Building a 555 Astable Oscillator: Achieving 50KHz Output

[helofrind]

So I built a 555 astable oscillator. I wanted to get 50 KHz on my output, which I have achieved, but the values of my components do not match the formula I have calculated. I have:
R1 = 98 ohms
R2 = 67 ohms
Cap = 100nF
So I have:
1.44/(98 + 2 67) 100nF
Which should be 62KHz. Also can anyone explain why I am getting 1volt on output with 5 volts in. Thanks in advance for any help.

 

[phyzguy]

You need to include the circuit diagram so we know which components you are talking about.

 

[Merlin3189]

Agree with Phyzguy. More info.

Reasons could be
for frequency error- component tolerances: 24% is a lot, but maybe possible. How are you measuring f?
for voltage output- output high is typically 1 to 1.5 V below supply
- mark to space ratio of about 2.5:1 could reduce the average output to about 50% of supply, ie. 2.5V
- but I can't get it down to 1V, so: how are you measuring it? what is your power supply? Could there be errors there?

 

[Svein]

So I built a 555 astable oscillator. I wanted to get 50 KHz on my output, which I have achieved, but the values of my components do not match the formula I have calculated. I have:
R1 = 98 ohms
R2 = 67 ohms
Cap = 100nF
So I have:
1.44/(98 + 2 67) 100nF
Which should be 62KHz. Also can anyone explain why I am getting 1volt on output with 5 volts in. Thanks in advance for any help.

I need to see the schematic, but I think one of the problems is that the values of R1 and R2 are too low. I would scale them both up by a factor of 10 and reduce the capacitance by the same amount. See if that helps.

 

[helofrind]

Thank you everyone for responding. Here is a snapshot off of electrodroid, its the same circuit. At the bottom it does say to use resistors above 1K for both R1 and R2, so I will scale up the resistance I'm using a 2072 rigol o-scope to measure, and a bench power supply. I haven't generated a frequency this high, so my first thought was a frequency that high things probly get a little unstable.

 

[Eddie Sines]

So I built a 555 astable oscillator. I wanted to get 50 KHz on my output, which I have achieved, but the values of my components do not match the formula I have calculated. I have:
R1 = 98 ohms
R2 = 67 ohms
Cap = 100nF
So I have:
1.44/(98 + 2 67) 100nF
Which should be 62KHz. Also can anyone explain why I am getting 1volt on output with 5 volts in. Thanks in advance for any help.

Do you have a pull up resistor on the output?

 

[helofrind]

No I don't have a pull up resistor, but ill try it. I wouldn't know what value to use though

 

[Eddie Sines]

Try a 10K.. its safe... tie it to +5VDC to output pin... then connect scope to output pin... let me know the result. The 10K will not hurt the 555...

 

[Svein]

No I don't have a pull up resistor, but ill try it. I wouldn't know what value to use though

Try 3.3kΩ.

 

[Eddie Sines]

Sure that will work... even a 1K will do the trick..

 

[Svein]

Try a 10K.. its safe... tie it to +5VDC to output pin... then connect scope to output pin... let me know the result. The 10K will not hurt the 555...

It just goes to show - a pullup resistor is not a critical component. My 3.3k is a relic from the TTL days.

 

[Eddie Sines]

It just goes to show - a pullup resistor is not a critical component. My 3.3k is a relic from the TTL days.

I have a book on the 555... let me see if I can find it...
the other thing is have you simulated it.. sometimes that is a lot of fun... and it saves time

One last thing is are you sure the parts are connected as like in the schematic? Often you will find that something is not connected... it may look connected, but it not.
I did breadboarding for many years... always cross-check

 

[Eddie Sines]

Thank you everyone for responding. Here is a snapshot off of electrodroid, its the same circuit. At the bottom it does say to use resistors above 1K for both R1 and R2, so I will scale up the resistance I'm using a 2072 rigol o-scope to measure, and a bench power supply. I haven't generated a frequency this high, so my first thought was a frequency that high things probly get a little unstable.

Helofrind,
The 555 chip will do a lot more that 65 KHz... all you need to do is adjust the RC...I have used this chip since the 70's... It a great chip.

 

[helofrind]

I have reconfigured the circuit. It looks a lot better mathmaticaly. But just for fun I am going to try a pull up resistor with a 10K pot from source to output. I have checked the connections multiple times and even reset the scope. Ill have to try these new things a little later when I get home. Ill keep everyone posted. New configuration in attachment.

 

[jim hardy]

1. You haven't said whether you are using the CMOS or bipolar version.

2. Your schematic shows 98 ohms for R1.
That means when your discharge pin 7 is "low", it must sink 50 milliamps through R1 plus some more from R2.
The IC should do that, but do not think pin 7 will go down to zero volts while sinking that much current.
So - if pin 7 is at a fraction of a volt instead of zero it'll slow down your discharge, which affects timing..

