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Twin-T dum oscillator circuit - trigger pulse bias
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alkopop79



Joined: Aug 21, 2008
Posts: 47
Location: London

PostPosted: Tue Mar 24, 2020 11:10 pm    Post subject: Twin-T dum oscillator circuit - trigger pulse bias
Subject description: trigger pulse voltage range
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This is a twin-T oscillator circuit I built years ago on breadboard. I would like to turn it into a PCB but there is one thing I'm confused about: according to simulations in SPICE, the trigger pulse from the 555 has be between the virtual ground and the maximum voltage, for instance 4.5V and 9V. If the minimum voltage of the pulse is 0V, it won't start kickstart the oscillation. Is it possible that the 555's ground pin should be tied to the virtual ground (4.5V) instead of the ground?

[img]https://drive.google.com/open?id=1icXFbkOP5CUHU8hcfvDyC51Mv-WXE3qu[/img]

https://easyeda.com/greg.lorincz79/twin-t-2
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IGR



Joined: Aug 31, 2014
Posts: 19
Location: Brno

PostPosted: Wed Mar 25, 2020 6:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Greg. Opamp -V (pin4) should be grounded to normal ground. +Input (pin3) should have 1/2V DC level, high impedance, therefore connect resistor >=1M betwenn +in and well filtered virtual ground. Then just bring short Exclamation (1-2ms) pulses (keyword PW) from the 555 through a capacitor (100n~1uF) to this input. Btw OpAmp 741 type is not the best choice there. 072,LF357 fits better due high input impedance. Good Luck!
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alkopop79



Joined: Aug 21, 2008
Posts: 47
Location: London

PostPosted: Wed Mar 25, 2020 8:38 am    Post subject: Reply with quote  Mark this post and the followings unread

IGR wrote:
Hi Greg. Opamp -V (pin4) should be grounded to normal ground. +Input (pin3) should have 1/2V DC level, high impedance, therefore connect resistor >=1M betwenn +in and well filtered virtual ground. Then just bring short Exclamation (1-2ms) pulses (keyword PW) from the 555 through a capacitor (100n~1uF) to this input. Btw OpAmp 741 type is not the best choice there. 072,LF357 fits better due high input impedance. Good Luck!


Wow, what a great reply! Thank you!
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alkopop79



Joined: Aug 21, 2008
Posts: 47
Location: London

PostPosted: Wed Mar 25, 2020 8:49 am    Post subject: Reply with quote  Mark this post and the followings unread

I added the resistor to the opamp input and the capacitor. Something like this?

[img]https://drive.google.com/open?id=1m05ev0iAJ8YdLuDccPav6DAcMQCmJPBp[/img]
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gabbagabi



Joined: Nov 29, 2008
Posts: 582
Location: Berlin by n8
Audio files: 23

PostPosted: Thu Mar 26, 2020 2:20 am    Post subject: Reply with quote  Mark this post and the followings unread

the combination of 100nF and 1M would give u a rather long impulse, make them smaller

u need to "kill" the negative part of the impulse, otherwise u get usually a doubleBD

in the upper "T" the cap should go to VGND, at least "others" are doing that, https://www.paia.com/ProdArticles/syndrum.htm
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IGR



Joined: Aug 31, 2014
Posts: 19
Location: Brno

PostPosted: Sat Mar 28, 2020 8:45 am    Post subject: Reply with quote  Mark this post and the followings unread

I recommended short impulses from 555. Duty cycle 99% or so. Then is the size of the coupling capacitor not so important.
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IGR



Joined: Aug 31, 2014
Posts: 19
Location: Brno

PostPosted: Sat Mar 28, 2020 8:55 am    Post subject: Reply with quote  Mark this post and the followings unread

https://syntherjack.net/555-trigger-cv-generator/ nice article with extensive description. Just replace resistor R1 with potentiometer to make clock speed variable..
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