Really simple 555 chip problem

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Froskoy

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Hi,

I've wired up an astable according to this:
555 Timer Astable Multivibrator Circuit; Pulse Generator Circuit
using a 555 chip.

R1 = 10MΩ, R2=1kΩ, C1=100µF, giving f=1.2Hz.

The thing is a get a really weird trace when connecting to the oscilloscope (see attachment) - 2V/div, 0.5s/div.

What could be wrong? I'm pretty sure it's wired correctly - have checked and double checked.

Many thanks,

Froskoy.
 

Attachments

  • 555 wired oscilloscope trace.jpg
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Thanks very much for the reply. Yep, taking output from pin 3, input looks like the attachment.

I'll swap R1 and R2 now and see what happens. Thanks very much for your help so far.

de MW3YMY
 

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  • 555 input.jpg
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Last edited:
What is to be seen is normal. (time base too short. (500ms/div) )

With R1=1K, R2=10M and C1=100µF the resulting frequency is 0.001Hz.

On-time is 693,216ms and Off-time 695,974ms (duty cycle 49.9%).

For a frequency of 1.2Hz use R1=358.9K, R2=6.011M and C1=100nF, resulting in On-time 416.7ms at 50% duty cycle.

Boncuk
 
Last edited:
If you swap R1 and R2, you have a duty cycle of about 55% which should be easier to spot on the scope.

Then you will know if there are more problems to find.
 
Try this it gives an excellent square wave.

R1 = 1K
R2 = 1M variable resistor
C= .001 uf
Diodes = 1N914

voltage 5 to 14 volts

**broken link removed**
 
Last edited:
Thanks everyone for your help! The circuit now works really well.

@gary350: Thanks for that link, I'll build that circuit in the future. What book is it from? It does look really good.

@Boncuk: Why do you use a smaller capacitance for this sort of circuit. Does it improve performance, just make the resistor values easier or just convention? Thanks very much.
 
Froskoy said:
@Boncuk: Why do you use a smaller capacitance for this sort of circuit. Does it improve performance, just make the resistor values easier or just convention? Thanks very much.
Click to expand...

Small value cacpacitors are available with a tolerance of 2.5%, whereas electrolytic caps have 40%. Timing is more predictable with smaller value caps.
 

That circuit is from TTL Cookbook by Don Lancaster. They also have one calls CMOS Cookbook and others. They have some really good information oots of ICs and lots of circuits. The 556 is a dual package with two 555 times in the same IC package.
 
Just one word using the NE556 chip:

I made the experience that having two oscillator circuits made with a 556 (one at 180Hz and the other at 400Hz) the first one made an output frequency close to 180Hz and the second had an output frequency of ~2.5KHz. Even changing the timer cap to a much larger value didn't change the frequency.

May be I purchased chinese copies of the 556.

Boncuk
 
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