Resistor turns waveform ugly, why?

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antknee

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I have a 555 timer, the output signal is fine and a typical square wave. It goes through an output resistor and suddenly turns ugly, why is this? I'd prefer it to stay the same or atleast similar. Thanks.



 

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hi ak,
Post a sketch, so we can see this dastardly resistor and its associated circuit.


EDIT:
has anyone else noticed that typed words in posts seem to disappear.???
 
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It looks like what ever the 555 is feeding is oscillating. As Eric has stated, post a diagram.
 
Hi Eric,

Resistors have the devil in them

I have made up the board in the post below. It works in that I get a reasonable output from the transformer. I'm trying to understand it a little better, to see what I need and what I don't need. The resistor in question is R5, I don't think I need it at all, I might just short it out.

This is the schematic, IC3 is a dual opamp, the tca0372, IC4 is the transformer.

https://www.electro-tech-online.com...at/108553-my-first-eagle-schematic-board.html

Thanks.

 

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The whole diagram seems pretty bizarre?, what are R10, R6 and R4 supposed to be doing?.

There also seems a total lack of the required decoupling capacitors.
 
The 555 is fed with a dual supply, I use R8 and R10 to get the volts down, as I'm using +/-9v the chip gives a bad output if I'm above about 16V. A regulator would be better, I didn't realise this at the time i ordered the board. R4 is a volume control, it does work but I dont think I need R4 and R5. R6 is required by the tca0372, not sure why, I've attached datasheet. I thought C3 and C4 were the decoupling capacitors.

Thanks.
 

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You can't just bung pots in like that to drop the voltage, and the decoupling capacitors need to be directly at the chips.
 
My 2 cents:
The coupling caps should go between pin 1 and 8 of IC1 and between pin 2 and 4 of IC3.

But that is probably only worth 2 cents, I'm sure there is more to add.

Mike
 
Nigel Goodwin said:
You can't just bung pots in like that to drop the voltage, and the decoupling capacitors need to be directly at the chips.
Click to expand...

I knew before I'd ordered there were a few things wrong, this board is just for testing, I will build 2 or 3 of these and then order another revision next week. I have a surplus of pots for sure, I will use the pots to decide what input and output of the 555 would be best. I don't know yet how to change amplification of the tca opamp so I have that at unity gain and i'm using the pots to attenuate the signal. Would it be better to replace those with a voltage divider or regulator?

lilimike said:
My 2 cents:
The coupling caps should go between pin 1 and 8 of IC1 and between pin 2 and 4 of IC3.

But that is probably only worth 2 cents, I'm sure there is more to add.

Mike
Click to expand...

I thought I could get away with fewer by having them where they were. Must check if the 555 even needs them.

Thanks for the comments in this thread.
 


You set the gain of an opamp with the feedback, there are tutorials all over the net.

And you DON'T vary the supply rails to anything with pots.

I thought I could get away with fewer by having them where they were. Must check if the 555 even needs them.
Click to expand...

Pretty well EVERYTHING needs them - electronics works on the bsis that the supply rails are at the same AC potential, that's what the decoupling capacitors do. For a 555 it's absolutely crucial, although less so for the CMOS versions.
 

I see. I should replace the pots with voltage dividers, I will need to look that up but I'm thinking use high values and have one resistor going to earth with the other to the output, somewhat like a pot but fixed. And I will have to keep the + and - supply the same.

I will put the decoupling capacitors close to the supply pins for both 555 and opamp.

Thanks!
 
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No, you don't need anything - a 555 will work quite happily off 18V anyway, but in this case your supply should be 9V - a 555 isn't a spilt supply device, it only needs a single supply.

What exactly is this circuit supposed to do?, it's confusing as you've not drawn the opamp out.
 
It is a piezo driver. I need a dual output to drive the piezo, on the prior stripboard I made up I used a single supply for the 555 and put the output through a cap to get the dual supply, but changed it around for this board because I only want to use 2 batteries. I have only just realised the purpose of decoupling capacitors but knew regardless it wouldn't be so good to tap off some volts from one battery and then use that one and another for the dual supply on the amp.

I couldn't find the device for the opamp or transformer in Eagle so I had to make do. In making do the transformer doesn't fit on the board because I chose a dissimilar package, its not a problem just a learning opportunity, I know I will be going through 3 or 4 more revisions before I get the circuit I want. Regards.
 
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