Two 555 timers, 2 different frequencies one and power supply

Thread Starter

monkeyhead

Joined Mar 5, 2007
45
Hi there,
I'm trying to make a circuit using two 555 timers set at two different frequencies. However I've set the two timers at different frequencies and the seem to conflict with each other. For example unless both are set at the same frequencies they work fine. If i alter one value of the capacitor for example then it'll effect the frequency for the other timer!

Is it even possible to use the same supply for somthing like this?

Many thanks
 

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Ron H

Joined Apr 14, 2005
7,014
555s put big current spikes on the power rail. The wiring impedance and/or the power supply impedance converts this current spike to a voltage spike, which tends to cause the oscillators to "injection lock" each other. This is actually a problem whenever you have two oscillators in close proximity if they are nearly the same frequency. With LC oscillators, electromagnetic radiation can cause injection locking, and sometimes extraordinary shielding, grounding, and supply decoupling measures are required to prevent it.
In your circuit, I would try a 100uF cap from +12V to ground, and 0.1uF caps on each 555 from pin 8 to pin 1, as close to each IC as possible, and with cap leads as short as possible. A ground plane would also help. If that doesn't work, you might have to add a small-value resistor (10 ohms or so) between +12V and pin 8 on each IC.
 

mrmeval

Joined Jun 30, 2006
833
I've seen the caps soldered to the IC pins in production units.

An *opps* later means some underpaid worker has to solder them on. Employed people yayz?
:)
 

GS3

Joined Sep 21, 2007
408
Some years ago I designed a simple timer for photographic enlargers and I ran into a strange problem. The output of the 555 monostable timer governed a relay directly and everything went fine except sometimes you'd get 2X or 3X the expected time. After some investigation I found out that when the 555 cut off supply to the relay this somehow triggered the 555 again and the process started over again. The problem was resolved putting a diode in series with the output and a capacitor to ground. See attached schematic.
 

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Thread Starter

monkeyhead

Joined Mar 5, 2007
45
Thanks for the feedback, much appreicated!

Basically what I'm trying to do is adapt a "heads or tails" circuit, using a 555 timer ,which sends its pulses to a 7474, which in turn acts as a divide by two counter. What ever the result is, either one led will light, or the other.

The way I want to adapt this circuit is so that say for example that the "heads" light is lit then a signal will be sent to an additonal 555 timer at a set freqency to output a tone for a few seconds. And if tails is lit, then a signal to be sent to another 555 timer at another frequency to output a tone, again for a few seconds.

In theory this should be possible right? I was told I might hit into some problems though..

Anyhow would I be correct in saying that I should use a monostable 555 timer for outputting the tones for only a few seconds? And connecting the trigger inputs from the 555s to the Q and G' of the 7474 right?

Many thanks
Matt

See attached for the circuit im adapting so far.
 

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bloguetronica

Joined Apr 27, 2007
1,372
Probably this is caused by a ground loop. A decoupling capacitor should solve the problem. Perhaps a 10uF cap should do. If the 555 timers are to switch substantial loads, you should use a 0.1uF cap near each timer.
 

GS3

Joined Sep 21, 2007
408
Probably this is caused by a ground loop.
Probably not. While "ground loop" is one of those terms loosely thrown around it actually has a precise meaning: http://en.wikipedia.org/wiki/Ground_loop_(electricity) I doubt that this is the cause of the problem in this case.
A decoupling capacitor should solve the problem. Perhaps a 10uF cap should do. If the 555 timers are to switch substantial loads, you should use a 0.1uF cap near each timer.
Well, that's like aspirin, it never hurts and it sometimes even helps. BTW, I assume you mean a decoupling capacitor connected to the power and ground terminals. It is always important to have a smooth supply voltage.

BUT

as I have shown in my previous post, transients can and do enter by way of other pins and all the power supply decoupling in the world is not going to help with that. Sometimes it pays to find out what is actually causing the problem and then solve it directly rather than just assuming common causes and throwing capacitors and loose terms at them.
 

Ron H

Joined Apr 14, 2005
7,014
Probably not. While "ground loop" is one of those terms loosely thrown around it actually has a precise meaning: http://en.wikipedia.org/wiki/Ground_loop_(electricity) I doubt that this is the cause of the problem in this case. Well, that's like aspirin, it never hurts and it sometimes even helps. BTW, I assume you mean a decoupling capacitor connected to the power and ground terminals. It is always important to have a smooth supply voltage.

BUT

as I have shown in my previous post, transients can and do enter by way of other pins and all the power supply decoupling in the world is not going to help with that. Sometimes it pays to find out what is actually causing the problem and then solve it directly rather than just assuming common causes and throwing capacitors and loose terms at them.
Yes, but if you have NO supply decoupling, it's the most likely problem, especially since he apparently is not driving any inductive loads, which I believe was the cause of your problem. Remember Occam's Razor. ;)
 

GS3

Joined Sep 21, 2007
408
Yes, but if you have NO supply decoupling, it's the most likely problem, especially since he apparently is not driving any inductive loads, which I believe was the cause of your problem. Remember Occam's Razor. ;)
I agree with you but decoupling the supply (which, as I said, should always be done) has little to do with a ground loop. They are two totally different things. That was the point I was trying to make.
 

