Human skin offers resistance to current and voltage that change in sympathy with the emotional states of the body. The circuit is based on changes in skin resistance followed by changes in mental states. In the relaxed state, the skin's resistance is as high as 2 Meg ohms or more which is reduced to 500k or less when the emotional state is too high. Reduced skin resistance is related to increased blood flow and permeability followed by physiological changes during high stress. This increases the electrical conductivity of the skin. So this circuit is useful for controlling the response of the skin while performing relaxation techniques.
Human skin offers some resistance to current and tension. This resistance changes with the emotional state of the body. The circuit proposed here measures changes in your skin's resistance after changes in your mental state.
In the relaxed state, the resistance offered by the skin is as high as 2 mega-ohms or more, which is reduced to 500 kilo-ohms or less when emotional stress is too high. Reduced skin resistance is related to increased blood flow and permeability followed by physiological changes during high stress. This increases the electrical conductivity of the skin. This circuit is useful for controlling the skin's response to relaxation techniques. It is very sensitive and shows response during a sudden moment of stress. Even a deep sigh will give response in the circuit.
The circuit uses a sensitive amplifier to detect variations in skin resistance. IC CA3140 (IC1) is designed as a voltage resistance converter that generates a variable voltage based on the conductivity of the skin. It is wired as an inverting amplifier to generate constant current to the skin in order to measure the resistance of the skin.
IC CA3140 is a 4.5MHz BiMOS operational amplifier with MOSFET inputs and bipolar output. Door-protected inputs have a high impedance and can detect a current as low as 10 pA. This device is ideal for detecting small currents in low current input applications.
The inverter input (pin 2) of IC1 is connected to ground (via the VR1 preset) and one of the touch plates, while the non-inverting input (pin 3) is grounded directly. The output of IC1 passes through the current limiting resistor R1 to the second touch plate. R1 act as a feedback resistor along with the skin when the touch pads make contact with the skin. So the gain of IC1 depends on the feedback provided by R1 and the skin.
In the reversing mode of IC1, a positive input voltage to its pin 2 through the feedback network causes its output to be low. If the skin offers a very high resistance in the relaxed state, the input voltage to pin 2 is reduced and the output remains high. Therefore, the gain of IC1 varies depending on the current passing through the skin, which, in turn, depends on the skin's response and emotional state.
In the standby state, the touch plates are free. As there is no feedback to IC1, it gives a high output (about 6 volts), which is indicated by moving the meter to the right side. When the touch pads are short-circuited by the skin, the feedback circuit is completed and the output voltage is reduced to 4 volts or less depending on the resistance of the skin. Since the feedback network has a fixed resistance (R1) and VR1 is set to a fixed resistance value, the current flowing through it depends only on the resistance of the skin. The output of IC1 is displayed on a sensitive moving coil meter (VU meter). By varying the VR2 preset, you can adjust the sensitivity of the meter.
To facilitate visual observation, an LED display is also included. IC LM3915 (IC2) is used to give a logarithmic display via the LED indications. You can pass current from pin 18 to pin 10 with each 125 millivolt gain on your input pin 5. Using VR3 you can adjust the input voltage of IC2, while using VR4 you can control the brightness of the LEDs.
In practice, the circuit provides meter reading and LED indications. If the LED display is not needed, IC2 can be omitted.
Mount the circuit in a general purpose PCB and lock in a suitable cabinet with pads glued on top, 5-10 mm away. The tactile pads may be any type of conductive plates, such as aluminum or copper plates, having dimensions of 1XC1 cm2. The mobile coil meter can be a small VU meter with 1 kilo-ohm coil resistance and reading from 0 to 10 digits.
After mounting the circuit, adjust the presets so that IC1 produces about 6 volts. None of the LEDs (LED1 to LED3) shines in this position with the touch panels open.
Now gently touch the touch pads with your middle finger. Hold your finger even allowing a minute to join with the pads and keep your body relaxed. Set VR3 until the green LED (LED1) illuminates and the meter shows full deflection. Adjust VR2 to obtain maximum meter deflection. This indicates a normal resistance of the skin, as long as the body is completely relaxed.
If you are stressed or feel bad, the skin's resistance decreases and the blue LED lights up followed by the red LED along with a deflection of the meter toward the underside. In summary, the red LED and zero meter reading indicate that they are stressed, and the green LED and high meter reading indicate they are relaxed. Practice some relaxation technique and observe how relaxed your body is.