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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Nor gated osc question & happy nye!
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mike page



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PostPosted: Tue Jan 01, 2019 4:42 am    Post subject: Nor gated osc question & happy nye! Reply with quote  Mark this post and the followings unread

I would like to make a gated osc like a 4093 one, but with the opposite response from the control. So it oscillates when the control is low, and is reset (sync) when it recieves a pulse.
Would a nor gate work for this?
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JovianPyx



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PostPosted: Tue Jan 01, 2019 7:40 am    Post subject: Reply with quote  Mark this post and the followings unread

If you have a breadboard, this should be easy to test.
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PostPosted: Tue Jan 01, 2019 8:13 am    Post subject: Reply with quote  Mark this post and the followings unread

V, true. I just thought I'd float the idea as I don't have any nor chips and I'm not able to get to my studio till the weekend.
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PHOBoS



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PostPosted: Tue Jan 01, 2019 8:17 am    Post subject: Reply with quote  Mark this post and the followings unread

In theory, yes.

However, for a one gate oscillator you need a gate with a schmitt trigger and to my knowledge there is no regular CMOS NOR gate that has one.
There is the SN74HC7002 but it's a bit pricey so unless you really need to make it as small as possible you might aswell use a NAND gate
(4093) and put an inverter in front of one of the inputs. (which can be done with a transistor). You might need an inverter on the output too.

There might be another problem though. You mention resetting it with a pulse but this might not completely work as you'd expect. If it is oscillating
then when the output is high it will charge the capacitor on the input through a resistor. Once the voltage over the capacitor has reached the upper
treshold level of the build-in schmitt trigger the output will change stage and become low. Now the capacitor is discharged untill the voltage reaches
the lower treshold level and than it becomes high again (In reality the treshold level itself probably changes instead of there being 2 of them). This
is also why you need the schmitt trigger otherwise when the output goes low it only has to discharge the capacitor a really tiny amount before it gets
below the treshold level and then it only has to charge the capacitor a really tiny amount to get above it again, resulting in a very fast oscillation.

Anyway, let's say the output is high so the capacitor get's charged. If you make the other input high the output immediately goes low and it starts
discharging the capacitor but the voltage over the capacitor now depends on 2 factors: the voltage it was at when you made the other input high
and how long you make the other input high (so the length of your reset pulse). If it's a very short pulse it will discharge the capacitor just a little bit
and then start charging again. So the output will be high, shortly go low for the duration of the reset pulse and then goes high again for an 'unknown'
amount of time. (before going low again and oscillate at a regular speed). Basically you just add a bit of a delay to it. You can even discharge it so
far that the voltage gets below the lower treshold level so when it starts charging again it takes longer than it does when it is oscillating and as a result
the output will be high longer after a reset than when it oscillates.

In other words if you send a pulse than how the oscillator responds depends on both the length of the reset pulse and when it is applied. If the pulse is
long enough the result should always be the same but it will never start oscillating at a regular speed immediatley after the reset. the first time the
output stays high will always take longer. This behaviour might be acceptable for whatever you want to use it for but it is something to keep in mind.

The same thing happens when using a NAND gate of course so you could test it out with that. Make an oscillator with a low frequency (< 0.5 Hz) with an
LED on the output and experiment with making the other input low at different times and for a different duration and see how it responds. Measuring the
voltage accross the capacitor probably helps to see what happens.

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mike page



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PostPosted: Tue Jan 01, 2019 8:22 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the reply! Looks like I just need to get busy on the bread board
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PHOBoS



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PostPosted: Tue Jan 01, 2019 8:30 am    Post subject: Reply with quote  Mark this post and the followings unread

There is another, somewhat dirty, way to achieve the same result.
Make an oscillator with a schmitt trigger inverter, add a diode to the input and use the other side of the diode as your reset input.
The orientation of this diode determines how it will respond:
- with the cathode connected to the input of the inverter a high reset signal will halt it.
- with the anode connected to the input of the inverter a low reset signal will halt it.
It's probably not a good idea to do this with oscillators that use very large capacitors.

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PostPosted: Tue Jan 01, 2019 8:33 am    Post subject: Reply with quote  Mark this post and the followings unread

Oh yeah! I had forgotten about that trick. Yup I reckon that will do what I need. Thanks again!
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PostPosted: Tue Jan 01, 2019 1:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

This is the idea I have. 3 oscillators sync each other via some 4520 dividers and some '&' diode logic. The output of the circuit isn't featured on the diagram. That will probably be all the outputs of the 4520 mixable using touch pads then buffered (like a ciat lonbarde tochante). I hope that makes sense! I can't seem to post the image for some reason so I popped the schematic here
http://mpanimation.blogspot.com/2019/01/sync-ring.html?m=1
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PHOBoS



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PostPosted: Tue Jan 01, 2019 5:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

interesting, I am curious what the results will be.
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