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Sanity Check please?
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Sanity Check please?
chaotician
Hi,

Can someone more sensible than me sanity check my schematic for a simple Arduino Digital Out to trigger/gate with indicator LED please?

Have I got the output resistor in the correct place? Or does it need to go above the LED protection resistor?

Have I gone mad, are these just lines and letters on a piece of paper.

Time for my Horlicks and a soft blanket I think...

Matt
mutronic
did you put it on a breadboard?
cackland
chaotician wrote:
for a simple Arduino Digital Out to trigger/gate with indicator LED please?


Why are you adding all this extra circuitry?

All you need is a resistor and LED split from the Arduino output pin to gnd.
kassu
I don't know what the indended behavior is, but the way you draw it the LED will be on when the gate is low (arduino pin high) and vice versa.

A bigger issue is that the gate will go only down to around 2.5V because of the LED voltage drop.

I think a 1k series resistor plus possibly diodes to ground and 5V provides reasonable output protection for the arduino.
chaotician
Quote:
did you put it on a breadboard?

Yes and it doesn't work correctly, lol

Quote:
Why are you adding all this extra circuitry?


1.) Inexperience and I'm trying to figure this out for myself
2.) I want to avoid external modules from drawing too much current from the Arduino (hence the transistor switching 5v rather than sourcing from the Arduino directly).
3.) I want to protect the module from having it's output patched to the output of another module (hence the diode on the output)

Quote:
A bigger issue is that the gate will go only down to around 2.5V because of the LED voltage drop.


Thanks, I hadn't even considered that, this is probably why it only triggers some of my modules.

Quote:
I think a 1k series resistor plus possibly diodes to ground and 5V provides reasonable output protection for the arduino.


Will this protect me from shoving reverse voltage up the output?

Thanks for the replies guys, I am very grateful for the responses, I am a real n00b at designing my own circuits and this is a massive help.
kassu
chaotician wrote:
Quote:
I think a 1k series resistor plus possibly diodes to ground and 5V provides reasonable output protection for the arduino.


Will this protect me from shoving reverse voltage up the output?


To a reasonable level yes. Suppose as a worst case that your module is trying to output +5V, and you plug the module into something that outputs -12V. That is a voltage drop of 17V over the 1k resistor, so a current of 17V/1kOhm=17mA will flow. If I remember correctly the arduino can source and sink 20mA, so that is safe for one pin.

Now if you have many trigger outputs and they all have to simultaneously sustain a 17mA fault current, the total current limit of the atmega will be exceeded and it will eventually break. If you want to be able to tolerate such level of abuse an external driver circuit is a good idea.

The optional diodes don't really have any function when the arduino pin is configured as output, but they may come to play if you have accdentally (or temporarily for example during startup) configured the pin as input. These should be Schottky diodes to work properly. The atmega also has internal diodes on each pin which are not intended for this purpose, but they will nevertheless likely withstand the few 10s of mA that is able to pass through the 1k resistor.
Boogie
chaotician
don't forget to put a 4.7k - 10k series resistor in the transistor's base.
Vbe is only about 0.6 - 0.7 V and will draw lots of current out of the output pin.
As mentioned by others, don't forget it's reversing now, so your software has to drive low to trigger.
Moving your trigger to below the led will cause the highest level to be Supply - Vf of the led. For a red one, that's in the order of 1.8-2.2V
Best to add a resistor (say 10k) from your supply directly to the collector of the transistor. (this is where you will connect your trigger as well)

You now should have:
limited current consumption out the output pin.
Negative logic in your software
and the trigger going up to supply minus (the current into the trigger pin x 10k).

good luck
chaotician
Thanks so much for the help everyone!

I have lots to learn, but I have boxes full of bits I've bought in the process of building my other module and doing other projects!

I really need to get my head around the maths involved.

Matt
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