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alkopop79
Joined: Aug 21, 2008 Posts: 49 Location: London
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Posted: 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
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Posted: Wed Mar 25, 2020 6:57 am Post subject:
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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 (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: 49 Location: London
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Posted: Wed Mar 25, 2020 8:38 am Post subject:
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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 (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: 49 Location: London
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Posted: Wed Mar 25, 2020 8:49 am Post subject:
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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: 651 Location: Berlin by n8
Audio files: 23
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Posted: Thu Mar 26, 2020 2:20 am Post subject:
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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
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Posted: Sat Mar 28, 2020 8:45 am Post subject:
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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
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