Suggestions for a circuit design

[MJK17]

Can anyone suggest a suitable electronic circuit for the below application.

0-6 ohm input switches on a Red LED
7-9 ohm input switches on a Green LED indication
10 to >100 ohm switches on a Red LED indication

Power supply needs to 9V cells only
LED type would be ideal as Bi-color LED.

Any advice would be most appreciated...

 

[alec_t]

Make a constant-current source of, say, 20mA. Using comparators set to different appropriate thresholds compare the voltage across the unknown resistance developed by the 20mA with those thresholds. Drive the LEDs with the comparator outputs. An LM324 has four opamps which could provide the current source and comparators.

 

[MikeMl]

The >10Ω range? Is it just ">10" or is it two subranges "10 to 100Ω" and ">100Ω"?

What is switched on if the resistance is exactly 6.50Ω? 9.45Ω?

 

[MJK17]

Thanks for the response Mike ML.

I was looking to use a Bi-color LED for the indicator.

The lower range resistance input is 0 - 6 Ohms - this should turn the LED red.
The mid range resistance input is >6 to <9 Ohms - this should turn the LED green.
The upper range resistance input is >9 ohms should turn the LED red.

If you could advise me where i could get a schematic of this circuit that would be most helpful...

 

[MJK17]

Thanks for the quick response alec_t

I was already considering the op amp circuit and glad you suggested what i was thinking. However, my electronic knowledge is only basic so i wil have to revert to some reference books for me to make a schematic of what you suggested!

Also, would it be possible to run your suggested circuit from 9V cells...?

 

[alec_t]

Yes this can be done with a 9V supply.
A bi-color LED of the 2-terminal type would be more difficult to use than a 3-terminal type.

Edit: Is this a school/college project?

 

[MikeMl]

I was looking to use a Bi-color LED for the indicator.

The lower range resistance input is 0 - 6 Ohms - this should turn the LED red.
The mid range resistance input is >6 to <9 Ohms - this should turn the LED green.
The upper range resistance input is >9 ohms should turn the LED red.

If you could advise me where i could get a schematic of this circuit that would be most helpful...

I have worked out a one-chip with two terminal bicolor LED solution. Look at the attached. Using a quad opamp, like a TLC2274. The simulation shows the current through the red and green leds (red trace and green trace) as a function of the unknown resistance at three different values of battery voltage (9,8, & 7V), showing that because the circuit is ratiometric, it is quite immune to variation of the battery voltage as it wears down...

obtw, my consulting rate is $250 per hour...