Question about a 555 Timer chip

Thread Starter

sydcomebak

Joined Dec 13, 2011
69
According to the 555 cookbook:

"4.7 RESTARTABLE ONE-SHOT

During normal operation, a 555 monostable circuit will ignore input triggers that may occur during the timing cycle and will produce uninterrupted output pulses. On occasion, the need may arise to bypass this mode of operation and to restart the output timing with the application of a trigger pulse during the normal timing period. A circuit that accomplishes this is shown in Fig. 4-7. In this circuit, the reset and trigger pins of the 555 are tied together and used as a single trigger input. When the input is driven to zero, it has reached the threshold level of the reset input. Since the reset function is overriding, the output is held low even though the trigger input is below its l/3V+ threshold. When the input rises, the reset function loses control and the trigger input assumes command. The output then rises, and a timing cycle begins."

This is really encouraging! I have a project I am dreaming up where jarring a motion switch turns on an LED for 60sec. If I jar it again in that time, the counter resets and triggers a restart. If ~60sec expire, the LED turns off.

I have simulated this successfully in an Arduino, but I'm told that a 555 chip is a cheaper alternative to my problem.

I'm trying to breadboard out a working product before I have a circuitboard made. Can anyone confirm that if I tie the 2nd and 4th pins, that this will function like I need it to?

Thanks!

-Dave in TX
 

GopherT

Joined Nov 23, 2012
8,012
According to the 555 cookbook:

"4.7 RESTARTABLE ONE-SHOT

During normal operation, a 555 monostable circuit will ignore input triggers that may occur during the timing cycle and will produce uninterrupted output pulses. On occasion, the need may arise to bypass this mode of operation and to restart the output timing with the application of a trigger pulse during the normal timing period. A circuit that accomplishes this is shown in Fig. 4-7. In this circuit, the reset and trigger pins of the 555 are tied together and used as a single trigger input. When the input is driven to zero, it has reached the threshold level of the reset input. Since the reset function is overriding, the output is held low even though the trigger input is below its l/3V+ threshold. When the input rises, the reset function loses control and the trigger input assumes command. The output then rises, and a timing cycle begins."

This is really encouraging! I have a project I am dreaming up where jarring a motion switch turns on an LED for 60sec. If I jar it again in that time, the counter resets and triggers a restart. If ~60sec expire, the LED turns off.

I have simulated this successfully in an Arduino, but I'm told that a 555 chip is a cheaper alternative to my problem.

I'm trying to breadboard out a working product before I have a circuitboard made. Can anyone confirm that if I tie the 2nd and 4th pins, that this will function like I need it to?

Thanks!

-Dave in TX

Do not directly connect pin 2 to pin 4. Nothing will ever happen. Pin 4 needs to be at positive supply to keep the chip out of reset mode, and pin 2 most be pulled near 0V to trigger the timer.

Google monostable 555. That is the circuit you need. If 60 sec is exact, do not use 555.
 

Thread Starter

sydcomebak

Joined Dec 13, 2011
69
It doesn't have to be nearly exact. I just need the LED to stay lit for ~60 more seconds if the vibration switch is jarred again, or again and again... If I keep jarring it, it stays on. After ~60 seconds of not enough motion to trigger the switch, the LED goes off again.

Googling now...
 

GopherT

Joined Nov 23, 2012
8,012
Here, this will be slightly longer than 60 seconds. 61 to 65. Connect an LED and a 330 ohm resistor to the output (pin 3). Connect resistor to ground.

image.jpg
 

Thread Starter

sydcomebak

Joined Dec 13, 2011
69
What program are you using for the circuit diagramming?

Thanks so much for the help! I'll report back ASAP... No component stores open tonight...
 

GopherT

Joined Nov 23, 2012
8,012
What program are you using for the circuit diagramming?

Thanks so much for the help! I'll report back ASAP... No component stores open tonight...
What component stores do you have to choose from in Dallas? They are non-existent around here. Everything has to be ordered and delivered.
 

crutschow

Joined Mar 14, 2008
24,111
Here, this will be slightly longer than 60 seconds. 61 to 65. Connect an LED and a 330 ohm resistor to the output (pin 3). Connect resistor to ground.

View attachment 97251
Your circuit is missing a resistor to charge the bottom side of the 100nF capacitor back to the supply voltage after it is discharged by the switch closure.
(And that's one of the reasons I simulate all my circuits. ;))
 

GopherT

Joined Nov 23, 2012
8,012
@sydcomebak
Here is revised version...

Earlier version trigger had an error and I didn't properly account for the re-trigger of the 555.

Any small PNP transistor will work (2N3906, 2N4403, ...)

The simulator on my iPad had some limitations (lots of limitations).
image.jpg
image.jpg
 

Thread Starter

sydcomebak

Joined Dec 13, 2011
69
Breadboarding hasn't gone well...

The only Transistor I have is this: S9013.

Do you have a virtual breadboard that you could take a screenshot of? I'm tearing everything up and starting over, just in-case.

Thanks for everything so far!

-Dave
 

Thread Starter

sydcomebak

Joined Dec 13, 2011
69
First I started by jumpering the leads for 6&7 as well as 4&8:
Capture_1.PNG
Next, I re-created the circuit between Output(3) and Control (5):
Capture_2.PNG
So far so good?

DAMN, looks like this transistor is an NPN... back to the store!
 
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Thread Starter

sydcomebak

Joined Dec 13, 2011
69
I don't have the right Transistor. but I guess I can continue building with the intention of replacing it this evening...
 

GopherT

Joined Nov 23, 2012
8,012
OK, the blue thing under the resistor is the 100uF cap.

I think this is everything?

View attachment 97298
Ok, dude. You are going to have to tidy things up a bit. Use some extra jumpers to avoid the diagonal lines. Also, spread things out a bit so I don't ave to guess about what is stacked up.

Finally, you need to make sure you understand how the power rails work. The two pairs of track that run down each side are 4 separate tracks. If you want a + and - on each sued, you need a jumper to connect the two + and a jumper to make the second -
 

Thread Starter

sydcomebak

Joined Dec 13, 2011
69
I'll try to clean up the drawing, Sorry. I'm not sure how to avoid diagonals, but I'll try.

If you could draw something up, it might help me,

Got the PNP, still not working.
 

Thread Starter

sydcomebak

Joined Dec 13, 2011
69
Thanks for your patience. It's been almost 15 years since I last made a circuit.Capture_4.PNG

I hope that's closer to what you wanted.
 

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GopherT

Joined Nov 23, 2012
8,012
@sydcomebak

Ok, it looks good (assuming the PNP pins are, from left to right, emitter, base, collector).

You need to add power to the 555 timer chip. Pin8 to + and pin1 to --.
 
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