a 0.1 μF parallel on the power points (pin 4 and 8) of the NE555 will give a lot more stability.


the 3 decoupling capacitors you will find it under the regulator



This is not a trigger problem at all. First is a start up problem. You can fix that by adding a 10K resistor from the positive supply to pin 4 (reset) and add a 10 uf capacitor from Pin 4 (reset) to ground. That will keep the chip from responding for a tenth of a second when the power starts up.
if you think that it 's not a trigger problem so why it trigger without press the button
 

bertus

Joined Apr 5, 2008
22,270
Hello,

Do you mean C1 and C5 on the PCB?
They are VERY far away from the regulator.
C1 is even on the very end of the trace.

Bertus
 

Thread Starter

pheonix992

Joined Jun 23, 2013
21
Because you stuck your finger in it!
You triggered the timer with your finger.
it doesn't trigger when touch it with my finger but when touch it with any metal
and the problem is that it triggered without touching it with any thing


Do you mean C1 and C5 on the PCB?
They are VERY far away from the regulator.
C1 is even on the very end of the trace.
they are c1 c3 c5 are filters for power supply
 
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