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Anonymous Poster

Monostable 555 Timer Problems

04/16/2007 11:57 PM

So I will start off saying that I am not a genius when it comes to this stuff quite yet, but i have done several EE classes(slow but its going) and tinker around for fun.

But for some reason this 555 Timer(i have a 556 Dual Timer) is just not working and i cant tell if its a faulty chip or what.

I am hooking up this circuit from figure-11 mid way down the page to create a monostable output for a circuit i am working on.

http://www.uoguelph.ca/~antoon/gadgets/555/555.html

Now from what i understand what should happen is when i get it all hooked up it should begin charging the capacitor to ~2/3rds of Vc as well as put the output high, then inside the timer it should flip back and thus drain the capacitor back to zero.

From what i can tell using those values it should last ~7 seconds. But this is what happens.

I setup my multimeter to watch across the capacitor, and short the trigger thus starting the charge process and turning on the output & LED. I watch it climb and climb until 2/3rds then nothing happens, the voltage SLOWLY starts to drain out of the capacitor but the output stays on. I am sure if i left it on long enough the capacitor would fully drain and turn off the circuit but we're talking atleast 5 minutes to do so.

What could i possibly be doing wrong? I have tried both sides of my 556 chip(that i got from Radioshack) and they both do the same thing. Forcing reset low does immediately drain the cap and turn off the output, but for some reason the stupid chip never does it itself.

Any ideas?

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#1

Re: Monostable 555 Timer Problems

04/17/2007 12:52 AM

I've used the 555 a lot and there's something I learned about it. When in monostable configuration:

  • the trigger input must return to V+ immediately after the being shorted to ground. All you have to do is touch it to ground and then release it. If you don't, you get the phenomena you saw.

I'm not sure if that's what you did (you didn't mention that you removed the short to the trigger input after making it) but if it is, then that might be your problem.

A way out of that is to connect a capacitor to the trigger input. The capacitor makes sure that only a short pulse enters the trigger input.

Another thing about the 555, it doesn't like to have the capacitor monitored. I guess the multimeter puts a load on the capacitor (especially the old analog meter types).

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#2
In reply to #1

Re: Monostable 555 Timer Problems

04/17/2007 1:06 AM

Thanks.

I am removing it as quickly as humanly possible, and from everything i have read this should work(hense that excersise telling you to do so).

I will try using a cap but i am not so sure that will fix the issue.

Also i have done it without monitoring the cap voltage it does the same thing :(

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#3

Re: Monostable 555 Timer Problems

04/17/2007 6:22 AM

Dear Guest,

I am really no genius either, but my experience has been that the most common problem in circuits, especially air-wired ones, is that there is a bad connection or a faulty connection someplace in the circuit.

If you've already rechecked all the connections, please forgive me.

Mike

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Anonymous Poster
#4
In reply to #3

Re: Monostable 555 Timer Problems

04/17/2007 2:52 PM

I guess i will just tear apart my circuit and rebuild it from scratch to see if that fixes any problems but from what i can tell the instant it hits 2/3rds voltage my capacitor should be discharged into the discharge pin to ground and the output should turn off.

Ill have to try afew more things i was hoping somebody here has seen the same issue :)

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#5
In reply to #4

Re: Monostable 555 Timer Problems

04/18/2007 12:11 AM

Just out of curiosity, what type of capacitor and what's its value?

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#9
In reply to #5

Re: Monostable 555 Timer Problems

04/18/2007 4:09 AM

680 nF: not sure why the author has shown it as polarised.

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#10
In reply to #4

Re: Monostable 555 Timer Problems

04/18/2007 4:14 AM

Don't tear it apart:-

Check that I've marked up this original drawing correctly: then check all the inputs are at the correct levels with your meter.

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#6

Re: Monostable 555 Timer Problems

04/18/2007 1:52 AM

I notice in the circuit you mention they show no connection for the reset pin. Best connect it and not let it float. Also, you do have the pullup resistor on the trigger pin as shown, yes? It is important that you do not let pins float or undesired results may occur.

