99 percent accurate. Really!

Today I threw an interesting challenge to the students to sensitize them on misleading nature of accuracies reported in some medical literature.

Question: A type of cancer can be contracted by 1 in 10000 people. A patient was tested positive for it by a test which gives 99% accurate results. What is the probability that the patient actually has the cancer?


As you can see the probability of having the cancer turns out to be less than 1% even with a 99% accurate test.

But it doesn’t mean that the test is useless. Let’s see what happens when we repeat the test just one more time.

Question: After testing positive for cancer if the patient repeats the tests and is found to be positive for cancer again. Now what would be the probability?


We can see that just by having one iteration of the test we can take the confidence to 50%. So sometimes a second opinion is necessary!

Action Potential from Cockroach Leg

Today we did a very interesting experiment on Cockroach leg samples to understand action potential. Cockroach leg was secured on a foam block using two electrodes. These two electrodes were placed across the Coxa such that there is significant amount of tissue gap between them.

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Figure source:


After this we stimulated the leg with a square wave generated from signal generator. Peak to peak voltage greater than 300 mV. We varied the frequency of stimulation first from 0.5 Hz to upto 10 Hz and back down to about 1 Hz.  We could see the matching leg flip activity with each stimulated pulse.


We also tried to see the reverse effect where we acquired the action potential induced due to tapping at the leg. For this we removed the stimulation line and only acquired the signals from oscilloscope. So whenever there was a small tap on the end of the leg we could see a distinct induced potential on the scope.


ECG using two chips

In the BM605 lab this time we made a working three lead ECG on a breadboard. We used just two amplifier chips. One of them, AD620 which is an instrumentation amplifier with following pin configuration:

And the other our very own μA741

We had tried earlier to implement this circuit with instrumentation amplifier designed out of discrete ua741 Opamps but had failed to obtain ECG signals due to enormous powerline noise pickup by body. In the following schematic we use an integrated instrumentation amplifier chip AD620 to amplify the differential signal from across the heart (Terminals 1 and 2). But still we have powerline noise pickup as common mode signal on both terminals 1 and 2. We sense the common mode voltage and feed it back to the body after multiplying with a factor of -1. This almost entirely removes the powerline noise and all features of ECG, PQRST are clearly obtained.

Our M.Tech. student Nilesh made a simulation design of the circuit implemented in this experiment.

And here is the system in action. A surprisingly clear ECG was obtained on the scope! The whole experiment was successfully completed by all groups in just two hours.

Thanks to Piyush for saving the CSV file of the data and sharing it with all.


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Pulse-oximeter on a breadboard

We had another very interesting experiment as part of our BM605 course. This time we designed a simple pulse-oximeter (more accurately a pulse-volume meter) on a breadboard. It was done by students in just a single two-hour lab session. Another interesting thing about this experiment was that it worked with just ambient lighting.

We used an LDR (Light dependent resistor) as the primary sensing element for this. The LDR went into a voltage divider circuit. The resistor connected with the LDR for voltage division was carefully chosen so as to have a value equal to that of LDR resistance during finger placement in ambient lighting. The output of this voltage divider was fed to a band pass filter with cutoffs set at 0.1 Hz and 100 Hz.In the above figure, R1 is the thevenin equivalent resistance of the voltage divider.

This is the output that was achieved by two of our students Samrat and Aman. They placed another inverting amplifier stage after the filter. The signal is clear enough to locate the dichrotic notch.

Great going guys. Keep it up!