555 oscillator circuit questions

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htor
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555 oscillator circuit questions

Post by htor » Sun Feb 23, 2020 12:12 pm

hi, electronics newbie here, asking for some good ol' tips! thanks in advance.

i attempted to make a square wave oscillator with the 555 chip, 9V battery powered. followed this tutorial: https://youtu.be/stN-ZzHfiO4. it works pretty good and spits out a decent sounding square wave, although it has some quirks. first issue i had was it being way too loud (outputted around 9V, good for direct speaker, bad for audio interface – poor thing, but it was totally cool with that) so i added a voltage divider to the output to get it down to ~0.9V. seems to work.
20200223_165856.jpg
the (ON)-OFF switch triggers the synth.
the 2 potmeters are controlling the frequency, but i'm thinking could i do it with just one? just one big sweep from low to high?
there was two controlling resistors, so that's why i went with this solution.

my oscillator has audible clicks at each trigger start/end. like a little noise peak at the beginning. can i get rid of those somehow?
Screenshot 2020-02-23 at 17.00.54.png
is this what you call a DC offset?
Screenshot 2020-02-23 at 18.34.56.png
it also has a -102 dB peak around 50hz when resting, so wonder what that can be? do i need to worry? i know abelton is not a scientific tool for this, but i don't have a proper scope :hihi:
Screenshot 2020-02-23 at 17.19.26.png
is there anything else i should consider before making this circuit a "synth" in my setup? i'd like to switch over to a DC connector and just plug it into my 9VDC power supply which powers my other pedals, plug in the audio jack (already tested) and just jam.

last question: what (free) software do you use for drawing circuit diagrams on your mac?
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Re: 555 oscillator circuit questions

Post by guest » Sun Feb 23, 2020 3:55 pm

what size is your pot? you should be able to use just one pot if you put it between pins6,7, and then use a small value resistor between pins7,8. also, im happy to see you put a resistor in series with your LED, which the video did not do.

as for the clicks, youve correctly identified it as the DC offset, and thats hard to avoid. a simple way would be to AC couple the audio signal with a capacitor, and then send it through the switch to turn it on and off. this way the oscillator is always running, and doesnt have a DC pop at start.
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Re: 555 oscillator circuit questions

Post by emmaker » Sun Feb 23, 2020 5:57 pm

I'm assuming that Germany has 50 Hz main power.

So I would guess that is where the 50 Hz issue is coming from. Either through the power supply or from EMI. So might see if there is any 50Hz ripple on the power supply and maybe look at short wires and shielding the circuit.

The bipolar (standard) 555 has grounding/power issues so do an internet search and read about that. The CMOS 555 still has the problem but it's a lot less of an issue. Deals with the output transistor shorting to ground when the circuit is switching and causing the circuit to use a lot of current.

Jay S.

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htor
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Re: 555 oscillator circuit questions

Post by htor » Thu Mar 26, 2020 5:06 am

guest wrote:
Sun Feb 23, 2020 3:55 pm
what size is your pot? you should be able to use just one pot if you put it between pins6,7, and then use a small value resistor between pins7,8. also, im happy to see you put a resistor in series with your LED, which the video did not do.
thanks, single pot works just great.
guest wrote:
Sun Feb 23, 2020 3:55 pm
as for the clicks, youve correctly identified it as the DC offset, and thats hard to avoid. a simple way would be to AC couple the audio signal with a capacitor, and then send it through the switch to turn it on and off. this way the oscillator is always running, and doesnt have a DC pop at start.
i tried placing the switch in the end, seems to work better with regards the click.
i'm curious to know more about ac coupling. as i understand it you block dc (low frequency signals) by adding a cap in the signal path. this means it would work for ac signals, but is my pulse signal an ac signal? as i understand it the 555's output is a periodic dc signal (with its voltage varying between 0V and 9V, say). but my signal definitely has a high frequency so does this ac coupling trick make it an ac signal by removing the offset/centering the signal around 0V?

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Re: 555 oscillator circuit questions

Post by guest » Thu Mar 26, 2020 5:23 am

yes, AC coupling removes the DC offset. you can think of our 0-9V signal as a +/-4.5V signal centered at 4.5V. by putting the capacitor in there, youre getting rid of the 4.5V. with any AC coupling circuit, there is a capacitor and a resistor. the values of these components determine what the circuit considers AC versus DC. lets say you use a 10uF blocking capacitor, and then drive into a 10k load. the cutoff frequency is 1/2*pi*10uF*10k = ~1.6Hz. so below this, you get more and more attenuation, until nothing passes at DC. so it can also be thought of as a highpass filter.
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Re: 555 oscillator circuit questions

Post by htor » Thu Mar 26, 2020 7:04 am

i see! my oscilloscope shows a significant dc offset in my signal, while the same signal from my audio interface is missing the offset and and comes out as an ac signal. this means the ADC in my audio interface must be ac coupled?
20200325_220647.jpg
Screenshot 2020-03-26 at 01.08.51.png
then i will try and experiment with different RC values for this highpass pass filter. really neat formula. by the way, i got a different answer to your equation: 0.5*pi*0.00001*10000≈0.6Hz. am i doing it right?
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Re: 555 oscillator circuit questions

Post by guest » Thu Mar 26, 2020 7:46 am

sorry, math is a bit hard to write correctly in plain text: 1/(2*pi*C*R)

the audio interface is most likely AC coupled, but it might also be DC coupled, with a DC voltage of 0V (using +/- powersupply).
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Re: 555 oscillator circuit questions

Post by EATyourGUITAR » Thu Mar 26, 2020 9:22 am

the 555 comes in 3 flavors. this is history. the first was BJT general purpose timer IC. because it was standard BJT you could actually pass enough current to make a charge pump. the 555 drives the diode + capacitor network for the charge pump circuit. then there was CMOS 555 that solved the problems caused by switching high current on off very fast when all you really want is a timer for logic or one LED. then there was TLC555 from texas instruments where the L is for low power and the C is for CMOS, T for TI. the only problem with TLC555 is that you can't drive big loads like a charge pump. since we have specific modern charge pumps like MAX1044 from maxim we really don't ever need to buy the original BJT version of the 555 ever. the only thing you would use it for is high current such as a big LED array 500mA with astable, bistable, monostable where it would reduce component count compared to TLC555+BJT or TLC555+MOSFET.
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