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Associate

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#16
In reply to #6

Re: Monostable 555 Timer Problems

04/18/2007 9:38 AM

I agree. The reset (pin 4) should be tied to Vcc. Max

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#7

Re: Monostable 555 Timer Problems

04/18/2007 3:13 AM

"I setup my multimeter to watch across the capacitor ..."

I dont know wether i`ve understood that correctly, but are you hooking your multimeter directly parallel to the capacitor ? In fig. 11 the pullup resistor has 10 MegOhms, and connecting your multimeter directly will influence your timing. Assuming your multimeter has also 10M input resistance then your Cap will never load to VCC but only to VCC/2.

If your MM is not so good and has only 1M then your load voltage across the Cap will never be more than VCC/11. Timing critical RC-Components should never be measured directly, if they are high-ohmic !
Hope that helps - Uwe

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#11
In reply to #7

Re: Monostable 555 Timer Problems

04/18/2007 4:15 AM

I missed the 10meg part. Actually, meter or not, that is quite high and depending on the cap you use it might not work. I would not rely on it working with an electrolytic or even tantalum. If you use 10 megs I would go with a film cap or else use a lower resistance and a bigger electrolytic but there are limits.

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#13
In reply to #7

Re: Monostable 555 Timer Problems

04/18/2007 6:48 AM

In post #2, our guest has already stopped monitoring the voltage across the capacitor but it's still not functioning.

A couple more things to check.

  • verify your component values. If you're as color blind as I am, your 10MΩ resistor might be 100M.
  • Try using different values for R and C. Try a smaller value for R first.
  • You mentioned that you're using the dual 555 version, the 556. Are your connections correct?
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#8

Re: Monostable 555 Timer Problems

04/18/2007 3:15 AM

AS ALREADY MENTIONED IS CRITICAL NOT TO LEAVE THE "RESET" AND "TRIGGER" PINS UNCONNECTED. AFTER ALL, YOU HAVE, ALSO, TO TRY ANOTHER "555" PART. MAYBE IT WAS A FAULTY PART OR SOMETHING BAD HAPPENED DURING THE TESTING. IF YOU HAVEN'T ALREADY DONE IT, THEN DO IT...JUST TO BE SURE.

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#12
In reply to #8

Re: Monostable 555 Timer Problems

04/18/2007 6:40 AM

Hi George,

If there's nothing wrong with your keyboard, please refrain from using all caps for your post. It's a bit difficult to read and all caps are the equivalent of "shouting" in printed form.

But, you're right, never leave the reset and trigger pins unconnected.

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#24
In reply to #12

Re: Monostable 555 Timer Problems

04/19/2007 3:23 AM

Thanks for your remarks, Voulkan... I didn't mean to "shout"... (And I hope that nobody thought so)...

And yes, Lambros, I'm Greek ( as you are. Right??? )

Maybe 10M is a large value for the time constant, but also I have to say that once I had used 12M & 1,5uF as a time constant in a monostable application of 556 (in order to extract a large pulse) and it worked fine... So I don't think that this is realy a problem...

What news about this circuit???...

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Anonymous Poster
#14
In reply to #8

Re: Monostable 555 Timer Problems

04/18/2007 7:52 AM

George didn't mean to shout out loud.

It's just the greek temper.

Swsta Giorgo?

As for the 555 circuit it's a good idea to check for bad connections or any faulty part.

If the circuit is implemented on a breadboard it's quite possible that the brdbord wire

lanes are not getting the job done anymore. Try to move the circuit to a new breadboard.

Lambros

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#15

Re: Monostable 555 Timer Problems

04/18/2007 9:16 AM

I've had similar problems with the old non-CMOS parts of the 555 series.

Inside there is a flip flop to change between discharge and charge timing, its a common problem to have the flip flop (can't remember whether its a JK or RS type) latch into an indeterminate state with both outputs (Q and /Q) at the same logic level.

