View Full Version : 3.3v to 5v level shifter

03-10-2011, 08:42 PM

i have tried to buy max3372, lm3350 and QS3861 Ic's in Mumbai.

i have to interface aubtm-20(3.3v supply) with 8051(5v supply)

does anyone here know where i can buy these IC's or are there any alternatives to this.

level shifting is needed for UART communication betweeen autbm-20 and 8051.

thank you,


03-11-2011, 01:09 AM
Option (A)
direct link without silicon. but protective resistor (5.1kohm~10k) between Tx_8051 and Rx_3.3ic.
similarly add resistor to any another output of 8051(5V0 to 3.3Vic input.
this method relies and works on -
-output levels of 8051, 5v are ok can be read by 3.3v logic inputs as long as you are not injecting damaging current in the lowevr voltage input
-input levels "maybe" ok, hopefully the threshold of 8051 logic input is about 2.5V.

for 8051 output, same as option-A, but to eliminate the "maybe" on inputs -
add a BJT with base connected to 3.3voutput through 1kohm. the collector goes to vcc5V, through 1kohm. the collector connects to 8051input.
the BJT could be your favourite bc547 etc.

03-11-2011, 01:11 AM
Question - if know what logic operation will be performed by using a BJT in option-B, you can go ahead and try the two solutions, if not, you are better off searching for a logic translator IC.

03-11-2011, 09:30 AM
Best use

I have used it with My MicroSD Module

03-11-2011, 10:01 AM

one more question:

do you think the circuits you mentioned can operate at 115200bps baud rate?

03-11-2011, 10:09 AM

74HC4050 is a high to low level shifter.
what about low to high level shifting?

03-12-2011, 03:15 AM
>do you think the circuits you mentioned can operate at 115200bps baud rate?

Well, first i answered this , then I realized I would steal the joy of designing and learning from you if I gave you an instant answer.
Rather Id show you how its done -

First - 115.2kbps is how many KHz in worst case ? (quiz). say it is "B"
Second- look up the datasheet of BC547, what is the gain bandwidth product of that transistor ? say "F"
Although you will run at gain of 1, in that case as long as F > B you are good.
Even if you run at gain of 10, then need to verify that F/10 > B.
As a thumb rule, simply verify is F/100 > B

besides that though -
I still see that it will require a little more thought and consideration than just trying the circuit directly -
-i.e. -- The UART works on reverse logic,
since you don't have a max 232, (which is basically inverters, with RS232 level shifters) - if you directly connect to 8051 following my suggestion, you might have to invert the signals to logic understood by 8051 uart pins.
Which simply implies, using more BJTs. (a bjt inverter)
So for 3.3v ---> 5V you already get and inverter built-in if you follow option-B.
for 5v --> 3.3v-, you can figure that out.

Best Regards

03-12-2011, 12:49 PM
i know it was a silly question from me. It's just that, i am under some much pressure and hardly any time-left to complete the whole project that I selfishly tried to reduce my calculation work and ask you directly instead.
anyways, I deeply thank you for helping me out. I will surely try out the methods mentions by you and reply if it works out.

Vivek M

03-13-2011, 07:20 AM
it was not silly question. it was far better than , asking a code or circuit outright.
good luck

03-13-2011, 10:55 AM
its not the fault of students who ask for direct codes or circuit layouts. Its the education system that sucks. Only theory is taught is colleges. but doing electronics in pratical and in theory is Completely different. Understanding a concept in electronics if far easier than implementing it in practical. All because we ignore simple basics in electronics. Hope one day the system focuses more on practicals.

03-14-2011, 12:04 AM
I don't agree.
One should not blame the system "only". one cannot.
students need to do their part, and learn to improve and work hard, even with poor system. then they will know the value of good vs bad system, and when that hard working generation grows into decision making adults, they will change the system.
I am not saying that our older generation is not hardworking, its just that they haven't seen the competition and flat world we see now. plus the desire to make our country a developed, industrial dynamic work house was missing, because people are contended with basic needs being, (or working towards basic needs first.)

I would like quote, with minor change - that "ask not what your country can do for you..."
but ask what you can do for the country and yourself.
the sooner you understand, that one cannot go around blaming the "system" and not doing their hardwork the better for them.
They say, even God doesnt help those who dont help themselves, then the system is just a bunch of humans.

Don't even think that I am lecturing because I can, but I am because I have seen all aspects, - of the system, the students and also endured that.
I was in a BE college where my teachers cleared their doubts in private with me, and all you can complain about missing facilities, and labs etc, it was all there.
but I still cannot blame them completely for the bad engineers churned out.
the zeal to work hard was missing.
genius is 99% sweat, and 1% luck.

and i can lecture it all, idealistic nonsense for some, because i believe it all.
I dont answer any posts asking for the complete circuits or solutions if I see that the poster has not done any homework.
Yours was answered, because you did some work, found alternative chips, and tried to find them in market.
Now, hopefull u see the difference, and stop sympathizing with those who dont put in their 18hrs a day.

anyway, enough digression.
yes, the bjts will work, if you didnt already figure that out.
you will need to correct the logic by inversion (again same bjt inverter) though.

good luck

03-14-2011, 07:44 AM
True Vivek, students do not want to study B.Tech for quest of knowledge. The B.tech stamp is a mere passport for a good future and an IT company. I am currently the project guide of few students in their final year, and it was painful to see that they could not differentiate between a buffer and a diode symbol, or a battery and a capacitor symbol. Although these students came from a some good college with decent marks.

03-14-2011, 11:31 AM
I can also say the same , Ive stopped taking trainees under me ... for ex - ECE Guys cannot read a resistor. Dont know how transistors work :) ... or how to use them. Yet all are placed at leading companies.

03-17-2011, 12:16 PM
Have you considered using a 3.3V uC instead ? 89LV51RD2 for example ?
Nice to see that people still bash the education system.In India getting a job in a corporate is a cakewalk if you can do two languages well : English and C.Its a stepping stone to the final destination in sales or management ( supervision and follow up).