Transcranial Random Noise Stimulation (tRNS) Circuit

In this post I have explained a basic Transcranial random noise stimulation (tRNS) circuit diagram which can be implemented externally for stimulating brain cells, resulting in an enhanced performance of the brain.

The design was requested by an avid reader of this blog "Ankita", as given below.

Design Request

I'm interested in building a tRNS device which produces an alternating current with randomly changing frequency and amplitude.

The frequency must remain within 100 to 640 Hz, and the amplitude should vary between -1.5 and 1.5 mA.

My question is: how can I generate this randomly altering current waveform?

Unlike tDCS (transcranial direct current stimulation), which uses a constant current, tRNS employs a dynamic, noise-like current waveform.

This randomness is often achieved through a Gaussian distribution centered around a desired mean intensity.

While the electrode placement and size considerations for tRNS are quite identical to tDCS, the current generating pattern is fundamentally different.

What is tRNS

Transcranial random noise stimulation (tRNS) is a method in which electrical current pulses are sent to the brain without involving surgery. 

It works by transmitting a weak electric current with random frequencies through the scalp via two small pads.

This kind of stimulation usually involve a wide range of frequencies, between from 0.1 to 640 Hz. 

This can also be applied at slow or fast frequencies, known as lf-tRNS or hf-tRNS (which may usually range between 0.1–100 Hz and 101–640 Hz).

Although this is a modern method, tRNS has become very popular in the last few years. 

Scientists across the world have focused on how tRNS affects different brain mechanism like movement, senses, and thinking. 

For example, tRNS might seem to enhance brain functioning to visualize things better, recognize faces quickly, and even do fast mathematical calculations. 

People who have certain health issues like multiple sclerosis or Parkinson's disease, tRNS could be successful in reducing pain and improving symptoms.

Basically, tRNS, which uses high frequencies, has proved quite beneficial in various situations. For example it makes our senses work better, and might help folks who have certain health challenges.

Circuit Description

The following figure shows the complete circuit diagram of our tRNS device. So I have explained the working of the design through the following explanation:

IC1 is an LM3915 IC which in response to a varying signal at its pin#5 generates a forward reverse sequential logic low across its output pins 1, 18, 17, 16, 15, 14, 13, 12, 11, 10.

Meaning, any varying voltage between 0 and 200 mV at pin#5 of the IC will cause the above output pins to generate a sequentially oscillating low logic.

Since we have 10 outputs for IC1, the input frequency at pin#5 will be divided by 10 across the 10 outputs.

As per the given specifications, the frequency of the tRNS must be between 0.1–100 Hz and 101–640 Hz. However, making a randomly varying frequency looks difficult.

Therefore, in our design we have used an innovative way of implementing this randomly varying frequency, through the IC2 UM66.

The IC2 UM66, is a melody generator IC and is capable of generating a musical frequency ranging from 200 Hz, and 3000 Hz.

Since it is a musical frequency, it has the tendency of randomly varying frequency between 200 Hz and 3000 Hz.

Now, as IC1 is supposed to divide the above frequency by 10, the resulting frequency across the IC1 outputs will be within the range of 20 Hz and 300 Hz. This frequency range seems to be exactly what we want for our tRNS circuit application.

This takes care of the randomly varying frequency specification. There's another parameter that needs to be satisfied for our tRNS circuit design. It is the randomly varying output current.

Implementing the randomly varying output current specification does not look too difficult and becomes possible by using randomly selected resistor values (R5 to R14) across the output pinouts of the IC1. The overall maximum output current range can be also tweaked by adjusting the R3, R4 values.

The above randomly varying frequency and current output is fed to the bases of the transistors T1 and T2.

These transistors suitably amplify the varying signals from IC1 to drive an output transformer TR1 (mistakenly labelled as T1).

The transformer amplifies the varying signal across its secondary winding to generate the intended tRNS output waveform.

Due to randomly switching resistor values at the T1, T2 bases, its collectors feed a randomly varying current amplitude across the secondary winding of the transformer.

How to Build the tRNS Transformer

It is a matter of some experimentation. It may be good to start with 30 + 30 turns for the primary side and 100 turns at the secondary side of the transformer. The wire should be super enameled copper wire, around 0.3 mm thick. The core of the transformer can be any toroidal ferrite ring.

You can experiment with the number of turns to get the most optimal response at the secondary side of the transformer, ensuring that the transistors and the transformer remain cool while operating.

How to Test

The testing procedure does not require too many steps.

After assembling the proposed tRNS circuit over a strip board, recheck all the connections and components polarity to ensure everything's done rightly without any mistakes.

Next, connect an oscilloscope across the secondary side of the transformer and switch ON power through a 12V regulated DC power supply.

You should be able witness a randomly fluctuating waveform on the oscilloscope.

Adjust the 10k preset P1 until the waveform functions optimally without clipping and without saturating the IC1.

You can also set P1 by measuring the peak voltage at pin#5 of IC1 which must not exceed 200 mV.

That's all, your tRNS circuit is now built and tested successfully.

The next step would be to get the circuit verified from a qualified medical engineer and start using it on needy candidates.

Warning: The explained circuit design of Transcranial Random Noise Stimulation (tRNS) has not been practically verified by the author of the post. It is strictly recommended to first verify the circuit working from a medical lab or from a medical professional before using the device practically on patients.

Parts List

• All resistors are 1/4 watt 5% CFR
• R1, R2 = 4.7k
• R3 = 2.7k
• R4 = 390 ohms
• R5 to R14 = select randomly different values between 10k and 100k.
• P1 = 10k prest.
• Semiconductors
• Z1 = 3V 400mW zener diode
• T1, T2 = 2N2907
• IC1 = LM3915
• IC2 = UM66
• Transformer = See Text.

References: Transcranial random noise stimulation (tRNS)