Simple Skin Biofeedback Circuit

This is a special circuit that helps people learn about their skin.

It uses sticky pads electrodes placed on your skin to pick up tiny electrical signals.

These signals change depending on how relaxed you are.

The circuit then shows you this change, like with lights or sounds so you can learn to relax more.

Circuit Working:

Parts List:

This basic circuit enables a specific type of biofeedback.

It operates on the principle that a persons skin resistance increases as they relax deeper.

This resistance variation affects the oscillator frequency in the circuit, which is built around a unijunction transistor T2.

Two electrodes, forming a ring are attached to two fingers of a hand.

A speaker produces a sound the pitch of which indicates the level of relaxation.

The deeper the relaxation the lower the sound frequency.

Another oscillator, using transistor T1 also produces a sound, which can be adjusted using potentiometer P1 to match the frequency of the first oscillator indicating the deepest state of relaxation.

For maximum relaxation, both tones should be at the same level when heard through stereo headphones, indicating synchronization between the left and right sides.

Formulas:

In the above circuit diagram, the signal from the electrodes is amplified by means of two transistors, designated as T1 and T2.

P1, the potentiometer, is used to modify the circuits sensitivity.

The transistors are biased by the 15k resistors, R1 and R2.

C1 and C2 two 1nF capacitors, filter out high frequency noise.

The following is the formula for the circuits gain:

Gain = (R2 / R1) * (1 + (R3 / R4))

where,

• Gain: is the relationship between the input and output voltages, put more simply, it refers to the circuits amplification of the minute electrical variations in skin resistance.
• R1, R2, R3 and R4: is the resistances of the resistors, capacitor, potentiometer, electrode and other individual circuit components.

In essence, the formula measures the circuits ability to convert minute changes in skin resistance into electrical impulses that may be measured and utilized for biofeedback.

How to Build:

To build a Simple Skin Biofeedback Circuit follow the below mentioned connection steps:

• Connect one electrode to the center terminal of T2 and the other electrode to a positive supply.
• Build the first oscillator circuit around T2.
• The frequency of this oscillator will be influenced by the skin resistance between the electrodes.
• Build the second oscillator circuit around T1.
• Connect the output of the T1 and T2 UJT oscillators to the headset.
• This oscillators frequency can be adjusted with potentiometer P1.
• Use the stereo headset to listen to the sounds produced by both oscillators.

Note:

• Adjust potentiometer P1 so that the sound from the second oscillator matches the frequency of the first oscillator when you are in a deep state of relaxation.
• This synchronization indicates the deepest state of relaxation.

Conclusion:

A skin biofeedback circuit is a valuable tool for monitoring and managing stress levels.

By measuring changes in skin resistance, this circuit can provide real time feedback to help individuals improve their relaxation techniques and reduce stress.

References:

Design and Development of GSR Biofeedback Device

Datasheet UJT 2N2646

Biofeedback