ionisiation?

Just put a high voltage on one terminal, with a 1K resistor in seriese with the input. Then wire a zenner between input and ground. The other end of the sensor goes to ground.

No like this.

probe.pdf (19.9 KB)

ok thanks
what do we call high voltage?

what do we call high voltage?

Dangerous.

Anything higher than 5V.
The higher the voltage the more current the flame will conduct.
Making the resistor higher will make any change in current a larger change in voltage and the easier it is to detect.

sorry to bring this back up again i had got this working but its extremely unstable as there is not much movement on the arduino, i was using 1500k resistor to even get it to work i cant seem to add any more than that or it seems to go the other way so i dont know how else we could make this work more reliably and give us more of a range on the arduino
i measured the resistance of the probe and flame was 1400k and my supply voltage i was running 17.11 and im getting back on the other end 1.65V so hell of a drop?

You need to use an op-amp to make the signal from the flame bigger.
Start of with a gain of about 100 and see how you get on.

ok thanks for that i have had a bit of a play and come up with this
http://s906.photobucket.com/albums/ac270/Joes-110/?action=view&current=Untitled.png
but now we have gone to the other way now it is to sensitive and can not seem to dye down any idea?

Three things wrong with that circuit.

  1. It only has a gain of 2, see how to calculate the gain of a non inverting op amp here:- Operational amplifier - Wikipedia
  2. You in effect have a floating input, that is the input is very high impedance and is picking up all sorts of interference.
  3. That sort of circuit will only amplify voltages, you are in effect putting a variable resistance on the input you are not inputting a voltage.

Try this. +ve input of amp connected to two 1M resistors, one goes to +5 and the other ground. Then feed this point and ground at your flame.
Change the resistor that goes from the -ve input to ground to a 10K. What sort of op amp are you using.

thanks for the reply i am using a LM358 as this is what i had lying around.
ok can you draw that as this is all a bit now to my and will be easier to understand thanks

You might have to play about with the value of the top 1M

amp.png

Sorry I forgot about the DC bias. Take the resistor that goes to the -ve of the op amp to ground and disconnect it from ground an put it to the wiper of a 10K pot with the two ends to ground and +ve.
This will allow you to adjust the output to be mid range with no flame, then you should see it move in the flame.

like that?

amp[1].png

No the end of the pot goes to the +ve supply not the +ve input of the op amp.
And the 100K still goes to the -ve input of the op amp.
And you still need the 10K resistor going between the wiper and the -ve input of the op amp.

ok cool i got it now thanks, will have to get some resistors in and have a play

right i tried that and it didnt seem to work when adjusting the pot it just seemed to be all the way one way or all the way the other. i dont know if i need a higher rotation pot so i can be more accurate with it? im currently using something like a 300 degree at the moment

I really don't think you need an op amp for this. The Arduino pins have very high input resistance (around 100 Mohms), so detecting the difference between 1.7Mohms and infinity is easy. I used a voltage divider made from two 4.7Mohm resistors with an atmega328p in a recent project. However, you will need to take precautions against picking up interference. I suggest you connect your ionisation probe between the input pin and ground, use a 4.7Mohm pullup resistor to +5V, and also connect a 0.1uF capacitor between the input pin and ground.

Joes:
right i tried that and it didnt seem to work when adjusting the pot it just seemed to be all the way one way or all the way the other. i dont know if i need a higher rotation pot so i can be more accurate with it? im currently using something like a 300 degree at the moment

You can get finer control by putting fixed resistors in the top and bottom of the pot so the wiper covers a smaller of voltages. Also try reducing the gain a bit.

I really don't think you need an op amp for this. The Arduino pins have very high input resistance (around 100 Mohms), so detecting the difference between 1.7Mohms and infinity is easy. I used a voltage divider made from two 4.7Mohm resistors with an atmega328p in a recent project. However, you will need to take precautions against picking up interference. I suggest you connect your ionisation probe between the input pin and ground, use a 4.7Mohm pullup resistor to +5V, and also connect a 0.1uF capacitor between the input pin and ground.

i did try this but did not seem to work

You can get finer control by putting fixed resistors in the top and bottom of the pot so the wiper covers a smaller of voltages. Also try reducing the gain a bit

i haven't altered the gain at all, i presume to lower the gain i reduce the value size of the 100k which is connected to the output of the op amp?
right i think i've got it working in a matter of speaking with out altering the gain the problem i have now is it seems to be too sensitive still so putting my finger between the probes is enough to activate it.

still so putting my finger between the probes is enough to activate it

That would be normal for a high gain high impedance circuit like you have here.