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Basic glucose meter schematic

A project log for Open source Arduino blood glucose meter shield

Open hardware and software blood glucose meter using electrochemical test strips

M. BindhammerM. Bindhammer 06/02/2016 at 19:067 Comments

Here is the first draft of the glucose meter schematic:

Mathematically interesting is only the op amp integrator (IC1D). The formula for determining voltage output for the integrator is as follows:

As the non-inverting input is not connected to GND (VR = 0V) but to VR = +2.5V:

Updated schematic:

I changed the MAX4575, which resets the op amp integrator by shortening the capacitor by the quad bilateral switch CD4066 (IC2), because this chip is much cheaper. Because integration involves a known start time and end time, a reset circuit must be included to establish the start time before each integration time period. The integration end time occurs when the measurement is read. The CD4066 simply short-circuits the capacitor C1 if digital output D6 goes HIGH. The not used control inputs are tied to ground. Unused CMOS input pins must never be unconnected, because they tend to float towards the dangerous region which is in the middle between VDD and GND.

The low-dropout regulator TPS76925DBVT (U1) provides a fixed output voltage of 2.5V.

The output voltage of the TPS76901 adjustable regulator (U2) is programmed using an external resistor divider. The output voltage is calculated using:

Where: Vref = 1.224V typ (the internal reference voltage).

As the applied voltage across two of the test strip electrodes should be about 400 mV, I chose R3 = 130k and R4 = 180k which results in a TPS76901 output voltage of 2.108V.

As mentioned above IC1D is configured as an op amp integrator. Currently I don't use the op amp integrator because the op amp integrator output voltage is not directly or indirectly proportional to the glucose concentration when using ONETOUCH test strips. This might be the case when using other test strips.

IC1B is configured as a transimpedance amplifier, where basically

The feedback capacitor C8 is required to improve stability.

IC1A and IC1C are configured as unity gain buffer amplifiers. In this configuration, the entire output voltage is fed back into the inverting input. The difference between the non-inverting input voltage and the inverting input voltage is amplified by the op-amp. This connection forces the op-amp to adjust its output voltage simply equal to the input voltage:

For IC1A this is only the case if an electrolytic bridge (blood) between the reference and counter electrode is applied.

As long as the strip is not inserted, input pin D2 is LOW. If a test strip is inserted, the two strip detect pins on the strip connector getting connected and D2 goes HIGH.

C4 and C5 are decoupling capacitors.

Discussions

erico4boys wrote 08/23/2016 at 02:45 point

Hello Bindhammer, can you please explain the significance of the integrator circuit (IC1D) and the IC2 circuit because I think the signal extracted from the ADC0 could be programmed to get the glucose concentration of a blood sample.

Thanks for your continous support.

Best regards 

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M. Bindhammer wrote 08/23/2016 at 06:50 point

Please take a look at 'Basic principle of operation' here: https://hackaday.io/project/11719-open-source-arduino-blood-glucose-meter-shield

and see above. Already explained.

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erico4boys wrote 08/23/2016 at 10:22 point

Thanks for your help, its much appreciated.

Best regards.

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M. Bindhammer wrote 08/21/2016 at 20:27 point

No. IC1C just forward the output voltage of the transimpedance amplifier 1:1 to ADC0. I just configured IC1C as a voltage follower so all of the 4 op amps, the LMC 6484 chip is providing, are used.

U1/U2 are necessary to provide the according reference voltages and the difference between the reference voltages (approx. 400mV). You could also use voltage dividers instead but they are less accurate.

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erico4boys wrote 08/22/2016 at 17:10 point

Hello Bindhammer, Thanks for the quick response it was very helpful. I simulated the IC1A circuit on proteus software and I realised that the voltage entering the reference and counter electrodes are 2.50896V and 2.5V respectively. Can you please explain this part. 

Can you please explain why IC1A pin 3 is assigned 2.1V and IC1B pin 5 assigned 2.5V.

Also is it true that the current generated due to the application of a blood sample on the test strip is fed to the working electrode?

Thanks for your anticipated response.

Best regards

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M. Bindhammer wrote 08/22/2016 at 17:58 point

2.50896V and 2.5V seems to be close enough to be actually equal. The applied voltage across two of the electrodes needs to be around -400mV. 2.5V is used as reference, so we need to create another voltage source of approx. 2.1V. Current is 'fed' to the working electrode.

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erico4boys wrote 08/21/2016 at 20:14 point

Bindhammer you are a star, I will always appreciate your effort towards this project. Have got two questions please: Is IC1C amplifying the output voltage of the transimpedance amplifier (IC1B) and then feeding it to ADC0?

Secondly, was the U1 circuit and U2 circuit only showing how to get the 2.5V and 2.1V used on the amplifers?

Thanks for your unrelenting support.

Best regards 

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