Jiggy-Ninja:
What objects are you working with that make you think you need a resistive sensor in this particular configuration?
the resistive plate is actually a rectangle-base box containing water
SumitAich:
the resistive plate is actually a rectangle-base box containing water
How do you measure the skin's impedance with that?
aarg:
How do you measure the skin's impedance with that?
i measured the skin impedance separately(finger to finger impedance) with a DMM came out to be 3 Mohm. My project aims at finding x-coordinates of point of contact of finger and water surface.
SumitAich:
i measured the skin impedance separately(finger to finger impedance) with a DMM came out to be 3 Mohm.
Yes but that is heavily dependent on how moist the skin is. This changes from person to person.
My project aims at finding x-coordinates of point of contact of finger and water surface.
I don't think you can do that. Pleas explain how you are going to attempt it. Water can be an insulator or very good conductor depending on contaminates like dissolved salts. As you continue to use this any water will become more contaminated and the resistance of the water will change. Certainly your finger to finger resistance will not stay the same if your fingers are in water.
Grumpy_Mike:
Certainly your finger to finger resistance will not stay the same if your fingers are in water.
Yeah so i need a larger input impedance so that slight variations in skin impedance does not matter much to the input voltage.
Refer the diagram in post #7
Can we have an IC(a buffer gate) , between finger and Arduino digital input pin , which has a larger input impedance than 100Mohm . I am aiming at 300Mohm so that the skin impedance to input impedance ratio is 1:100.
Use a JFET op amp. The input impedance will be about 1000G (1012) ohm. Good luck.
Yeah so i need a larger input impedance so that slight variations in skin impedance does not matter much to the input voltage.
No you don't. You seem not to understand how electricity works. You already have a very large input impedance on the analogue pins.
Grumpy_Mike:
No you don't. You seem not to understand how electricity works. You already have a very large input impedance on the analogue pins.
Sorry for arguing. Yeah ADC worked fine. Just that dipping my fingers depper into water changes skin impedance and hence the analog reading. Any way round this can you suggest?
I think only viable way would be some capacitive sensing. You measure capacitance between some points at the sensor and from result at different points you guess location of the finger. I never did anything much close to this but in my (only) experiment with capacitive sensing & water capacitance increased a lot when I touched the water. So maybe it could work. But expect lot of experimenting and theory to study before it will work reliably.
Smajdalf:
I think only viable way would be some capacitive sensing. You measure capacitance between some points at the sensor and from result at different points you guess location of the finger. I never did anything much close to this but in my (only) experiment with capacitive sensing & water capacitance increased a lot when I touched the water. So maybe it could work. But expect lot of experimenting and theory to study before it will work reliably.
can you provide a link to your experiment blog or video.
SumitAich:
can you provide a link to your experiment blog or video.
It is only in this thread. But you may find links to some interesting app notes.
Smajdalf:
I think only viable way would be some capacitive sensing. You measure capacitance between some points at the sensor and from result at different points you guess location of the finger. I never did anything much close to this but in my (only) experiment with capacitive sensing & water capacitance increased a lot when I touched the water. So maybe it could work. But expect lot of experimenting and theory to study before it will work reliably.
I don't think that will work. The capacitive slider designs I've seen all use two or more pads, and the controller computes finger position based on the different reads from the pads. That would require partitioning the water, probably not possible to do.
Treating the water volume like a potentiometer and the finger like a wiper might actually be the best method to doing this, but that's not saying much since it's still a really bad method. Having your body in the mix is going to make it noisy as all hell, and the high impedances involved in the input of the amplifier circuit will make this a huge challenge. At a minimum you're going to need some instrumentation amplifier or a FET input op amp on the input stage to use as a buffer.
SumitAich:
Yeah ADC worked fine. Just that dipping my fingers depper into water changes skin impedance and hence the analog reading. Any way round this can you suggest?
No it doesn't change the skin impedance, it changes the resistance because you are increasing the the contact area which of skin / water and so reducing one element of the series resistor chain you have. This is not mitigated by increasing the input impedance of your A/D.
If you want to measure how far one finger is along the track, then you need to put a voltage across the water cell and connect an electrode to one hand, use a metal contact and something like KY Gell to make a good contact. Then put your other finger in the water and measure the voltage. Then reverse the direction of the voltage across the water and measure again. The difference in the two measurement could give you the distance along the water. You might have to use distilled water to be able to establish a good potential across the water.
I think due to high resistances OP is measuring more (parasitic) capacitances in the circuit than real voltages. Adding small (1nF?) cap between ADC pin and GND may make it better? It would charge slowly via large resistances of body and then quickly fill the sampling cap of ADC? Just ideas, maybe it is nonsense... Because if the ADC channel is not switched the sampling cap (and pin capacitance) should keep the same charge and does not need refilling?
Grumpy_Mike:
. Then put your other finger in the water and measure the voltage. Then reverse the direction of the voltage across the water and measure again. The difference in the two measurement could give you the distance along the water. You might have to use distilled water to be able to establish a good potential across the water.
Alright I'll use distilled water . But I didn't understand how the difference in the two reading gives distance? Distance can be calculated from just one reading.
Distance can be calculated from just one reading.
Only if all the other factors are constant, which in your case they are not.
It looks like it is not so hopeless after all. I tried this with my lunch box and tap water. I had 9V battery as voltage source, 3k3 current limiting resistor and jumper wires dipped in the water as electrodes about 10 cm apart. Voltage over the resistor was about 4.5V (~1.5mA current) while dipping electrodes of DMM into water showed about 0.5V difference when they were close to jumper wires and the reading changed nicely when I moved the electrodes. I have no idea where rest of the 9V supply voltage was lost - probably metal/water contact (funny bubbles formed on the GND wire).
When I touched GND and connected black wire of the DMM to it and touched red electrode of the DMM with another finger of the same hand and dipped another finger of the other hand into the water the DMM showed a bit more than 1V. When moving the finger in water closer/further from GND jumper wire the voltage decreased/increased - total difference was about 300mV. Since I have some cheap DMM with internal resistance around 10Mohm it should be doable on Arduino. But what troubles me are the funny little bubbles - I suspect they were eating the jumper wire. Probably something more durable is needed for some long therm solution.
BTW I did not notice any change if I dipped my finger more into the water. I don't know how DMM ADC work but AFAIK it is not SAR which Arduino has. Probably it is more immune the unknown thing that caused the change you described. Or my setup was too different from yours.
(funny bubbles formed on the GND wire).
That is the electrolysis of water, each electrode will bubble, one hydrogen and the other oxygen. That is one of the reasons you must keep swapping the polarity over. As the bubbles form they insulate the wires and cause the contact resistance between water and wire to increase. By reversing the polarity constantly you prevent this.
I suspect they were eating the jumper wire.
When you pass a current through a liquid also ions will transfer from the anode to the cathode, that is how electro plating works. Electro plating
If you collect this gas mixture it will explode violently when lit.
Distilled water is an insulator. But the sweat from your skin and metals leaching from the wires will change that.
Grumpy_Mike:
That is the electrolysis of water, each electrode will bubble, one hydrogen and the other oxygen. That is one of the reasons you must keep swapping the polarity over. As the bubbles form they insulate the wires and cause the contact resistance between water and wire to increase. By reversing the polarity constantly you prevent this.
A big thanks to you that greatly increased the precision 