hi,
the attached circuit reliably tests 1ohm and below.
i want to test 3 ohms and below, what change in component values would i have to meke to achieve this please ?
(i think they would only be resistor value changes but i am not that clever with understanding how to calculate what is required.)
i basically want a "go / no go" quick tester for less than 3 ohms

thank you dave

 

Hi,
try R5 to 6,8 kohm

 

i basically want a "go / no go" quick tester for less than 3 ohms
thank you dave

View attachment 355890

Is this for an igniter tester?

 

hi sghioto,
yes it is, BUT only the bare chip, no pyrogen, so no danger,
can you help ?
will the change of R5 to 6.8k ohm work ? i have not tried it yet
dave

 

I believe so as that value of 6.8K is appx 3 times the value of R5.
BTW the max current through the igniter is only 16ma, should be safe with the pyrogen.

 

ok i will give it a go

 

That circuit schematic looks wrong to me. The transistor is merely switching on power to the op-amp, although there is constant leakage current from battery to the IC's (+) input. The buzzer does not connect to the LED but rather to the op-amp output pin 1.
Another variation on the web adds an LED to the input. It could use a decoupling capacitor on power.

Just mentioning it's a circuit with mistakes IMHO.

 

Just mentioning it's a circuit with mistakes IMHO.

I don't see any of those issues.
Igniter is connected, the power switch is turned ON.
If the buzzer sounds and the LED lights up then the igniter resistance is above 3 ohms.
After all the TS mentioned it was reliable in post #1.

 

Looks suspiciously like this design, without the circuit protections.
The connection to the LED and buzzer looks strange? is that to clamp the buzzer voltage at ~3Volts?
In my design R11 sets the threshold of detection.

 

I believe so as that value of 6.8K is appx 3 times the value of R5.
BTW the max current through the igniter is only 16ma, should be safe with the pyrogen.

it actually works better with R5 as 5k ohms, it will the light the LED at 3ohms and under, but will not light it at 3.3 ohms.
excellent, job done. (i decided not to use the buzzer).

 

Looks suspiciously like this design, without the circuit protections.
The connection to the LED and buzzer looks strange? is that to clamp the buzzer voltage at ~3Volts?
In my design R11 sets the threshold of detection.

i have not seen that design and i am not an electronics guru like some of you guys.
that is why i occasionally ask for help.

 

it will the light the LED at 3ohms and under, but will not light it at 3.3 ohms.
excellent, job done. (i decided not to use the buzzer).

As the circuit is shown in post #1 the LED should only light if the resistance is above 3 ohms.
Pins 2 and 3 must be reversed to light the LED below 3 ohms, shown below.

 

As the circuit is shown in post #1 the LED should only light if the resistance is above 3 ohms.
Pins 2 and 3 must be reversed to light the LED below 3 ohms, shown below.
View attachment 355957

yes you are right, the led only lights when 3 ohms or less, which is what i want.
thank you for your help
dave