Help with Skin Resistance Output

[pkg9991]

Hello friends,

I'm using the attached circuit for getting the galvanic skin resistance. The bridge circuit is working fine and I'm able to obtain resistance output as required (points Out_1 and Out_2 - varying between 0.8M - 1.5M ohm) but the op-amp circuit is not functioning. I'm not getting the required voltage levels (not shows any voltage levels at all at pin 14 of IC) which I then need to interface to ATmega16 microcontroller by internal ADC of uC to get this reading on LCD. Is something wrong with the circuit. Some help would be really appreciated guys. I'm stuck.

Values for various components are:
R1 = R2 = R3 = 1M ohm
R4 = R5 = R6 = R7 = 1K ohm
IC used is LM324

 

[jiripolivka]

In your schematic the Input 1 and 2 are shorted together, so the opamp does not have anything to amplify.
Otherwise, if you break the short, the circuit should work. Try to use a 1 MOhm potentiometer for opamp gain, and start from a lower gain. With too high gain the opamp can lock on one of the "rails", Vcc or ground.

 

[pkg9991]

Thanks for the reply.
I'm uploading the corrected schematic. This is what I'm exactly using to make the circuit. I'm a newbie so please let me know where exactly should I use the 1Mohm Pot?

 

[jiripolivka]

R7 is the gain-control resistor I would replace with a potentiometer. 1 MOhm sets a full gain, I would advise to start from ~10 kOhm then the gain will be low. Gradually increase the gain (slowly turning R7 to higher resistance) while observing the output voltage response to skin-resistance variations.
The LM324 is a good opamp but sometimes locks "on the rail" when the input voltage exceeds the gain-set level,then fails to get back to normal operation. If you can please find a "rail-to-rail" opamp type which can be driven out-of-range and returns back to normal function.

- - - Updated - - -

For initial testing I would connect a shorting switch between the "skin" terminals. Then adjust R7 (and possibly also R6) to get the output swing between Vcc and ground.
Real skin conditions then may require readjusting the R7 and R6 for the best output.
In real conditions, the circuit may pick up some interference from the body and surrounding power line. Try to connect a 1 nF to 1 uF capacitor in parallel with R7 to suppress the interference. This capacitor, however, will also slow the amplifier response. The best way to see the output variations and possible interference is to connect an oscilloscope to it.
Use also a battery or a good power supply for Vcc.