Artificial pancreas

GopherT

Joined Nov 23, 2012
8,012
http://www.sciencebuddies.org/science-fair-projects/project_ideas/HumBio_p040.shtml#procedure
Hi
We are trying to do this as the Science project.The problem is that the pump does not even start.We followed everything,but the circuit is not doing anything.please help
Look closely at the breadboards to the right-hand side of each photo of a breadboard. Make sure your circuit is the same. Also, make sure of your resistor values. Don't just look at the color codes. Make sure of the values by measuring them.

Finally, make sure your transistor is the same model and oriented correctly.
 

wayneh

Joined Sep 9, 2010
16,162
Wow, with no photos or diagrams of your actual work, we're supposed to diagnose your problem?

Here are some things to get started:
Does your pump work when powered directly?
Are your batteries fresh?
What MOSFET are you using? (Check and double check the pinout. It's very easy to get it wrong, or inserted backwards into the breadboard.)
Do you have a multimeter to check some voltages? If so, measure some your nodes and report what you find. The MOSFET gate voltage especially.
Also, check to see if you can see an ohms variation across your sensor.
Divide and conquer - figure out what is working and what is not.
 

ronv

Joined Nov 12, 2008
3,770
http://www.sciencebuddies.org/science-fair-projects/project_ideas/HumBio_p040.shtml#procedure
Hi
We are trying to do this as the Science project.The problem is that the pump does not even start.We followed everything,but the circuit is not doing anything.please help




Mods edit:
Please don't do double post, one thread is enough.
I think there may be an error in the circuit. The 100k and 300k resistors should be swapped. Then of course the sensor must be in the acid.
 

cmartinez

Joined Jan 17, 2007
6,676
After seeing the diagram:

artificial-pancreas-circuit-diagram.jpg

Wouldn't it be best if PWM were to be used instead of trying to control the voltage at the mosfet's gate? In fact, I have very serious doubts about the practicality of the circuit above.
 

wayneh

Joined Sep 9, 2010
16,162
I think there may be an error in the circuit. The 100k and 300k resistors should be swapped.
I was going to question that, because the MOSFET has a threshold voltage of just 2.5V, but the schematic is for a 12V supply. With just 6V, I think you're right. There's no way to pull the gate high enough otherwise.
 

wayneh

Joined Sep 9, 2010
16,162
...I have very serious doubts about the practicality of the circuit above.
It'll work. Just mentally replace the sensor with a wire, or a switch. In fact, that would be a good test for the TS to run. If that doesn't trigger the MOSFET, he can rule out the sensor. The 100k resistor could probably be eliminated. Just absorb it into the pot.
 

cmartinez

Joined Jan 17, 2007
6,676
It'll work. Just mentally replace the sensor with a wire, or a switch. In fact, that would be a good test for the TS to run. If that doesn't trigger the MOSFET, he can rule out the sensor. The 100k resistor could probably be eliminated. Just absorb it into the pot.
But wouldn't it be better to use a traditional bi-polar transistor instead? Just wondering, I'm not the expert here.
 

ronv

Joined Nov 12, 2008
3,770
It'll work. Just mentally replace the sensor with a wire, or a switch. In fact, that would be a good test for the TS to run. If that doesn't trigger the MOSFET, he can rule out the sensor. The 100k resistor could probably be eliminated. Just absorb it into the pot.
Removing the 100k is a good idea.
So @maliha tamkeen why don't you try that?
 

sailorjoe

Joined Jun 4, 2013
361
I would really appreciate if someone suggest the solution.
First, use a spare wire and put a short circuit across your mosfet, from drain to source. Does the pump turn on?
If that doesn't work, then there is either something wrong with the pump or with power to the pump. Fix whichever it is and let us know.
If that works, then remove your spare wire and use it to put a short circuit across your conductivity sensor. Does the pump turn on?
 

Alec_t

Joined Sep 17, 2013
10,734
With the circuit shown there is a good chance that the FET will not turn on fully and will overheat. It really could benefit from a snap-action gate-driver. I suggest you research 'comparator' and 'hysteresis'.
 

wayneh

Joined Sep 9, 2010
16,162
Right... how about a darlington then?
I don't see a reason the avoid the MOSFET. Getting voltage should be easy, it's current that is lacking.

It's possible it might work best without either of the bias resistors. The pot at ~500K should pull down the gate enough when the sensor goes "open". Then the sensor should have no problem getting nearly full 6V onto the gate when it is immersed.
 

atferrari

Joined Jan 6, 2004
3,643
Mr Maliha is not telling what tests were carried out nor their results.

Or just a clear picture of how the circuit was implemented.
 
Top