The principle of magnetic stimulator (source: chapter 22)

Friday, 9 September 2011

My Final Year Project's topic is about Digital Taste and Smell Communication and I have been assigned to Smell Communication side. In this project, I will work on developing a simple Transcranial Magnetic Stimulation (TMS). Since this is a quite new topic for me, so, I started off the project by reviewing some materials which had been assigned by my project's mentor.

From my readings, I found that Transcranial Magnetic Stimulation (TMS) is an intriguing technology for investigating function in the brain by using a rapidly changing magnetic field and using electromagnetic induction to induce weak electric current, it could modulate neural activity. The magnetic stimulation can be identified through Electroencephalohraphy (EEG), Super high density scalp EEG, and Functional Magnetic Resonance Imaging (fMRI).

In this project, we are aimed to develop a simple TMS design to regenerate smell sensation in the human mind, the basic principle is by using magnetic user interface on adjacent human tissues and neurons, using the magnetic stimulation on human tissues, it could be possible to form similar kind of sensations and experiences in human mind in a controlled environment.

By looking at the basic principle behind TMS whereby a large current waveform is driven through a coil place adjacent to the tissue of interest, the induced electric field would penetrate the tissue and induce eddy currents on conductors, such as nerve fibers. Thus, we argue this could form similar kind of perception in human mind.

The principle of a magnetic stimulator is illustrated in the figure above. A magnetic stimulator which includes a coil would be place on the surface of the head or the adjacent neuron. Thus, addressing this problem requires knowledge of the distribution of the electric field induced within the head and also the mechanism of interaction between the induced electric field and neural tissue. To induce a current into the underlying tissue, a strong and rapidly changing magnetic field must be generated by the coil. In practice, this is generated by first charging a large capacitor to a high voltage and then discharging it with a thyristor switch through a coil.(source:

For the next two weeks, I and my teammate are gonna work together to produce the initial TMS circuit design, if possible, we could come up with the beardboard prototype. But, before that, we have to do a research on the desired coils that we will be using for our TMS circuit.

Friday, 16 September 2011

This week, we were discussing about possible circuit designs to choose for TMS. However, we have not decided one as we have to take some important considerations such as what minimum current and voltage should flow in to the circuit, and what pulse would be generated from the circuits, etc.

From my reading, I found some plausible circuit designs, for examples:

Screen shot 2011-09-16 at AM 12.01.15

Circuit 1

Magnetic stimulation requires moving enough charge through an electrically sensitive nerve membrane to depolarize it, this means that the membrane voltage must be increased from its normal resting potential. This circuit offers design considerations from coupling analytically the electromagnetic circuit to that within the nerve. A coupled magnetic circuit-biological has been formulated in order to predict the conditions for optimizing the nerve membrane stimulation. The circuit was formulated using Laplace transform theory. There are number of calculations to calculate the magnetic fields, induced current and voltage etc. This circuit was a sufficiently to reach high resonant frequency near 10 kHz. In addition to that, the stimulation core must always be designed to deliver the minimum magnetic circuit reluctance under load; this is sufficient to avoid undue saturation

The second circuit which I found is:

Screen shot 2011-09-16 at AM 12.15.38

Circuit 2

The magnetic stimulation system consists of charging loop, discharging loop, control system, stimulating coil. High voltage DC supply, charge control and storage capacitor compose the charging loop. The control system can detect the voltage on capacitor real-timely, and then control the turning on and off of charge control. Storage capacitor, discharge control and stimulating coil form the discharging loop. The control system controls the turning on and off of discharge control according to input parameters. Charge control and Discharge control cannot be turned on at the same time. In the meantime, the control system also detects the temperature of stimulating coil, because the coil wire has a certain threshold of high temperature resistance. The embedded system can receive parameters inputted by the user through a LCD touch screen, and then design control signals to control the frequency of charging and discharging. Switching power supply is chosen to perform as system high voltage DC supply. Internal work of switching supply is under high frequency condition, and its own power dissipation is very low. As a result, the efficiency of supply can be very high; generally can reach 80%, and leaves out the bulky power frequency transformer at the same time, making large power supply, which reaches several kilovolts easier to be produced.

We also looked at some possible coil design of magnetic coil in TMS, and it’s believed that the 8-shaped coils’ performance is better than the other coils. The 8-shaped coils can produce several focuses, and it has better ability of focus that circular loop coils. In the next work, we will make our great efforts to analyze the magnetic stimulations coil array.

Friday, 23 September 2011

Screen shot 2011-09-23 at PM 01.01.55

In this week, after researching and reviewing some possible circuit designs, we finally decided to start building our TMS circuit based on a research paper "Circuit and Coil Design for In Vitro Magnetic Neural Stimulation System" (source:, since we all are quite new in this TMS and neuron stimulation area, we are going to refer to this research paper.

This is the complete circuit. Transistors are Advanced Semiconductor Q1, Q8 = 2N2907 Q2, Q7 = 2N2222 Q3 = 2N3866 Q5 = 2N5160 Q4, Q6 = 2N3632. Input diodes are 1N4148, clamping diodes are BAV99, zener diodes are 1.5KE16A, power supply capacitors are low ESR electrolyric, 2200uF. circles are optional single turn ferrite beads that can reduce high frequency feed through. Vcc and Vee are +/- 16V respectively.

We also looked for what components we are going to use for this circuit. Thus, I made a list and noted down some components from farnell's website for us to use. Later, Mr. Kasun will order the components for us after, Dr. Nii approved the components list. There are two components that I could not find in farnell's website, those are transistor 2N5160 and 2N3632.

Friday, 14 September 2011

Breadboard Prototyping

TMS Breadboard Prototyping

In this week, we have assembled the breadboard prototyping for the TMS like you can see on the picture. We have not stimulated the output results of this circuit, since we have not yet designed the stimulating coils. We have thought to use figure-8 coils or butterfly coils type, but we need to think more and research about the technical designs of the coil before we could proceed with the stimulation test, so in the mean time, we are thinking to test the output by using osciloscope. In addition to that, we have done the continuity test, by using multimeter, to make sure all the pins are connected properly, this is done to avoid any short circuits in the real stimulations tests.

Furthermore, we supposed to use LT1468 feedback amplifier, however, it is not available in Farnell, so for the time being, we have replaced it with LT6231. Moreover, as I mentioned in the previous journal entry, Transistors 2N5160 and 2N3632 are no longer available in the market, thus, we also have replaced them with 2N2905 and 2SD1802S-TL-E. Those alternative components were chosen by considering the closest technical specifications.

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