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 Forum index » DIY Hardware and Software » Circuit Bending
V-trigger to S-trigger stops when devices share ground?
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Wirl



Joined: Jan 22, 2018
Posts: 6
Location: Philadelphia

PostPosted: Mon Jan 22, 2018 5:18 am    Post subject: V-trigger to S-trigger stops when devices share ground? Reply with quote  Mark this post and the followings unread

Hi all,

Many of you may are likely familiar with the following simple circuit: https://www.synthesizers.com/learn/gates/strigcircuit.jpg It takes the gate output from one piece of gear, and uses that signal to close a switch on another piece of gear (using a transistor).

Recently I've used this little circuit to trigger the "push" of the One-Key-Play button on my old Casio PT50 (allowing it to be sequenced in time). It works great, for the most part. More specifically it works while playing sound out of the Casio's speaker, and with headphones plugged in. The problem arises when I plug the Casio's audio output into a mixer which also receives audio output from the triggering device on another channel (in this case from a LepLoop). In other words, the LepLoop sends gate-out to the Casio and sends it's own audio to the mixer. In this setup the Casio is no longer triggered.

Some strange things to note:

- The actual gate pulse can be faintly heard on the Casio channel on the mixer. I'm guessing this means that the gate pulse found an easier path to ground through the Casio rather than returning to the LepLoop through the gate cable?

- I can also get the S-trigger to stop working by simply touching the grounds of the audio out cables from the LepLoop and Casio together (mixer aside).

- Checking the transistor emitter and collector with a multimeter, I get beeps in rhythm with the gate (but notes are not triggered). I think this means that the switch is still closing when the devices share ground, although I'm not sure how accurate this test is given that the test leads may be affecting the circuit.

- Using a multimeter on the LepLoop, I tested between the ground from the gate out and audio out, and found that I get beeps when the gate fires a pulse, so this means they share ground.

I've tried modifying the circuit in a hundred ways using different arrangements and values of resistance and diodes with no luck. At this point I'm pretty stumped so any help would be greatly appreciated. If I can't figure this out I might be forced to explore solid state relays for this application. Any suggestions there would also be appreciated. Thanks!
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PHOBoS



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PostPosted: Mon Jan 22, 2018 6:13 am    Post subject: Reply with quote  Mark this post and the followings unread

welcome party!

Does the One-Key-Play button originally connect to GND ?
Personally I try to avoid tranistors for remotely switching things and use an optocoupler instead to have complete electrical isolation.
I have good experiences with the CNY17 but there are probably plenty that work well. You do have to check if the drive circuit can provide
enough current but usually it doesn't need a lot to function. A digital switch like the CD4066 could work too but is not as isolated

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Wirl



Joined: Jan 22, 2018
Posts: 6
Location: Philadelphia

PostPosted: Mon Jan 22, 2018 6:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the quick reply Smile

At one end of the One-Key-Play button I measured 2 MegaOhms to ground, and the other 9 MegaOhms to ground. Not sure that counts as being connected?

I'm going to take your advice and order some optocouplers. I will definitely check out the CNY17. I might even pair that with an LTC6993 to make the gate pulse wider for when the Casio is clocked way down (it's a circuit bent project). The clock gets too slow to recognize that the button was "pressed".
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PHOBoS



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PostPosted: Mon Jan 22, 2018 8:07 am    Post subject: Reply with quote  Mark this post and the followings unread

no, megaohms are not really considered a connection. It would have to be something very close to 0 ohms.
In these kind of devices switches are often wired in a matrix which is controlled and read by a processor.
Simple toys with only a few buttons sometimes use connections to GND in which a transistor will work fine.
Even then I prefer using an optocoupler as there can be a difference in supply voltage between devices
which could potentially fry the circuit and it also keeps any noise out.

I can't guarantee the CNY17 will work, I recently had a circuit where it didn't and had to use a different one
but that was the first time in many years. It has a build in transistor and you might have to experiment which
way around you have to connect it, sometimes it doesn't matter but if it doesn't work right away try swapping
the connections. If you have an old switched power supply laying around (like a power supply from PC) they
usually, if not always, have an optocoupler inside. I've also found them in computer monitors and TVs.

There is one other thing to watch out for although it is not very common. Sometimes the audio output does not
actually have a GND connection but is connected to the positive supply or 2 outputs pins comming directly from
the processor (usually one of those black blobs). This is generally not a problem untill you power multiple devices
from the same power source or connect them together in some other way and also connect them to a mixer.
In those cases you could use an audio transformer on the output which will isolate them as well.

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Wirl



Joined: Jan 22, 2018
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PostPosted: Mon Jan 22, 2018 12:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks again for the in-depth reply.

The One-Key-Play button is in fact connected to a button matrix that is read by the main NEC chip on the board. What happens inside that is anyone's guess. Ultimately one side of the One-Key-Play button is getting shorted to Casio ground (through the mixer), which halts proper operation. Based on this conclusion, I'm surprised I haven't seen optocoupling mentioned in online circuit bending articles regarding trigger inputs.