Take look at the spec sheets.
Look here for cmos version http://www.ti.com/lit/ds/symlink/tlc555-q1.pdf
page 5 of 15 , specifications with 5 volt supply
parameter "Discharge-switch on-state voltage" about middle of page
it's allowed half a volt at just 10 milliamps.
Next, page 6 shows that with 15 volt supply it's allowed 1.7 volts at 100 milliamps
see page 8 fig 2, chart of "Discharge switch on state resistance" and observe it's probably around 10 ohms at 50 ma. That's a half volt.Look here for bipolar version http://www.ti.com/lit/ds/symlink/ne555.pdf
page 5 0f 36, parameter "DISCH switch on state voltage" about middle of page
it's allowed ⁴/₁₀ volt at just 8 milliamps.
Now look at page 7 figure 1 , the curve of output volts versus current with 5V supply . Assuming the output and discharge transistors are similar, at 50 milliamps you might see more than a whole volt.

I think that's the source of your discrepancy.
Try making your discharge pin currents smaller by using larger R1 and R2.
Then the device will do a better job of discharging C1 so you should find yourself operating closer to the formulas.

 

[jim hardy]

i see svein already suggested that.

Those spec sheets should let you pick values that'll work better.

Let us know how it works out ?

Late entry - i see you came to same conclusion while i was typing... 10:21 pm..

always glad to see 'hands-on' experimenters. We learn best by doing.old jim

 

[Eddie Sines]

Thank you everyone for responding. Here is a snapshot off of electrodroid, its the same circuit. At the bottom it does say to use resistors above 1K for both R1 and R2, so I will scale up the resistance I'm using a 2072 rigol o-scope to measure, and a bench power supply. I haven't generated a frequency this high, so my first thought was a frequency that high things probly get a little unstable.

Helofrind,

See attached... I didn't take a lot of time... but this circuit does simulate. I can send you the simulation model if you provide an email. You can download a Free Spice simulation tool via web go to http://www.linear.com/designtools/software/ down load LTSPICE... now you can simulate before you build... good luck.. -e

 

[Eddie Sines]

1. You haven't said whether you are using the CMOS or bipolar version.

2. Your schematic shows 98 ohms for R1.
That means when your discharge pin 7 is "low", it must sink 50 milliamps through R1 plus some more from R2.
The IC should do that, but do not think pin 7 will go down to zero volts while sinking that much current.
So - if pin 7 is at a fraction of a volt instead of zero it'll slow down your discharge, which affects timing..

Take look at the spec sheets.
Look here for cmos version http://www.ti.com/lit/ds/symlink/tlc555-q1.pdf
page 5 of 15 , specifications with 5 volt supply
parameter "Discharge-switch on-state voltage" about middle of page
it's allowed half a volt at just 10 milliamps.
Next, page 6 shows that with 15 volt supply it's allowed 1.7 volts at 100 milliamps
see page 8 fig 2, chart of "Discharge switch on state resistance" and observe it's probably around 10 ohms at 50 ma. That's a half volt.Look here for bipolar version http://www.ti.com/lit/ds/symlink/ne555.pdf
page 5 0f 36, parameter "DISCH switch on state voltage" about middle of page
it's allowed ⁴/₁₀ volt at just 8 milliamps.
Now look at page 7 figure 1 , the curve of output volts versus current with 5V supply . Assuming the output and discharge transistors are similar, at 50 milliamps you might see more than a whole volt.

I think that's the source of your discrepancy.
Try making your discharge pin currents smaller by using larger R1 and R2.
Then the device will do a better job of discharging C1 so you should find yourself operating closer to the formulas.

Hi Jim,
New member here.. just stared today... looks like a nice place to spend some time. later -e

 

[helofrind]

Jim Hardy that was a great and helpful explanation, thank you. I am using a CMOS. so I increased the resistance and the components
better match the formula now. also the pull up resistor had no affect. I did however ad a capacitor from source to ground to filter noise,
and that help clean up the square wave (not shown in the picture).
R1 is now 1488
R2 is now 4754
C1 is 2.2 nF
I said before I was getting 1V, but actually its more like 500 mV. can I amplify this output?
(below I attatched the output signal)

 

[Svein]

I said before I was getting 1V, but actually its more like 500 mV. can I amplify this output?
(below I attatched the output signal)

It looks to me as if you are using a 10x probe - which means multiply the amplitude by10.