Ron H

Joined Apr 14, 2005
7,014
Thanks for the feedback, much appreicated!

Basically what I'm trying to do is adapt a "heads or tails" circuit, using a 555 timer ,which sends its pulses to a 7474, which in turn acts as a divide by two counter. What ever the result is, either one led will light, or the other.

The way I want to adapt this circuit is so that say for example that the "heads" light is lit then a signal will be sent to an additonal 555 timer at a set freqency to output a tone for a few seconds. And if tails is lit, then a signal to be sent to another 555 timer at another frequency to output a tone, again for a few seconds.

In theory this should be possible right? I was told I might hit into some problems though..

Anyhow would I be correct in saying that I should use a monostable 555 timer for outputting the tones for only a few seconds? And connecting the trigger inputs from the 555s to the Q and G' of the 7474 right?
I'm not sure exactly what you have in mind. You need to draw a schematic, and we can critique it.
Below is another way to do it. The monostable at lower left triggers when you release the pushbutton. It allows the astable at lower right to run until the monostable times out. I included 2 pots to set the two tones. The pots do not interact, but since the duty cycle is necessarily low so that the pots will have plenty of range, I ran the output of the astable through another toggle flip-flop, so the output duty cycle will be 50%, independent of frequency.
Note that this only uses one output transducer (speaker), and you may need a (simple) power amplifier, as the 7474 doesn't have nearly as much output drive capability as a 555. If you were intending to use a separate speaker for each tone, you will need two astables.
Note that I have not tested this, or even simulated it. It could have errors.

Many thanks
Matt

See attached for the circuit im adapting so far.
 

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bloguetronica

Joined Apr 27, 2007
1,372
Probably not. While "ground loop" is one of those terms loosely thrown around it actually has a precise meaning: http://en.wikipedia.org/wiki/Ground_loop_(electricity) I doubt that
Then again, probably it is. Faulty grounds can increase the influence of spikes. Ground loop is a very broad term. Resistance in the power supply conductors may have the same effect of a ground loop, and indeed, those factors are correlated.

Don't forget that a ground loop is not only formed by the ground conductor, it is also formed by a power or signal conductor, or else it wouldn't be called a loop.
 

GS3

Joined Sep 21, 2007
408
Faulty grounds can increase the influence of spikes.
And bad wine can cause headaches but neither of those things is a ground loop.
Ground loop is a very broad term.
No it's not. You are using it in a broad unwarranted way. Do you know the meaning of "loop"? How can there be a "ground loop" if there is no "loop" to begin with, ground or otherwise?
Resistance in the power supply conductors may have the same effect of a ground loop, and indeed, those factors are correlated.
Oh man. :rolleyes: Accepting for a moment just for the sake of argument that the assertion is true, having the same effect as X is not the same as being X.
Don't forget that a ground loop is not only formed by the ground conductor, it is also formed by a power or signal conductor, or else it wouldn't be called a loop.
Now you are contradicting what you said before and saying what I just said. OK, we are in agreement now.
 

bloguetronica

Joined Apr 27, 2007
1,372
And bad wine can cause headaches but neither of those things is a ground loop.
Indeed. And good wine too!

No it's not. You are using it in a broad unwarranted way. Do you know the meaning of "loop"? How can there be a "ground loop" if there is no "loop" to begin with, ground or otherwise?
Ground has resistance, as any other conductor. Think in broader terms.

Oh man. :rolleyes: Accepting for a moment just for the sake of argument that the assertion is true, having the same effect as X is not the same as being X.
Correlation is an implication, not an assertion. Assertion is the equivalent of equality, and I didn't prove an equality.

Now you are contradicting what you said before and saying what I just said. OK, we are in agreement now.
No, a faulty ground is a ground where the ties are not in the same AND the ground path between them has too much resistance for the current biasing it. So, where is the contradiction? The caps solve the problem, because they do ground decoupling, but I bet my sweet potatoes that just using a big cap with the grounds tied together will solve the problem.

Concluding: Resistance IS the cause of ground loops.
 

thingmaker3

Joined May 16, 2005
5,084
Your question was welcome Monkeyhead, as will be any you have in the future.

You did not touch anything off. We debate among each other from time to time. There is no animosity between GS3 and Cumesoftware - only a passion for our shared discipline. And perhaps a divergence of viewpoint. We all can learn from good, polite debates.
 

bloguetronica

Joined Apr 27, 2007
1,372
Your question was welcome Monkeyhead, as will be any you have in the future.

You did not touch anything off. We debate among each other from time to time. There is no animosity between GS3 and Cumesoftware - only a passion for our shared discipline. And perhaps a divergence of viewpoint. We all can learn from good, polite debates.
Indeed. That is true. We are discussing, not fighting.

...
I'm now wondering whether i should have posted this thread I've appeared to triggered something off!
You didn't triggered anything. Any doubt, feel free to ask.
 
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