In response to the LepLoop gate output having enough power to drive the LED inside the CNY17: I measured the LepLoop gate output using my multimeter. Despite trying to measure the fixed high "note on" voltage, my DC readings were too low to make sense (maybe the pulses are too fast for this multimeter setting). I took some readings in AC which made more sense. With my test leads connected to the gate out in series I came up with a max voltage of 2.4V, a max current of 6.67mA, and a max resistance of 361Ω. Based on the CNY17 Data Sheet, the internal LED has a forward voltage of 1.45V (1.65 max) and a forward current of 60mA. Using the LED resistor formula, the LepLoop gate signal can only provide the CNY17 LED with 2.6mA which is nowhere near the 60mA on the spec sheet. Who knows, maybe it would still close the swtich. Interestingly I connected a regular small yellow LED to the LepLoop gate output and got a fairly bright output in rhythm with the pulse. Maybe I'm missing something here.

I'm wondering if it makes sense to also add something like a power regulator for the pulse. This would not only allow the CNY17 LED to receive adequate current, but it could also allow different gate voltages from different gear to be used. Hopefully by the end of this I'll have the ultimate trigger in circuit for circuit bent gear. Smile

There's a good electronics dumpster at the technology institute down the street so I'll have a look for some of those PC power supplies and monitors. On Mouser and DigiKey it looks like there are many cheap options as well.

PS Very cool circuits you've made. Nice and neat. Great to see parts being recycled. *<
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Wirl



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PostPosted: Mon Jan 22, 2018 12:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

I also just discovered a useful tidbit about the LTC6993 pulse stretcher:

"The OUT pin swings from GND to V+ with an output resistance of approximately 30Ω. When driving an LED or other low impedance load a series output resistor should be used to limit source/ sink current to 20mA."

I believe what this means is that when the LTC6993 receives a pulse, it creates it's own separate pulse at whichever voltage you are using to power it (2.25V-5.5V). In essence, this would regulate gate pulses from a wide range of input trigger voltages and output a set voltage every time. With the stated output voltage and resistance, supplying 60mA to the CNY17 would be no problem.

Ultimately it's possible that an LTC6993 feeding into a CNY17 is all that's required for a complete and functional trigger circuit.
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PHOBoS



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PostPosted: Mon Jan 22, 2018 1:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

I remember the LED current mentioned in the datasheet indeed to be rather high. I've personal never used
it anywhere close to that. It probably has to do with how much current you want to be able to flow trough the
output side which in your case is hardly anything. Only way to find out is of course to just test it out.

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Last edited by PHOBoS on Fri Jan 26, 2018 8:05 am; edited 2 times in total
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Wirl



Joined: Jan 22, 2018
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Location: Philadelphia

PostPosted: Mon Jan 22, 2018 1:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

That's reassuring that the CNY17 can run at lower currents. I'll post back after I acquire parts and give it a spin. *<
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Grumble



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PostPosted: Tue Jan 23, 2018 5:19 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm a fan of the H11F1, this is a opto FET, which means that you don't have to worry about polarity of the voltage across the FET connections (obviously the LED needs a certain current in a certain direction to have the FET to conduct)
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Wirl



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PostPosted: Fri Jan 26, 2018 7:02 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Grumble for the suggestion! I'll keep it in mind if I order some in the future.

For now I just stopped by the dumpster at a technology university down the street and it was full of junk PCs with power supplies inside. I unscrewed two supplies to take home. Inside I found 6 optocoupling chips total. Thanks again for the PC tip PHOBoS. Smile

These are the chips I found. They have a forward voltage of 1.17V and forward current of 80mA. The LepLoop gate out is only able to give the the internal LED of these chips about 3mA. Using a multimeter I found that this only reduces the resistance on the other side to 4-5MΩ. With nothing to lose I connected it with the Casio and surprisingly it closed the button every time thumb up. I guess it does't take much current flow for the matrix chip to register a button "press".

After staying up all night recording wacky Casio music I'm looking forward to getting some LTC6993s for longer pulses to trigger those nice clocked-down circuit bent sounds. cat :
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PHOBoS



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PostPosted: Fri Jan 26, 2018 8:01 am    Post subject: Reply with quote  Mark this post and the followings unread

good to hear you found some optocouplers and it works Very Happy

instead of the LTC6993, which I have no experience with but looks to be a nice chip for it, you can do it with a simple 555.
It's actually what I use most of the time when I want to change the pulse length of something. If you do a search for
monostable 555 you should find some example circuits. Keep in mind that it triggers on a high to low transition so you
probably have to invert your signal but that's easy to do with a transistor. Oh and it also has a CV input. Wink

btw. I don't think you can measure resistance that way when using a transistor. It controls the amount of current that can
flow but if you measure it with a DVM there should only flow a very small amount of current resulting in what looks like a
high resistance.

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