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Topic: resistors before outputs/inputs jacks (Read 475 times) previous topic - next topic

capicoso

Oct 25, 2013, 07:24 am Last Edit: Oct 25, 2013, 07:27 am by capicoso Reason: 1
Hello.
I've seen in many circuits(mostly audio, modular synths) every in/out jack has a resistor in series just before it, or after it's an input. They're generally 100k, sometimes 1k, 220 ohms, etc. I think someone told me this is for impedance, but I don't remember.
Now, I want to ouput some voltage from my arduino to other device. A square wave from a digital output(no PWM), with a max frequency of 10hz or so, the same digital output will blink a LED too... It'll act as a clock, to drive other sequencers.
It'll have an input too, with a square wave of the same range too, into a digital input.
So, what should I put before/after the ouput/input jacks? What happens if I don't put any resistors?
And also, can I use the same digital pin to blink the LED and at the same time output the square wave?
thanks

EDIT: oops, sorry, I thought I was at the electronics subforum.

Grumpy_Mike

A resistor is not necessary if I understood your application correctly but post a schematic to remove any ambiguity.

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And also, can I use the same digital pin to blink the LED and at the same time output the square wave?

Yes with the caveat output the square wave to what? 

capicoso


A resistor is not necessary if I understood your application correctly but post a schematic to remove any ambiguity.

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And also, can I use the same digital pin to blink the LED and at the same time output the square wave?

Yes with the caveat output the square wave to what? 


Well now I added a resistor and two diodes to the input. I read the article you wrote on your page, because the square wave input could be between -5v and more than 5v. I have no schematics.
The square wave from the "outside world" ideally should be 0-5v. The idea is that I could connect any clock signal from synthesizeres or so, like a Korg monotribe, monotron. But there are exceptions like a modular synth, which generally outputs a square wave of -5v to 5v. That's why I added your circuit.
  The square wave output can go to any other sequencer/synthesizer, for example, this one from MFOS:

At the bottom of the schematic is U1-D 40106. XCLK is the clock input. So the square wave my Arduino generates, goes to that input.

I forgot to explain the project ... IT's a MIDI sequencer, now it can generate it's own clock and output MIDI clock to sync other instruments. It has MIDI in too, so with MIDI clock from other instruments it can be synced as slave.
The objective of the square wave input/ouput is the same as the MIDI clock in/out, but with analog instruments instead of digital. Because I have older hardware(modular synths) which doesn't have MIDI. So I want to be able to sync multiple digital and analog devices at the same time.


Grumpy_Mike

Those 40106 inverters are running off 12V. Therefore before you can successfully put an arduino output into it you have to boost it up from a 5V signal to a 12V signal. This can simply be done using a transistor and two resistors.
Take the first schematic here:-
http://www.thebox.myzen.co.uk/Workshop/Motors_1.html
replace the motor and diode with a resistor ( 1K ) and connect the +ve to your 12V. Then connect the input of your inverter to the collector of this transistor.

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I have no schematics.

Then these is no way you can possible build it. You will have nothing to tell you where to put the wires.

DVDdoug

I'll just throw some possibilities at you....   All of this is optional, or it depends on the particular circuit design...

You'll almost never find a resistor in series with the input, so we'll ignore that possibility.

You might find a "protection resistor" in series with a preamp-output, or headphone-output.   It's purpose is to limit current (and prevent damage) if the output is accidentally shorted to ground.   A preamp (designed to drive loads of 10K & up) might have a 1k resistor.   A headphone amp might have a series-output resistor of 20 Ohms or less.   The higher the resistance the more in interacts with the headphone impedance (which is not uniform across the frequency spectrum).  I've seen one headphone design with a current limiting resistor is inside the feedback loop.   That creates a low effective output impedance (for flat frequency response) while still limiting current when the amp clips or is shorted.    (When an amplifier clips, the negative feedback no longer has any effect.)

You won't find a resistor in series with the output of a power amp.  A power amp needs to deliver maximum power to the 4-Ohm or 8-Ohm load.    If a power amp has short circuit protection, it's more complicated.

A parallel resistor in parallel with an input or output to ground is usually there to "pull" the output to ground when nothing  is connected, or when something with a series capacitor is connected.   This is usually 10K - 100K, but on the output (especially the output of a headphone or power amp), it might be around 1k.   If the input or output is capacitively coupled and one end of the capacitor is not connected to anything, the unconnected end can "float"  up or down.     Then if you connect something that suddenly pulls it to ground, you'll get a "snap" or a "pop".   I know of one case where a soundcard-input was floating-up during recording and it started to distort after about a minute.   In this case, I think it was whatever was connected to the soundcard causing the problem, but a resistor to ground at the soundcard input solved the problem.

Pro microphones are low impedance.    On a preamp, there is usually a resistor of around 1K in parallel with the (differential) mic input.     That resistor properly "loads" the microphone (for the proper sound/frequency response), and a lower impedance also reduces noise pick-up, especially when no mic is connected.  On some high-end preamps, this input resistance is variable.

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