Need a way to measure ohms

Hi,

I'm building a circuit where I need to connect a sensor to a microcontroller
(AVR MEGA128). The sensor has a full value range of 5-100 ohms. I'm assuming
because of the low resistance building a voltage divider would waste too
much power so I need something else. The best I've found so far is:

http://www.circuitsonline.net/circuits/view/27

Only problem is that I would need to use two AD converters to measure the
voltage and it doesn't run off 5v.

Any Ideas??


Paul.

 

Dud assumption.

One way to measure resistance is to apply a constant current
and measure the voltage dropped in the variable resistance.

Those just cause steam to pour out of the ears.

 

PèéJäy threw some tea leaves on the floor

A constant current source and measure the voltage using one a-d within
the Mega128 ?

 

Hi,
the a-d on the atmel is 10 bit, what sort of resolution do you need, 1 ohm,
0.1 ohm ?

for example

if you connect a 1k resistor in series with your sensor and connect this to
5V then you will have a range of ~ 25mV to 0.45V to send to the A-D. The
a-d has a resolution of 5mV giving about 1 ohm resolution. You just need to
do some math inthe micro to convert back to a resistance

You could put an opamp with a gain of say 2 or 3 or 5 in between this cct
and the atmel if you wanted better resolution.

+5
|
1k
|
-------A-D
|
sensor
|
GND

Cheers
Greg

 

Hmm, didn't realise I would get such good resolution with a high value
resistor. I was thinking something like 100 ohms which is a waste of power
and might damage the sensor. ~1 ohm is fine.

Thanks!

 

If the Mega has an AD, then use the voltage divider. If you are worried
about power, use a spare port pin to supply power to the sensor. Switch the
pin on when you are making a measurement.

Alternativly, if you have no ADC, charge up a cap and measure how long it
takes to discharge through the sensor.