project help - interfacing microcontrollers with sensors

[tomas632]

Hi Guys
I have a project where i need to interface some sensors with a microcontroller. However the micro runs off 3.3V and so does one of the sensors. However one of the sensors is 5V and gives a voltage output that I need to read in via the microcontroller ADC. Could anyone suggest a way of interfacing this sensor? The microcontroller pins are not 5v tolerant and the only way I could think of doing this would be to do a potential divider but wasnt sure if this would be accurate enough. Hope someone can help

 

[ericgibbs]

Hi Guys
I have a project where i need to interface some sensors with a microcontroller. However the micro runs off 3.3V and so does one of the sensors. However one of the sensors is 5V and gives a voltage output that I need to read in via the microcontroller ADC. Could anyone suggest a way of interfacing this sensor? The microcontroller pins are not 5v tolerant and the only way I could think of doing this would be to do a potential divider but wasnt sure if this would be accurate enough. Hope someone can help

hi,
A resistive divider would work OK, ensure that the divider impedance [resistance] is less than 10K.
A top resistor of 1.8K and a lower resistor of 3.3K would lower the 5V to 3.3V.

For an exact reduction, split one of the resistors into a fixed value and a variable.

 

[tomas632]

hi, thanks for the response. Why is it necessary to have the resistance less than 10k? Also would it be better to put a buffer after the voltage divider?

 

[ericgibbs]

hi, thanks for the response. Why is it necessary to have the resistance less than 10k? Also would it be better to put a buffer after the voltage divider?

hi,
The charge time of the internal capacitor from the external source is determined by the series resistance.
To ensure that the internal capacitor fully charges during the acquisition time of the adc the external resistance has to be less than 10K or so.

You dont need a buffer after the divider.
What is the +5V voltage source.?

 

[tomas632]

the 5v source is going to be a standard voltage regulator, undecided on which to use yet though

 

[ericgibbs]

the 5v source is going to be a standard voltage regulator, undecided on which to use yet though

hi,
This is a simulation I did last year, for a PIC, shows the effect of the source resistance.
The circuit is based on the PIC datasheet.