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intermittent 555 monostable circuit help

plutes

Mar 4, 2021
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I keep coming back to this book and for three days been stuck trying to figure out circuit on page 9, I cant move forward until I get it working. If I use one switch/push button it works intermittent, could be 10 seconds could be 5mins. try another push button, it just stays on.

I have managed to get other circuits around monostable working from yt vids etc, thing I notice different form this is pin 5 is not being used. not sure if this is the reason why its not playing fair.
 

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PETERDECO

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I haven't dabbled with 555's in a long time but every monostable circuit I've seen has a cap from pin 5 to ground. 10nF to .01uF. Try it.
 

plutes

Mar 4, 2021
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I haven't dabbled with 555's in a long time but every monostable circuit I've seen has a cap from pin 5 to ground. 10nF to .01uF. Try it.
that has worked really well, 100 nf. any thoughts why this book schematic not use pin 5?
 

bertus

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Nov 8, 2019
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Hello,

Using a capacitor on pin 5 will make the reference voltages more stable.
Have a look at the functional schematic:
NE555_functional_diagram.png
Bertus
 

plutes

Mar 4, 2021
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Hello,

Using a capacitor on pin 5 will make the reference voltages more stable.
Have a look at the functional schematic:
View attachment 51213
Bertus
something odd is happening I can get it too light the led without touching the switch, not every time but every so often. will that be static off me? it doesn't seem to like being filmed
 

Audioguru

Sep 24, 2016
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It is important for a 555 to have a supply bypass capacitor as shown in its datasheet, and a supply and wiring that can produce the 400mA of "shoot-through" current produced by the old 555.
The datasheet for a 555 shows that the trigger pin2 MUST have a pullup resistor.

The tangle of wires all over the place on the solderless breadboard does not clearly show what connects to what.
The circuit also shows hundreds of "intermittent" breadboard connections. Isn't this thread is about intermittent problems?

The very important power supply voltage is not shown. Maybe it is too low.
 

Audioguru

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It is not odd that the long wire connected to the trigger pin 2 picks up interference since your circuit is missing an important pullup resistor.
 

plutes

Mar 4, 2021
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It is important for a 555 to have a supply bypass capacitor as shown in its datasheet, and a supply and wiring that can produce the 400mA of "shoot-through" current produced by the old 555.
The datasheet for a 555 shows that the trigger pin2 MUST have a pullup resistor.

The tangle of wires all over the place on the solderless breadboard does not clearly show what connects to what.
The circuit also shows hundreds of "intermittent" breadboard connections. Isn't this thread is about intermittent problems?

The very important power supply voltage is not shown. Maybe it is too low.
9 volts, I did attach a circuit drawing in file 4 above. I can attach more photos if you desire
 

plutes

Mar 4, 2021
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It is not odd that the long wire connected to the trigger pin 2 picks up interference since your circuit is missing an important pullup resistor.
the book circuit diagram on page 9 of the book never used a pin two resistor, my question is not about another way too get a monostable to work, it is why the books schematic doesn't work. in that they use 1 resistor and 1 cap pins 6 and 7

2nd reply
shortened the pin 2 lead and now its not turning off....
it is now was shorting
 
Last edited:

Harald Kapp

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why the books schematic doesn't work.
Because it is incomplete:
  • The lack of a capacitor on pin 5 is a common mistake (don't ask me why), but often not a big issue, especially with CMOS versions of the 555.
  • The lack of a pull up on pin 2 is probably due to the intended trigger signal: It is expected to be either logic high or logic low, not a simple pull-down pushbutton. Look at the waveform below the circuit diagram. It shows trigger being either high or low. If you connected a pushbutton to gnd you can force the input to gnd and thus logic low, but there is no way to force the trigger input back to logic high. This is where you need an additional pull-up resistor.

For a full discussion of the 555 as a monostable multivibrator see e.g. here.
 

plutes

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Because it is incomplete:
  • The lack of a capacitor on pin 5 is a common mistake (don't ask me why), but often not a big issue, especially with CMOS versions of the 555.
  • The lack of a pull up on pin 2 is probably due to the intended trigger signal: It is expected to be either logic high or logic low, not a simple pull-down pushbutton. Look at the waveform below the circuit diagram. It shows trigger being either high or low. If you connected a pushbutton to gnd you can force the input to gnd and thus logic low, but there is no way to force the trigger input back to logic high. This is where you need an additional pull-up resistor.

For a full discussion of the 555 as a monostable multivibrator see e.g. here.
Thanks will read through that, I won't pretend I fully understand whats going on here, but should be able to move forward through the book.

Remade circuit added a resistor before the push button and changed the cap to 100uf, seems a lot more stable.

fullsizeoutput_71.jpeg
 

plutes

Mar 4, 2021
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massive thank you for all your help, I'm plowing through the book now multivibrator astable operation, normal and equal mark space astable have been easy to follow
 
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