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Passive Or combiner diodes question
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Passive Or combiner diodes question
Drilldoughzer
Hi,

Can I put 5mm red leds (with the right resistances before each led) instead of
4148 diodes to do this?

Thanx!
PHOBoSapiens
It can be done, but LEDs have a higher forward voltage (2V~3V) compared to regular silicon diodes (0.6V~0.7V).
As a result there is a larger voltage drop which could cause some problems for things like gates/triggers. Also if
you want to use LEDs to have more blinkenlights you might be dissapointed as the current is usually too low to light
them. However, that depends on the LEDs you use and I had succes doing something similar using blue LEDs which
hardly need any current to light up.
Drilldoughzer
Thanx! The resistors should be around 500 ohms?
PHOBoSapiens
I personally wouldn't go below 1K but it depends on what voltages you are expecting to use and also a bit on the LEDs.
Here's the thing though,. the current will also be limited by the input resistance of whatever you connect the output to
and this wil probably be much higher (10K~100K seems to be a bit of a standard range). This is also why I mentioned
the current will probably be too low to light them. You do need the resistors though as an extra protection. For example
if you would connect a cable to the output and plug that into a 3.5mm socket* you will most likely hit the ground connection
first (unless you play operation with your patchcables) which might fry the LEDs if there is no resistor in series. Outputs
often already have (and they should) current limiting resistors btw, but it's better to be safe than sorry.

Another thing to take into consideration is bipolar voltages. It works fine as long as the voltage is positive and although
they would block a negative voltage like a regular diode they are not always happy when the voltage is reversed and this
could actually damage them. If the current is low this is unlikely to happen but still it might be wise to put some regular
diodes in series (so resistor + diode + LED). Of course adding diodes causes an extra voltage drop but you could use
schottky diodes which have a lower forward voltage.

* (also goes for 6.3mm of course and even with bananas there is a risk for an accidental ground connection Dead Banana )
Drilldoughzer
Hey thanx I’m learning alot with this!

Stupid question: The Led at the end of the chain, the cathode is soldered to the jack out or the ground?

If it’s to the ground, the cathode of the diode would be soldered to the anode of the Led and to the out jack right?
Drilldoughzer
So I have bread board it with 1k res+4148 diodes(I don’t have other diodes at home) and blue leds. All in series...


They blink but when I disconnect the output they stop blinking why?

I use trigs from the QCD expander as voltages.

Directly from the QCD expander it outputs 5 volts.
From the output of my circuit, I read 0.4 volts. That won’t cut it...

Quadra, Ginko Sample Slicer, Ginko TTLFO and Galilean Moon fires up but other modules (Turing, Synthrotek SEQ, Rampage) don’t advance...

Is it the diodes? I’ll go get some schottky‘s tomorow...
nigel
Drilldoughzer wrote:
So I have bread board it with 1k res+4148 diodes(I don’t have other diodes at home) and blue leds. All in series...


They blink but when I disconnect the output they stop blinking why?

Because there is no longer a path to ground?

Drilldoughzer wrote:

I use trigs from the QCD expander as voltages.

Directly from the QCD expander it outputs 5 volts.
From the output of my circuit, I read 0.4 volts. That won’t cut it...

Quadra, Ginko Sample Slicer, Ginko TTLFO and Galilean Moon fires up but other modules (Turing, Synthrotek SEQ, Rampage) don’t advance...

Is it the diodes? I’ll go get some schottky‘s tomorow...


The Blue LED drops about 3.5 V, the diode another 0.6 or so. Not much left out of 5V.
PHOBoSapiens
Drilldoughzer wrote:
Stupid question: The Led at the end of the chain, the cathode is soldered to the jack out or the ground?

to jack out, together with the cathodes from the other input chains.

nigel wrote:
Drilldoughzer wrote:
They blink but when I disconnect the output they stop blinking why?

Because there is no longer a path to ground?

Exactly. You could add an extra resistor to ground at the end so it will still work with nothing connected to the output
but don't make it too small as the current has to be provided by the modules. This might also be a reason why you
only read about 0.4V.

I just did a quick test with a blue LED and a resistor in series.
with 100K it still lights up very visible and the voltage drop is 2.5V
with 10K it's already getting too bright and the voltage drop is 2.7V
with 1K it hurts my eyes and the voltage drop is 3.0V.

