Good afternoon All About Circuits,

Posted feedback on '555 Monstable Vibrator'
http://www.allaboutcircuits.com/vol_6/chpt_8/4.html

It is the the fourth post in a thread marked '555 Timer 110402' in
the The Projects Forum.

Allen in Dallas

 

Howdy,

These are my articles, when I find your post I'll respond. Other good places to leave feedback include...

http://forum.allaboutcircuits.com/showthread.php?t=26530

and

http://forum.allaboutcircuits.com/showpost.php?p=82319&postcount=4

I'm having a little trouble finding it. Using the address line as above is the best way to go. I'll keep looking.

 

OK, found it.

On the figure marked ILLUSTRATION there is a red jumper wire.
When that jumper is connected between R4 (in the connection marked 'B')
and the connection marked 'Gnd' the green LED glows. When the red jumper
is removed, the red LED glows. When the red jumper is placed between the
Gnd connection and the connection marked 'A' the red LED glows but that is the same as when the red jumper is not connected so not sure what that proves.

Since the circuit is titled '555 Monostable Multivibrator' it was expected
that it would light one of the LEDs for a certain amount of time and then
go off. But I seem to have missed in the exposition an explanation of how
the circuit should operate. Perhaps my understanding is so modest that the
annotation is aimed at readers with greater knowledge. Before a basic
perception of how the circuit operates and the logic behind is provided in the elucidation the breakdown jumps into 'illegal conditions' and 'edge only triggering'.
Later in the article it is stated that the circuit is designed 'for 3 seconds timing duration'.
Since the circuit would not work for me it is wondered which LED was supposed to come on for three seconds, the green or the red?
Any recommendations on troubleshooting to try and get the three second duration to occur?
Thanks.
Allen in Dallas

The red jumper wire is a switch. Its function was described in the text, it is there to show why the conditioner is needed. You have to remember that I won't junk up the schematic with switches unless I have to, when a simple piece of wire will do. This is a tutorial, meant to teach the sublties of this circuit.

Timing starts when the jumper is put into the circuit. With the conditioner (position A) it will proceed normally and time out. When it bypasses the conditioner and illegal condition occurs and the timer will stay high even after it has timed out. This is basically repeating what the text (Instructions) said.

You can just touch the jumper to point A and ground and get a normal timing operation. With the conditioner, you can leave it connected. On point B, touching the jumper will also start the timing. Where it goes wrong (and is explained with a show and tell) is if the jumper stayes in place.

This is a non retriggerable monostable, as opposed to a retriggerable type.

 

Which LED was supposed to come on for three seconds, the green or the red?

 

One stays on, the other stays off, they flip.