richacm

Hi,

I have a PIC chip that I want to output a digital signal from (easy). However the component that receives this signal has a 3V max on it. How do you normally go about reducing the voltage on the line? Should I put 3 diodes (1N914) in series on the line to drop the voltage by 2.1V?

Thanks,

Craig

dknguyen

You can do many things:
-diodes
-resitive dividers
-level translation ICs
-comparators
-op amps
-pull-down transistor with a pull-up resistor
-resistor with zener diode
-resistor with positive rail clamp diode

http://www.newark.com/pdfs/techartic...ricksBrchr.pdf

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richacm

What a great link. Thanks!

Nigel Goodwin

You really need to give more details of EXACTLY what you're trying to feed, that makes a huge difference - but your diodes suggetsion is probably the worst possible idea.

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Boomslang

Have a look here: http://www.sparkfun.com/datasheets/S..._datasheet.pdf on page 17 for a circuit using the 74HCT08, Ive used it on RS232 and SPI signal lines without problems.

ericgibbs

hi,
You could use a HEF4050 non inv CMOS buffer, will tolerate input voltages higher than Vcc.

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Krumlink

I agree with eric, but a different chip. Use a 74AC245 supplied with 3VDC. It is a non inverting buffer that takes the 5VDC input and outputs the 3VDC signal, creating a effective level converter. the 245 can sink/source up to 50ma in some cases and can be supplied from 2VDC to 6VDC, but in your case use 3VDC.

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Mikebits

This may have worked for you in the past but this is not a good idea as it exceeds the Vin spec of the part.

The LVC logic family can operate at 3v and tolerate 5Vin.

Krumlink

hmm I wonder why I said AC, maybe because its what I have on hand, sorry about that.

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Mikebits

They both have a C in them, easy mistake

petrv

Hi,
there are many approaches for logic level translation, one of the simplest is to use open-collector (or open drain respectively - LS,HCT...) logic - gates, inverters, buffers (e.g. 74xx03,74xx05,74xx07 ...) - for low speed logic it is ok, just connect the pull-up resistors to the voltage you want for Vh (3.3 or 5V) for the output.

Another approach is to use specialized ICs, for example from MAXIM: (MAX337x, MAX339x)

MAX3372E, MAX3373E, MAX3374E, MAX3375E, MAX3376E, MAX3377E, MAX3378E, MAX3379E, MAX3390E, MAX3391E, MAX3392E, MAX3393E ±15kV ESD-Protected, 1µA, 16Mbps, Dual/Quad Low-Voltage Level Translators in UCSP

or there is a chip from TI: (SN74LVC2425)

Octal Bus Transceiver And 3.3-V To 5-V Shifter With 3-State Outputs - SN74LVC4245A - TI Product Folder

just an example, there are more possibilities to implement the level translator

STMicroelectronics - Level Translators

Petr

Krumlink

Petrv, it has already been covered! thank you for the extra info but I think the OP has figured it out hopefully

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petrv

I know I just wanted to point out the additional possibilities. By having more choices it is possible to select the solution most suitable for the original problem

And perhaps it can help someone else who is dealing with a similar problem as logic level translation is a common issue, especially between 5V and 3.3V logic.

Petr

Boomslang

This is one of the best 5V-3V docs I found so far: http://www.microchip.com/stellent/gr...c/en026368.pdf