The project that I am working as my Final Year Project is titled as Digital Taste and Smell Communication. It is proposed as a new communication medium, which allows people to digitally share taste and smell sensations with a remote person through existing networking technologies, for example the Internet. This project is mainly divided into two parts, Taste Sensation as one and Smell as another. I am responsible for the Smell Communication side.

I started off by reviewing materials on Transcranial magnetic stimulation(TMS), repetitive transcranial magnetic stimulation (rTMS) and Bioelectromagnetism. Our task on the project is mainly focussing on the simulation of the neurons using TMS, a method of stimulation using current induced by electromagnetic field. We are going to develop a TMS circuit to regenerate smell sensations to the human brains. For the next two weeks, my groupmate and I will be working together to produce a preliminary design of the TMS circuits.

Transcranial magnetic stimulation

Transcranial Magnetic Stimulation

From my readings, I found that the TMS works when an electromagnetic coil is placed against the scalp. This coil generates a rapidly changing magnetic field to induce the current into the brain tissues to stmulate the neurons. Typically, the process of the TMS circuit is firstly charging a large capacitor to high voltage and discharging it with a thyristor switch through the coil that passes the current to the brain tissues. The brain cells sustain a potential difference between intra and extra-cellular space. An externally applied electric field may deviate the cell's membrane potential, that is, depolarize the membrane and hence activate excitable tissue. The typical magnetic field strengths of about 1-2 Tesla in magnetic stimulation are achieved by driving the stimulating coil with brief current pulses of several kiloamperes. A coil with one stimulating edge is placed perpendicularly about 5 cm from the vertex causes the thumb to twitch quickly after stimulus onset. There will be a clicking sound from the coil and the subject may feel an uncomfortable sensation of scalp being drawn up, which probably results from the force between the coil current and the induced current in the scalp.

We are looking at chances to create a TMS circuit that uses the minimum magnetic fields, current and voltages. The design of the coil determines the pattern of the electric field that is induced into the brain. We are currently still in the process of researching on the most desired design of the circuit that we could use and hopefully by next weekend we could come out with a conclusion on the design through a discussion with our project leader, Mr Kasun.


The basic instrumentation of the TMS circuit consists of a capacitor, a thyristor switch and a stimulating coil. Together with the resistance R in the components and cables, the capacitor and coil form an oscillating RLC circuit. When it is activated, it produces a brief exponentially decaying sinusoidal, or biphasic, current pulse I(t) (bottom). Referring to the figure below, the energy is returned through the diode D from the coil to the instrument, which reduces coil heating and power consumption.


Basic Instrumentation and Sinusoidal Current Pulse

We are still considering the types of components and the suitable instrumentation that we could use to produce the circuit. It is very important to go through the details carefully as the circuit is going to be used for stimulating the human brains. We cannot just go on using the breadboard and test out the circuit without studying and reviewing the considerations.

For the stimulating coil design, the considerations include the type of material for the core, the shape and the pulse characteristics generated by the coil. The geometric shape of the coil affects the focality, shape and depth of cortical penetration of the magnetic field. The main issue for the TMS method is the focality of the stimulation. Another one is the low efficiency of power transfer from the coil to the tissue. To address these issues, we need to design a suitable shape of the stimulation coil. Some examples of the stimulation coils are round coils, figure-eight(butterfly) coils, double-cone coil, four-leaf coil. The figure-eight coil results in a more focal pattern of the stimu

lation, double-cone is suitable for deeper penetration and four-leaf is used for stimulation of peripheral nerves.

We are planning to use the figure-eight also called as butterfly coil for the neurons stimulation as it has a smaller focal point. This kind of coil consist of 2 round coils tied together forming the shape of the number 8. It elicits a maximum current at the intersection of the 2 round coils. The specifications of Butterfly Coil MRi-B90-II can be found here.

It has been found that different frequencies of magnetic stimulation will have different effects. Typically, inhibitory effects are seen in subject's motor cortex with rTMS frequency between 0.2 and 1 Hz (cycles per second). The intensity of the electrical field to use to generate the magnetic force can vary. As a rough guide, intensities of about 1 tesla (T) with an upper limit of about 2 T are typically generated. This is the problem that we are looking at, i.e, 1 tesla is a very high magnetic field, about 40,000 times earth's magnetic field, we need to produce a TMS circuit that uses much weaker magnetic fields that is sufficient to induce unusual experiences in the stimualtion process.


After reviewing the circuit designs that we have been researching for the past weeks, Mr Kasun has given us the green light to test out on the circuit design from this research paper since the circuit design is low scaled, closer to what we want to achieve. As we have very little knowledge in neuron stimulation, we are going to start off with the design of the circuit by referring to the research paper and then test out with multiple rounds of experiments. Through the experiments, we aim to develop the circuit step by step catering to the objective of the project, i.e: minimizing the magnetic field, and able to stimulate smell sensations.

Circuit Design
TMS schem

Schematic diagram of the TMS circuit

A current ramp of ~0.1A/μs is well within the range of a power amplifier topology. The figure on the right shows the schematic diagram of the TMS circuit that we are going to use as the preliminary design.

List of Components

1 x current feedback amplifier = LT1468

Q1 & Q8 = 2N2907

Q2 & Q7 = 2N2222

Q3 = 2N3866

Q5 = 2N5160

Q4 & Q6 = 2N3632,

4 x input diodes = 1N4148

4 x clamping diodes = BAV99

2 x zener diodes = 1.5KE16A

2 x power supply capacitors, low ESR electrolytic = 2200uF

2 x capacitors = 10pF

1 x capacitor = 47uF

1 x resistor = 22k ohm

2 x resistor = 0.22 ohm

1 x resistor = 2 ohm

4 x resistors = 10 ohm

5 x resistors = 100 ohm

2 x resistors = 1.5k ohm

3 x resistors = 1k ohm

Vcc = +16V

Vee = -16V


LT1468 feedback amplifier is currently unavailable. In the meantime we have replaced it with LT6231. Transistors 2N5160 and 2N3632 are no longer available and we have replaced them with 2N2905 and 2SD1802S-TL-E respectively. The alternative components are chosen according to the closest specifications.

The assembled circuit is shown in the figure below. We have just done with the continuity test and some testings of the circuit output need to be done before we could proceed with the stimulation tests.
IMG 2672-1-

Assembled Stimulation Circuit