So the voltage drop is not fixed and it also depends on the LED. But yeah, with a 5V input there is not going to be a lot
left which is why I mentioned that it could cause some problems for things like gates/triggers.
Of course you could make an active (powered) circuit but I'm not sure of you want to go that route.
Drilldoughzer
So I tried with different combinations of leds, resistors and Diodes but I always get something under 1 volt... So I’ll make this Or combiner with schottky Diodes, no resistor for each channel and no Leds... With this configuration I have 1.3 volts at the output...

Last question: When I put a 100k resistor from the output to the ground I have 0.9 volts at the output... But it’s safer to put a resistor there for protection right?

And no I’m not ready to do this project active... I got much to learn before I do that.

Thanx PHOBosapiens for your time!!
PHOBoSapiens
Drilldoughzer wrote:
So I’ll make this Or combiner with schottky Diodes, no resistor for each channel and no Leds... With this configuration I have 1.3 volts at the output...

If that's with a 5V input I would expect something around 4.5V, definitely not something so low. May I ask what kind of signal you are using ?
And just to be clear, for a simple diode OR you connect the input signals to the anodes of the diodes and connect all the cathodes together
as the output. Of course you could first test with a single diode.

Quote:
Last question: When I put a 100k resistor from the output to the ground I have 0.9 volts at the output... But it’s safer to put a resistor there for protection right?

You don't need a resistor on the output to ground for protection but there might be situations where it doesn't work without it. Although if it doesn't
work for standard gate/trigger inputs then those would be bad designs IMO. Also if you are measuring the output voltage without anything else
connected to it a resistor might be needed.
Drilldoughzer
QCD Expander trigs out are my voltage sources.

Yes I connect the input signals to the anodes of the diodes and connect all the cathodes together.

The modules that where not advancing do now with this configuration... With the 100k protection resistor, I get like 0.4 volts drops... I’ll try if the modules get trigged with the resistor...

BTW it’s a 3 inputs circuit...

Just to be sure, the resistor goes from the output to the ground or from all the cathodes to the output?
PHOBoSapiens
Drilldoughzer wrote:
QCD Expander trigs out are my voltage sources

ok, I can't really find it in the manual but the outputs are probably 0/+5V and it looks like a good source for testing.
edit: oh you actually mentioned it is 5V

Quote:
The modules that where not advancing do now with this configuration... With the 100k protection resistor, I get like 0.4 volts drops... I’ll try if the modules get trigged with the resistor...

a voltage drop of 0.4V sound right. cool

Quote:
Just to be sure, the resistor goes from the output to the ground or from all the cathodes to the output?

yes, from output (the cathodes) to ground, so just parallel to your output jack. Unless you are using bananas which don't
have a ground connection, but judging by the QCD I am guessing it's for euro-rack.
PHOBoSapiens
offtopic: I just checked your video for Black Bow (first one of which I spotted a modular in the thumb), very nice !! It's peanut butter jelly time!
Drilldoughzer
Thanx man! I’m doing some passive utilities to put on the top and bottom gaps of my road rack. I use this system to receive trigs from my acoustic drums.

https://www.youtube.com/playlist?list=PLqX4E2C2jeDnZM2gT-zKdE9SrLnebO5 cy
PHOBoSapiens
awesome applause checking some more videos.


maybe a discrete XOR could also be fun https://www.edn.com/design/other/4373765/Perform-the-XOR-XNOR-function -with-a-diode-bridge-and-a-transistor
Although not really passive it doesn't need any external power. Might not work with all modules though.
Drilldoughzer
Cool thanx I’ll try that!
Drilldoughzer
Finished... https://www.modulargrid.net/e/other-unknown-half-utils






Next step: Put 3 x 2hp modules (Midi, Mix and LFO v2) horizontaly behind a panel on the top gap... w00t
PHOBoSapiens
That case looks like a lot of fun! hyper

(you need a green knob though wink)
Drilldoughzer
I know!! LOL I’ll get one on my next Thonk order.
Drilldoughzer
Off topic but here is my top half 1u panel:



3 x 2hp modules under a single half 1u panel:

Midi, LFO v2 and Mix
PHOBoSapiens
nice, is that MIDI through USB ?
Drilldoughzer
Yep usb mini

http://www.twohp.com/modules/midi
Drilldoughzer
And I got

LFO
http://www.twohp.com/modules/lfo

And Mix in there to...

http://www.twohp.com/modules/mix
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