Should the frequency of the square wave and the sine wave be the same?

[Joshbessant2]

Hi all, I'm current doing an electrical engineering course and am currently having to use an oscilloscope in order to generate a square and sine wave from a sine wave generator with a 555 timer chip. When calculating the theoretical frequencies they are both very similar at 7Hz. However when putting this into practice practically, I'm receiving a value of around 20Hz on the square wave and the theoretical 7Hz on the sine wave. Should they be the same? am I going wrong on the calculations or on the practical with the oscilloscope? Thanks in advance.

 

[DPG]

I think this is because of the severe distortion of the square wave, but I'm not 100% certain.

 

[Joshbessant2]

I think this is because of the severe distortion of the square wave, but I'm not 100% certain.

By distortion do the mean the high and low peaks and troughs? I believe this is caused by the capacitors and is a match to the example image we have on the learning resource so I believe its meant to be like that. Thanks for the reply

 

[Lucien Nunes]

I'm receiving a value of around 20Hz on the square wave

You state Hz, but I think you mean kHz, as the displays show.

It is hard to see in the pics but the control voltage decoupling capacitor on pin 5 does not seem to be correctly connected to 0V. There's a green link wire to the top blue busbar of the prototyping board but this is not connected to the bottom one so the capacitor is not doing anything. Not decoupling the control pin can cause the 555 to misbehave.

The circuit shown drives the timing network R, C from the output pin. From memory its frequency should be 0.722/RC, which with the values shown on the diagram should be 32kHz. What are the actual values and formula you are using?

One general caution with the 555: The non-CMOS variety is extremely prone to multiple output transitions if the power rails are not effectively bypassed. Basically there is a minor design flaw that results in a very momentary short-circuit across the rails as the thing switches, which can upset the internal comparators. Consider putting a 0.1uF ceramic from +Vcc to 0V near to the IC, for most stable operation. They will work without, they just sometimes get a bit twitchy especially when the output is loaded.

Because the input to the pocket scope is just a simple lead without a divider probe, it can add significant loading to the output of the circuit under test. Therefore, if something is on the brink of malfunction or is just marginally stable, moving the scope connection from say the output pin 3 itself, to downstream of the lowpass filter, can change the operating conditions of the IC and therefore the frequency.

 

[Joshbessant2]

You state Hz, but I think you mean kHz, as the displays show.

It is hard to see in the pics but the control voltage decoupling capacitor on pin 5 does not seem to be correctly connected to 0V. There's a green link wire to the top blue busbar of the prototyping board but this is not connected to the bottom one so the capacitor is not doing anything. Not decoupling the control pin can cause the 555 to misbehave.

The circuit shown drives the timing network R, C from the output pin. From memory its frequency should be 0.722/RC, which with the values shown on the diagram should be 32kHz. What are the actual values and formula you are using?

One general caution with the 555: The non-CMOS variety is extremely prone to multiple output transitions if the power rails are not effectively bypassed. Basically there is a minor design flaw that results in a very momentary short-circuit across the rails as the thing switches, which can upset the internal comparators. Consider putting a 0.1uF ceramic from +Vcc to 0V near to the IC, for most stable operation. They will work without, they just sometimes get a bit twitchy especially when the output is loaded.

Because the input to the pocket scope is just a simple lead without a divider probe, it can add significant loading to the output of the circuit under test. Therefore, if something is on the brink of malfunction or is just marginally stable, moving the scope connection from say the output pin 3 itself, to downstream of the lowpass filter, can change the operating conditions of the IC and therefore the frequency.

You sir are a legend! Absolutely spot on, I've just gone back to take a look after seeing you're comment and you're correct. The wire from that capacitor was put on earth but as you say it wasn't actually earthed. Thank you so much. Everything else adds up now. Very much appreciated!

 

[DPG]

You sir are a legend! Absolutely spot on, I've just gone back to take a look after seeing you're comment and you're correct. The wire from that capacitor was put on earth but as you say it wasn't actually earthed. Thank you so much. Everything else adds up now. Very much appreciated!

Lucien has proved many times on here that his knowledge is exceptional. He also appears to have good eyesight!

 

[Joshbessant2]

Lucien has proved many times on here that his knowledge is exceptional. He also appears to have good eyesight!