Sounds like this is what's happening with your circuit.

John.

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#17

Re: Monostable 555 Timer Problems

04/18/2007 11:16 AM

Try using 680K resistor with 100uF TANTALIUM capacitor.

Leakage input current in your 555 maybe high for 10M resistor.

I make this circuit in proto-board and work fine with these values.

Not forget terminal #4 must be high. Connect to Vcc

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#18
In reply to #17

Re: Monostable 555 Timer Problems

04/18/2007 2:26 PM

Good response.

Several posters have mentioned related things, but no one has directly said to make sure the capacitor is a low-leakage type. 10MΩ is very high, so the charging current may be entirely lost to capacitor leakage once the voltage reaches some value (charging current is constant, but leakage current is proportional to the capacitor voltage). If you don't have a very low leakage capacitor, then increase the size of the capacitor and decrease the value of the resistance proportionately, as Langdom indicated.

Dick

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#19

Re: Monostable 555 Timer Problems

04/18/2007 2:41 PM

Hi

If you are using Al Electrolytic cap for time base timing resistor shuold have low value best choice is bellow 1M.

Time period of 555 in monostable mode is 1.1RC seconds.Use 62K resistor and 100uF electrolytic capacitor that will provide pulse of 7 Sec.

If you connect multimeter accross capacitor with the said circuit, capacitor is going to discharge through multimeter called loading effect.Timing resistor of 10M and multimeter input resistor forms voltage divider and capacitor will discharge through multimeter.

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#20

Re: Monostable 555 Timer Problems

04/18/2007 4:39 PM

If you really want to do it right dump the 555/556. These chips are 30+ years old and they never were that great. Get an 8 pin Microchip part, same size or you can even get one in an SOT-23 package. No external components except a bypass cap on the power pin and you can program it for any delay and to behave any way you want. It may cost you a little more to get a programmer to get started but the development software is free and it all works great. Check out www.microchip.com

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#22
In reply to #20

Re: Monostable 555 Timer Problems

04/18/2007 11:44 PM

Well, each to his own.

I've used the 555 forever and I love them. I've even used them as RS flip-flops when I didn't have one handy. And the mere fact that they're 30 years old and still produced in large quantities is testament to its popularity.

About the only thing I don't like about them is the arrangement of the pins. Other than that, it's one of the most useful ICs ever produced.

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#23
In reply to #22

Re: Monostable 555 Timer Problems

04/19/2007 1:03 AM

True. I'm not averse to old parts I just prefer more functionality. Virtually anything that can be done with a 555/556 I can do with a 74C14/40106 hex Schmidt trigger inverter including RS flip flops (three in one package), oscillators (six in one package), mono's, retriggerable mono's, and logic functions. Quit a useful part. And it draws virtually no power when idle but I know they have some improved versions of the 555, which in its original form used a good bit of power (relatively speaking - I'm a let's run it off a battery for four years kind of designer).

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#21

Re: Monostable 555 Timer Problems

04/18/2007 9:46 PM

I found a Clue!

"556 chip(that i got from Radioshack"

"You got Q's [Why their second source chip DNF], they got answers..."

Lucent used a chipset around their DSP's that maintained gorgeous, crisp square waves at long tc's and 5, 20 and 60mhz

QED

Try Mouser or Digi Key -rgo

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#25

Re: Monostable 555 Timer Problems

04/19/2007 4:15 AM

I've just noticed that there is something missing from the otherwise wonderful tutorial website you're using (I haven't read it all so I might have missed it somewhere).

There is no power supply de-coupling.

I don't know what you're using as a supply but normally you should add low and high frequency decoupling (say a 10 µF electrolytic and a 100 nF multilayer ceramic cap) where the supply comes onto the board, and, high frequency decoupling (say a 100 nF multilayer ceramic cap) close to the power supply pins of the IC.

I doubt if this is the problem, and, of course you may have already added your own decoupling. But it is always good practise.

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