Need a way to measure ohms

[PčéJäy]

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.

 

[Rod Speed]

PčéJäy <wiseguyatiinetnetau> wrote in message
news:3fd027b4_3@news.sydney.pipenetworks.com...

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

Dud assumption.

so I need something else. The best I've found so far is:

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

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

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??

Those just cause steam to pour out of the ears.

 

[Terry]

PčéJäy threw some tea leaves on the floor
and this is what they wrote:

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??

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

Paul.

--
Kind Regards from Terry
My Desktop is powered by GNU/LinuX, Gentoo-1.4_rc2
New Homepage: http://milkstone.d2.net.au/
** Linux Registration Number: 103931, http://counter.li.org **

 

[gcd]

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


"PčéJäy" <wiseguyatiinetnetau> wrote in message
news:3fd027b4_3@news.sydney.pipenetworks.com...

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.

 

[PčéJäy]

"gcd" <gcdnoSPAM@austarmetro.com.au> wrote in message
news:3fd07457@news.comindico.com.au...

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

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!

 

[The Real Andy]

"PčéJäy" <wiseguyatiinetnetau> wrote in message
news:3fd027b4_3@news.sydney.pipenetworks.com...

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??

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.