Thank you both! Yeah I agree, I must need to get my glasses prescription double checked, haha. Its my first time on this forum but I thinks its safe to say I'll definitely be sticking around! Thanks again

 

[DPG]

Thank you both! Yeah I agree, I must need to get my glasses prescription double checked, haha. Its my first time on this forum but I thinks its safe to say I'll definitely be sticking around! Thanks again

Yeah stick around, lots of good knowledge on here.

 

[Lucien Nunes]

it wasn't actually earthed

Not technically earth, BTW. Call it 0V, DC negative, signal common, even 'ground' when used to mean local common terminal. But one should reserve the term 'earth' for something that is directly connected to the mass of earth. Many pieces of equipment do have the 0V rail earthed e.g via the mains power cable, but in this case the circuit is sitting isolated on the table with no connections to the outside world. You could actually earth any point in the circuit, e.g. DC +ve. If you earthed pin 3, the whole thing including the battery and scope would swing up and down at the output frequency with respect to earth.

He also appears to have good eyesight

I have spent 40 years debugging circuits at the prototyping stage, sometimes with many dozens of ICs. One gets drawn to particular errors that we all make. Floating inputs, outputs tied together, missing power connections, missing pullups, things shifted one hole along or in mirror image. Sometimes before you can even articulate what is wrong, you get a sense of 'there's an error around there somewhere' or 'that looks funny but it won't be that.'

I remember one night at uni, I had spent all afternoon and evening programming, and I couldn't debug a math routine. My good friend and expert programmer Richard popped by the lab, scanned the code and went straight to it - I had declared an array as a constant and later tried to use it as a variable. I realised later that not only had he come to it fresh, he automatically formed a clearer mental image of the data structure than I did myself and had a much better insight into its ramifications and in this case discrepancies.

 

[DPG]

Not technically earth, BTW. Call it 0V, DC negative, signal common, even 'ground' when used to mean local common terminal. But one should reserve the term 'earth' for something that is directly connected to the mass of earth. Many pieces of equipment do have the 0V rail earthed e.g via the mains power cable, but in this case the circuit is sitting isolated on the table with no connections to the outside world. You could actually earth any point in the circuit, e.g. DC +ve. If you earthed pin 3, the whole thing including the battery and scope would swing up and down at the output frequency with respect to earth.



I have spent 40 years debugging circuits at the prototyping stage, sometimes with many dozens of ICs. One gets drawn to particular errors that we all make. Floating inputs, outputs tied together, missing power connections, missing pullups, things shifted one hole along or in mirror image. Sometimes before you can even articulate what is wrong, you get a sense of 'there's an error around there somewhere' or 'that looks funny but it won't be that.'

I remember one night at uni, I had spent all afternoon and evening programming, and I couldn't debug a math routine. My good friend and expert programmer Richard popped by the lab, scanned the code and went straight to it - I had declared an array as a constant and later tried to use it as a variable. I realised later that not only had he come to it fresh, he automatically formed a clearer mental image of the data structure than I did myself and had a much better insight into its ramifications and in this case discrepancies.

Always good to have a fresh set of eyes on things. I had a rather embarrassing moment a few weeks ago where I was struggling. I sent a vbs file to someone none technical and didn't prompt them with my pre conceptions. They found the problem even though they didn't understand it. I owe them a beer

 

[xmetal]

I have spent 40 years debugging circuits at the prototyping stage, sometimes with many dozens of ICs. One gets drawn to particular errors that we all make. Floating inputs, outputs tied together, missing power connections, missing pullups, things shifted one hole along or in mirror image. Sometimes before you can even articulate what is wrong, you get a sense of 'there's an error around there somewhere' or 'that looks funny but it won't be that.'

I remember one night at uni, I had spent all afternoon and evening programming, and I couldn't debug a math routine. My good friend and expert programmer Richard popped by the lab, scanned the code and went straight to it - I had declared an array as a constant and later tried to use it as a variable. I realised later that not only had he come to it fresh, he automatically formed a clearer mental image of the data structure than I did myself and had a much better insight into its ramifications and in this case discrepancies.

Sometimes I find if I get stuck on a fault, is to sleep on it. Many a time the next morning, I go straight to it and find the problem.