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Topic: ionisiation? (Read 2309 times) previous topic - next topic

Joes

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

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

dc42

#32
Dec 01, 2012, 07:48 pm Last Edit: Dec 02, 2012, 10:33 am by dc42 Reason: 1
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.
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Grumpy_Mike


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.

Joes

Quote
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


Quote
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.

Grumpy_Mike

Quote
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.

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