 

[Eddie Sines]

Jim Hardy that was a great and helpful explanation, thank you. I am using a CMOS. so I increased the resistance and the components
better match the formula now. also the pull up resistor had no affect. I did however ad a capacitor from source to ground to filter noise,
and that help clean up the square wave (not shown in the picture).
R1 is now 1488
R2 is now 4754
C1 is 2.2 nF
I said before I was getting 1V, but actually its more like 500 mV. can I amplify this output?
(below I attatched the output signal)

Helofrind,
The circuit works...sim's nice, just a value change and you're done.
-e

 

[Merlin3189]

Just a minor issue.
There seems to be ringing on the rising edge of the output: I wonder how this arises? I've not noticed it on 555 output (bipolar) before and with no load except a 10k pullup, where is the inductance?

The 10x probe seems to explain the low output. Presumably the 612mV p-p shown is acknowledging the overshoot.

 

[helofrind]

yes, I forgot I reset the o-scope to its default settings, the 10x probe was the issue. haha. thanks.
to get I have rid of the ringing, I added a capaccitor filtor to the source input.
attached the new signal output

 

[Eddie Sines]

Helofrind,
The circuit works...sim's nice, just a value change and you're done.
-e

Helofrind,

Your scope probe is slightly out of calibration. Most scope's have a calibration source on the front, a signal... connect the probe to this source and with a small jewelers screwdriver turn the small tunable cap on the BNC connector until the signal has a complete flat top. That will take out the little distortions in the signal. Good luck..-e

 

[jim hardy]

I did however ad a capacitor from source to ground to filter noise,

Always, with 555's. They take a big gulp of current at switch time.

Keep on experimenting. Learning to interpret spec sheets will become part of your skillset. I try to go straight to manufacturer's websites, those "middleman" sites are really annoying and i hate to give them revenue by clicking. Graft and corruption, mumble grumble... they make you go through a gauntlet of advertisements to reach a half legible copy.

 

[tkjtkj]

So I built a 555 astable oscillator. I wanted to get 50 KHz on my output, which I have achieved, but the values of my components do not match the formula I have calculated. I have:
R1 = 98 ohms
R2 = 67 ohms
Cap = 100nF
So I have:
1.44/(98 + 2 67) 100nF
Which should be 62KHz. Also can anyone explain why I am getting 1volt on output with 5 volts in. Thanks in advance for any help.

Rarely do components actually possesses the value the engineer wishes it to have ..
So, consider tolerances, including wire/connection/temperature/position/proximities/etc .. If a DM says 5.14v , its nearly always processed as being 5.0

 

[tkjtkj]

Helofrind,

Your scope probe is slightly out of calibration. Most scope's have a calibration source on the front, a signal... connect the probe to this source and with a small jewelers screwdriver turn the small tunable cap on the BNC connector until the signal has a complete flat top. That will take out the little distortions in the signal. Good luck..-e

But what is the 'scope itself using for calibration? On how warm a day? with what effect on it's 'truth component' due to the scope's calibrating source being affected by temp fluctuations? ... Perfection is a nice thought ...

 

[Eddie Sines]

But what is the 'scope itself using for calibration? On how warm a day? with what effect on it's 'truth component' due to the scope's calibrating source being affected by temp fluctuations? ... Perfection is a nice thought ...

Send me the model of your Osc... and I will will up the details. I was referring to the small down-up turn of the front of the pulse. This is the cap in the BNC connection area. A slight adjustment will flatten this out... as for the volts per division, I do not know this... you can look it up.

 

[tkjtkj]

Send me the model of your Osc... and I will will up the details. I was referring to the small down-up turn of the front of the pulse. This is the cap in the BNC connection area. A slight adjustment will flatten this out... as for the volts per division, I do not know this... you can look it up.

Actually, my comment was only a general comment, on the fact that 'real values' are not necessarily what are defined by such things as 'color codes' or 'standard values' of components .. I am not concerned here with particular devices, etc, only with the issue that real values can be significantly different from what the 'charts' state...
My Owon scope serves me well .. thanks ..

 

[Eddie Sines]

Just trying to be helpful..

 

[Mark Harder]

So I built a 555 astable oscillator. I wanted to get 50 KHz on my output, which I have achieved, but the values of my components do not match the formula I have calculated. I have:
R1 = 98 ohms
R2 = 67 ohms
Cap = 100nF
So I have:
1.44/(98 + 2 67) 100nF
Which should be 62KHz. Also can anyone explain why I am getting 1volt on output with 5 volts in. Thanks in advance for any help.

Have you tested the 555 itself? For instance, if you swap your current chip with another keeping all other components, does it behave exactly the same? The one volt output from 5V in seems strange. BTW, without having the 555 specs in front of me, I wonder if 5V is within the specified Vpower of the chip. Is it at the lower limit of specified voltage? Did you measure the Vpower IN CIRCUIT, or did you assume that was 5V because you used a 5V supply? Are you using the LM555, a CMOS or multiple 555 on a chip version? I'm just casting about here, since I don't know more about your circuit.

 

[Mark Harder]

Sorry about my redundant post. Somehow, I thought there was only 1 reply...