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### Topic: using 5v logic with 3.3v devices (Read 935 times)previous topic - next topic

#### recklessrog

##### Jan 28, 2014, 12:01 am
As I'm new to Arduino,I was wondering if there is any good reason why, when connecting the output from arduino @ 5v logic level, to 3.3v level devices I could not just use a resistor potential divider (say 3.3k in series with a 6.8k, Nearest preferred values ( 6.8k to neg.))  from the output pin to neg. and take the feed out across the 6.8k resistor?  This would save using level shifting IC's, etc.
Ive seen some designs where there is just a resistor in series with the output pin, but when it is coupled to a mosfet input device, the voltage at each end of the resistor is still 5v as no current is being drawn through it, possibly risking breaking down the input mosfet.

#### Caltoa

#1
##### Jan 28, 2014, 12:42 am
Sure, a resistor divider is often used.
But sometimes a high frequency signal is needed, and a resistor divider can slow down the signal because of the impedance.
In many cases not only the output but also the input needs to be level shifted, that is why an IC can be handy.
For the I2C bus, a resistor divider can not be used, since the I2C bus uses a signal that is active when pulled low.

So in many cases a resistor divider is not the best solution.
One more thing, why would you want to save using a level shifter IC ? I can turn that question around : Why would you use resistors if you can save using resistors by using a level shifter IC ?

#### recklessrog

#2
##### Jan 28, 2014, 12:58 am
AhA!, I see it all now, (well some of it) I get your point but the reason I asked about resistors instead of I.C's is because I have literally thousands of resistors, and have to BUY I.C'S at a pound a go! that's a lot when you're only on a pension  =(
Guess I'll have to get some though. Thanks for the explanation

#3
##### Jan 28, 2014, 01:23 am
Chips like 74HC125 or 126 make good level converters, usually < 50c for a quad pack.

_______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

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