Distortion in a 555 timer's output

Semiconductor Integrated Circuits (ICs)
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1

Attached is a photo of my 555 timer output. I was thinking it should look more like a square wave. I believe this circuit may have suffered the effects of a lightning strike. I had to replace two of the filter capacitators on the power supply. Take a look, let me know what you think.

Distorted output of a 555 timer as measured on pin 3
Distorted output of a 555 timer as measured on pin 31920×2560 280 KB

2

Hello,

I am in agreement with you. I would say it should be a square wave. Did you have protection on the input for this kind of event?

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3

There’s no protection that I know of. Here’s

Internal view of a (80’s) vintage voltmeter featuring a 555 timer.
Internal view of a (80’s) vintage voltmeter featuring a 555 timer.1920×2560 457 KB

a photo of the circuit board. The 2 electrolytic capacitators are the ones that I replaced so far. They tested bad after removal from the circuit.

4

Hello Andrew,

May I assume this is on the 555 timer’s output (pin 3)? If so, you are correct, it should be a square wave.

I’ve seen this before typically when something has gone wrong with power supply connection for the 555 timer or the load it is attempting to drive. For example, if the load(s) (collection of logic chips) are not properly powered, the 555 does it’s best to provide power to the circuit often via the input protection diodes associated with the pins on the other logic chips. The result is a distortion of the square wave as different loads applied to the output of the 555 timer.

Thoughts:

  1. I’m curious if the pin 8 voltage is steady. Perhaps you could check with an oscilloscope.

  2. Have you attempted to reseat the socketed components including the large chip and the LED to main PCB connections.

  3. Please double check that you have the oscilloscope grounded the displays, common ground e.g., pin 1 of the 555 timer.

  4. Other? I hope other readers can share their observations.

Looking forward to your reply.

Best Wishes,

APDahlen

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5

Thanks for your reply. Yes the pin 8 voltage is steady at 5 volts. I have not tried reseating the chip yet. Double checked that the oscilloscope was hooked up correctly.

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So it looks like the 555 timer is working as the manufacturer wants. Ra is 4.7 kohm, Rb is 12.47 kohm, Cap is 333pf RL is 1.47kohm.
555 astable

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7

Understood, Andrew.

Any success?

Also, is there any other way we can assist?

Sincerely,

Aaron

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No, I think we’re good. So we replaced 2 filter caps and a -5 volt dc regulator. It looks like it’s working as it should. Thanks for your help. DigiKey was a big part of our success getting us the parts we needed quickly.

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Excellent!

Glad we could help.

Two follow questions:

  1. Out of curiosity, what are we looking at? It appears to be an industrial display of some type.

  2. May I edit your post ever so slightly by changing picture captions and the name so that the post has greater 'net visibility?

Thank you and have a good weekend.

Sincerely,

Aaron

10

Yes, it’s an old (80’s) vintage voltmeter. We’re using it to convert an analog 0 to 5 volt input that comes from an air flow meter. The analog signal is converted to BCD by the main chip on the board and is multiplexed and sent to a different display that converts the BCD to a reading in LBS/Day. Yes you can edit the post as you wish. Thank you.

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11

Thanks again for the post.

Minor changes to make it more discoverable for our readers.

Please let us know if we can assist in the future.

Sincerely,

Aaron

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12

Thanks for the feedback!

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