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From 3V to 5V -- how to operate ICs working with 5V from Microbit / Audrino 3V pins


grossdan
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Hello,

I just noticed that ICs such as and/or gates, etc. operate around 5V while the output and input pins of the microbit and audrino (i hope to get as well) run with 3V ...

 

I imagine that this is a common issue -- how is this typically solved to drive circuitry without too much addition of other components.

I listened to something called open collector -- which seem to be a way to use a transitor controlled by 3 V to run a circuit with 5V -- there seem also be chip variants such as and/or gates that use open collector as output pins and require external pullup resistors and "drain" resistor.

Is this the way to go -- to add for every control path such additional transitors / resistors - or is there an more effective way to handle this ...

 

thank you,

 

Dan

 

This topic was modified 3 weeks ago by grossdan

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grossdan
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Noticed for example the following two variant gates:

 

https://www.futurlec.com/74LS/74LS03pr.shtml   -- open collector nand

and

https://www.futurlec.com/74LS/74LS00pr.shtml    -- regular

It seems that the latter is neverthless operated by min 4.75V and high level voltage output is min 2 -- whatever that means -- i guess, its at least 2 but could be higher depending on some factors (not sure which).

So, perhaps the simple solution is to have two power sources one 3.3 V from the microbit and one 5V for powering these non open collector devices -- and, so long the signal is propagated to other logic gates all is well -- but, when something larger needs to be driven -- then some open collector version or circutry needs to be put inbetween.

 

 

This post was modified 3 weeks ago by grossdan

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Will
 Will
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@grossdan 

You can also get inexpensive level converters which do this for you ...

https://www.sparkfun.com/products/12009


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grossdan
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@will In one of the youtube videos a buffer was mentioned -- to facilitate fan out.

 

Looking up the MM74HC125 3-STATE Quad Buffers, thought to add these between a output pins of a 4 bit full adder and 5 leds.

However, i was then wondering if i could power the quad buffer from a 5V source and thereby also effectively draw up the output for the leds ...

Could this be an alternative to using a resistor to control a 5 V input to each led, or the sparkfun level converter?

thanks!

Dan

 


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Will
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@grossdan 

The purpose of the level converter is to safely connect the 3.3V pins to your microbit to any 5V devices connected to those pins. The converter will take the output pin values (0V,3.3V) on it's LV (low voltage) side and convert that to (0V,5V) on the HV (high voltage) side.

That allows you to send data from your 3.3V microbit pins to other devices or components which require 5V inputs. It also works the other way around, converting HV signal to LV signal so your 5V devices or components can send their signals to the HV side and that will be switched to the appropriate voltage on the LV side.

The level converter is intended to provide a safe and easy method for converting between two devices which operate at different voltages. All it takes to operate the LED is a resistor of suitable value to limit the current to a desired value. You can also drive one from 3.3V if you wish, so either voltage will do; you just need an appropriate resistor value.

I've never used the 3-state buffers so I can't comment on their suitability.


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grossdan
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Thank you.

I got confused ...

I was thinking about fan out and unnecessarily conflated it with up-leveling. 

Connecting each of the five output pins on a 74LS83 4 bit adder to drive 5 leds -- all the while the Adder is powered by a weak 3 V source. 

So, there will likely not be enough push to drive the led directly from those pins -- and i guess, trying to drawing too much electricity through the adder is also not healthy.

So, I see several options for such a boost:

1. introduce a buffer between each output pin and led -- each buffer is dedicated to each lead and is directly powered by a 3 V source and hence provides enough to boost each led - resistor segment -- without drawing it from the Adder.

2. use one of those up-levels to 5 V for each pin -- but this would do more than necessary since the leds could be powered by 3V as well -- up-leveling is not needed ... so, i now understand that its over kill.

3. use a transistor to use the weak output from the Adder to drive a 3 V source through the led

4. use an open collector version of the 74LS83 (if existing) to avoid the need for an additional transistor

 

(edit: i now notice that all options might actually be the same -- they all use a transistor to buffer between the output pin and the led -- just the "form" on the circuit is different)

 

Lessons i learned are:

1. sometimes voltage on a pin is intended to drive a device -- this however requires drawing electricity through that pin and its IC

2. sometime voltage on a pin is a signal only and voltage to drive something needs to come from a different source.

3. an IC always has a max electricity that can be draw through it - independent of level of its voltage source -- this must be kept below in the circuit design.

4. sometimes electricity flow must be further limited via a resistor (e.g. to protect a led) and sometimes electricity flow must be boosted -- usually via a transistor.

5. sometimes high voltage without electricity flow is a signal, can be achieved through a pull-up resistor connected to a voltage source

6. sometimes ground voltage without electricity flow is a signal, can be achieved through a pull-down resistor connect to ground.

7. high impendance (resistance) on an IC pin cuts electricity flow and is a signal to remove the pin from the circuit -- its can be used for multiplexing.

8. I think the concept of "buffering" between parts of a circuit plays a significant role -- to ensure carefully controlled electric interaction between subcircuits -- its usually achieved by a transistor packaged in an IC or placed outside.

Perhaps circuit designers "chunk" their thinking in terms of buffered subcirucits what they are and how they interact electrically to achieve the desired purposes.

that's as far as i got now 🙂

 

 

Does what i am thinking make sense ...

Dan

This post was modified 3 weeks ago 5 times by grossdan

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jker
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Posted by: @grossdan

I just noticed that ICs such as and/or gates, etc. operate around 5V while the output and input pins of the microbit and audrino (i hope to get as well) run with 3V ...

One simpler solution is just not to use LS gates if you're working with ~3V logic. The 74HC CMOS series of gates can run anywhere in the 2V-6V range for Vcc.

"A resistor makes a lightbulb and a capacitor makes an explosion when connected wrong"
"There are two types of electrical engineers, those intentionally making antennas and those accidentally doing so."


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grossdan
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@jker thank you.

Yes, i realized this the other day, what the difference between those are -- but already bought quite a few of the LS components -- the cost is part of my learning process 🙂

But, i also noticed that the LS series seem to have more components available. For example, I was not able to find an HC equivalent for 74LS83 ...

Dan

 


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jker
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For the 74LS83, I wonder if most people just switched to the 74HC283.

"A resistor makes a lightbulb and a capacitor makes an explosion when connected wrong"
"There are two types of electrical engineers, those intentionally making antennas and those accidentally doing so."


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