Post by thread:12 Volt Logic to 5V Logic

A PNP transistor + 1 resistor! Andrew On Thu, 9 Mar 2000 15:57:11 -0000 Nigel Porter <spam_OUTnigelporterTakeThisOuTKELVEDON99.FREESERVE.CO.UK> writes: {Quote hidden}> I have 12 Volt logic signals (0 Volt or 12 Volt signal) which I wish > to convert to 5 Volt logic (0 Volts or 5 Volts signal) levels. I am > considering the use of:- > > Opto-coupler > > analog switch (4066 or similar) > > resistor and 5V1 zener > > Can anyone suggest a good method? > > I intend on interfacing automotive system to a PIC16F84 > > Thanks > > Nigel ________________________________________________________________ YOU'RE PAYING TOO MUCH FOR THE INTERNET! Juno now offers FREE Internet Access! Try it today - there's no risk! For your FREE software, visit: dl.http://www.juno.com/get/tagj.

Hi You might want to use two resistors. This is not a completely fail-safe way implementing this, but I''ve used it previously with varying results. Connect as follow: +12V (Vin) | | | = | | R1 = | |------- +5V (Vout) | = | | R2 = | | | GND The formula works as follow: Rt=R1+R2 Lets say we want Rt (total resistance in the series) to be 10K Then Rt=R1+R2=10K Vout=5v (wanted) Vin=12V (given) The relationship is: R2/Rt=Vout/Vin Therefor, R2=Vout/Vin*Rt R2=5/12*10K R2=4166.66 Ohm Then, because Rt=R1+R2, then R1=Rt-R2 R1=10Kohm - 4.16666Kohm R1=5.833.34 Kohm Hope it helps! Werner Soekoe .....wsoekoeKILLspam@spam@global.co.za ----- Original Message ----- From: Nigel Porter To: PICLISTKILLspamMITVMA.MIT.EDU Sent: Thursday, March 09, 2000 5:57 PM Subject: 12 Volt Logic to 5V Logic I have 12 Volt logic signals (0 Volt or 12 Volt signal) which I wish to convert to 5 Volt logic (0 Volts or 5 Volts signal) levels. I am considering the use of:- Opto-coupler analog switch (4066 or similar) resistor and 5V1 zener Can anyone suggest a good method? I intend on interfacing automotive system to a PIC16F84 Thanks Nigel

or use diode and variable resistor: 12v ! ! variable res ! !------------5.1volts diode ! ! ground then calculate res for adecuate current needed. Lucky, Ismael. Werner Soekoe wrote: {Quote hidden}> Hi > > You might want to use two resistors. This is not a completely fail-safe way > implementing this, but I''ve used it previously with varying results. > Connect as follow: > > +12V (Vin) > | > | > | > = > | | R1 > = > | > |------- +5V (Vout) > | > = > | | R2 > = > | > | > | > GND > > The formula works as follow: > > Rt=R1+R2 > > Lets say we want Rt (total resistance in the series) to be 10K > Then Rt=R1+R2=10K > Vout=5v (wanted) > Vin=12V (given) > > The relationship is: > R2/Rt=Vout/Vin > Therefor, > R2=Vout/Vin*Rt > R2=5/12*10K > R2=4166.66 Ohm > > Then, because Rt=R1+R2, then > R1=Rt-R2 > R1=10Kohm - 4.16666Kohm > R1=5.833.34 Kohm > > Hope it helps! > > Werner Soekoe > .....wsoekoeKILLspam.....global.co.za > > {Original Message removed}

> I have 12 Volt logic signals (0 Volt or 12 Volt signal) which I wish to > convert to 5 Volt logic (0 Volts or 5 Volts signal) levels. I am considering > the use of:- I would probably use a current limiting resistor (about 3.3-10K) and a pair of clipping diodes. It would be safe and work with anything from 5V up. Well, not exactly, it could burn something if you plug it to 5,000V ;) Best regards, Brusque

<x-rich>The safest bet is to use optocouplers. Voltage spikes and noise tend to bite you in the rear end when you least expect it. Unless it's going to be produced in high volume, it's worth the money to get optocouplers. Cheers Terry At 03:57 PM 3/9/00 -0000, you wrote: >>>> <excerpt>I have 12 Volt logic signals (0 Volt or 12 Volt signal) which I wish to convert to 5 Volt logic (0 Volts or 5 Volts signal) levels. I am considering the use of:- Opto-coupler analog switch (4066 or similar) resistor and 5V1 zener Can anyone suggest a good method? I intend on interfacing automotive system to a PIC16F84 Thanks Nigel </excerpt> </x-rich>

Nigel; You must judge the method by the quality and speed of the signal. If you have fast signals, you should avoid the plain old resistor dviders or transistor solutions. If you have noisy signals, and cannot withstand brief false logic states, then you also must avoid the above. If these cases are true, try using quad comparators and cover each input with a comparator. This would be one proper way to do it. Then you can add a threshold and hysteresis if you desire. The output can be pulled up to 5V, or use a totem pole output comparator and no pullup. You can invert the state by reversing + and - on the comparator. I have done the transistor method before, and they tend to start to turn on at 0.7V. This is not much, and a bit of noise can whack you over the head. You can add a zener in series, but leakage currents can sneak up on you and damn near turn it on anyhow. I have found myself sprinkling capacitors to cover up noise problems. G'luck. Chris Eddy Nigel Porter wrote: > I have 12 Volt logic signals (0 Volt or 12 Volt signal) which I wish > to convert to 5 Volt logic (0 Volts or 5 Volts signal) levels. I am > considering the use of:- Opto-coupler analog switch (4066 or > similar) resistor and 5V1 zener Can anyone suggest a good method? I > intend on interfacing automotive system to a PIC16F84 Thanks Nigel