Monostable 555


Joined Apr 20, 2004
If, by "according to the attached PDF", do you mean you have put such a kit together?

If so, have you carefully inspected your soldering to insure no pads got bridged?


Joined Apr 5, 2008

Is the 555 fitting well in the socket?
If you are using the type of socket as shown on the pictures, they may make bad contact.



Joined Dec 5, 2009
The pin layout on the PDF is not the layout as the actual 555, so if you used this as a wiring diagram and not the kit:

Be sure you have the package in the right direction, and have identified pins 1 through 8.

Now be sure you attach the wires to the proper pins.


Joined Mar 24, 2008
You should be able to see the capacitor charge up. It starts from ground, then slowly (slow enough to be seen by a meter) charges to 2/3 the power supply voltage. If this isn't happening then there is where the problem is.

Pin 4 should be Vcc, and stay there.

This circuit is very similar to the 555 Monostable in the AAC book.

I would recommend a thorough read of the theory of operation.


Joined Aug 7, 2008
When you apply power, does the red LED light? When you press & hold the trigger switch closed does green LED light & stay on?

Thread Starter


Joined Apr 15, 2010
The 555 chip is up the correct way.
The capacitors, diode and transistor are facing the correct way.

When applying power LED2 light ups. When the trigger is set off, LED1 lights up and LED2 is switched off.

I will try testing with the capacitors with a volt meter.


Joined Mar 24, 2008
I just took a closer look at your PCB vs the schematic, they don't match. You have Q1 on the PCB, which appears to be a driver transistor. Bad form that, a schematic should always have all of the parts, including part numbers (which yours doesn't).

Don't forget to check the voltage on Pin 4, it is important, and this PCB swaps sides several times getting the voltage there. It should be at Vcc.


Joined Aug 7, 2008
Every thing seem to be working right except for the charging ofC3 470 uF. On schematic 1k from VR1 is R5 which by parts list is 100k. When checking for V rise on C3, be sure to hold trigger sw closed to keep pin 7 from discharging C3.