The only stupid question is the one you don't ask.

You can often use a voltage divider made from resistors to reduce the voltage like that.

 

You simply use an inverting opamp, this can give positive or negative gains - a non-inverting one can't go below a gain of one. Not that it's a problem, you just use an attenuator to drop the gain.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

You simply use an inverting opamp, this can give positive or negative gains - a non-inverting one can't go below a gain of one. Not that it's a problem, you just use an attenuator to drop the gain.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

so is it better to use a opamp instead of a voltage devider to measure sensor signals over 5v, i think they are all 5v but i want to measure battery voltage so i will have to use a opamp or voltage devider to do that as the voltage can go up to 15v when the engine is running.

is there a formula to calculate the resistors to be used in the opamp circuit to get the correct inverted voltage output

many thanks

jason

 

No point using an opamp, a simple voltage divider is all you require - it's a VERY low impedance source, so you can use fairly low value resistors, and keep the impedance to the PIC low.

Yes, it's VERY, VERY simple - try googling for "741 tutorial" if you know absolutely nothing about opamps.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

the only problem i think i might have with just using a voltage devider is that the 12v battery feed has lots of spikes ect on it, do i just need to add a zender diode into the devider circuit or will that not work in a voltage devider circuit

 

You can add filters, including zeners - but bear in mind the attenuator will reduce the spikes as well. and the PIC input includes protection diodes.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

i'v never used attenuator or filter circuits, can you recommend any good web sites to learn about them, the electronic book i read talked alittle about filters but most of it went in and back out again. but iv never even heard of attenuator.

 

An attenuator (in this case) is simply a potential divider (two resistors).

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

The problem with using a potential divider on car sensors is that it can alter the characteristics of the original sensor.

For example a Vauxhall GTE temperature sender has a resistance of 3410 ohms at -10 degrees and 2030 ohms at 0 degrees. The PIC requires around a 2k input to the A2D converter (well the 18F series do) so your potential divider is going to have to be something like a 1k+2k resistor. This will effectively halve the sensors resistance at -10 degrees leading to a misreading.

For measuring battery voltage this isn't an issue at all but for some of the more sensitive sensors it could be.

 

Which was why I specifically mentioned that reason - if it's for a high impedance source then simply use a higher value potential divider followed by a buffer (as my tutorial hardware does).

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

Want a fight ?

Actually thats how I've designed the input stages for one of my new products.

High resistance voltage divider for the input stage on a 1/10 divider giving me an input to an LM358DT opamp ranging from 0-1.6 volts (typical vehicle voltage range) and an X2 gain on the opamp giving me out a 0-3.2v output from the LM358DT running from a supply of 5V. The LM358 will not give out any more than around 3.5v from a 5v rail so thats the PIC protected. It will also handle up to 30v on its input pins with a 5v supply (confirmed by ST) so any spikes on the system up to 300 volts will not kill the opamp. Output from the opamp to the PIC is Opamp -> R -> C -> R -> PIC to smooth out any nasty little glitches.

Works for me

 

Oh and the fact that I picked up nearly 5000 x LM358DT for a tenner makes it even more attractive to use as a buffer

 

Jason - I've just had a quick browse of Nigels tutorials to see if I could find the circuit he was talking about above - you should be able to use it as it is for what you want to do.

http://www.winpicprog.co.uk/pic_tuto...ogue_board.htm

 

Notice how I neatly avoid the requirement for a rail-to-rail opamp (which don't really go fully to either rail anyway).

As for a fight?, a little extra training always comes in handy!

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk