HOW TO MAKE A HEARING AID?

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DESCRIPTION

Commercially available hearing aids are quite costly. Here is an inexpensive hearing aid circuit that uses just four transistors and a few passive components.

On moving power switch S to ‘on’ position, the condenser microphone detects the

sound signal, which is amplified by transistors T1 and T2. Now the amplified signal

passes through coupling capacitor C3 to the base of transistor T3. The signal is further

amplified by pnp transistor T4 to drive a low impedance earphone. Capacitors C4 and C5

are the power supply decoupling capacitors.

The circuit can be easily assembled on a small, general-purpose PCB or a Vero board. It operates off a 3V DC supply. For this, you may use two small 1.5V cells. Keep switch S to ‘off’ state when the circuit is not in use. To increase the sensitivity of the condenser microphone house it inside a small tube.

This circuit costs around Rs 65.

DOWNLOAD THE CIRCUIT DIAGRAM

FINGER PLETHSYMOGRAPH TO MEASURE BLOOD RESISTIVITY-BIOMEDICAL PROJECTS

SUMMARY ABOUT THE PROJECT

Impedance plethysmography can be used to measure arterial volume change that occurs with propagation of the blood pressure pulse in a limb segment. For this measurement, we assume a constant value of blood resistivity.

However, blood resistivity may change under both physiological and pathological conditions. Use of an impedance plethysmograph on a finger immersed in a saline filled beaker may yield a method for determining this change in blood resistivity.

This may develop into a method that diabetics can use to measure glucose levels non-
invasively.  The goal of our project is to design a finger plethysmograph to measure blood resistivity.

PROBLEM STATEMENT
Our goal is to design a finger plethysmograph to measure blood resistivity.  In order to
accomplish this, we will need to design and build a data acquisition device to acquire the signal
from the finger.

The device should mechanically immobilize the test subjects’ finger such that motion artifacts are kept to a minimum.

This device should be able to detect the electrical potential (voltage) change across the finger so that the change in resistance may be determined.
It should be able to detect the velocity-dependent change in blood resistivity due to arterial blood pulsations.
In addition, we will need to build an electrical circuit to perform signal processing and
analysis.  This circuit should be capable of rectifying the alternating current (AC) signal from the finger data acquisition device and modulate it into a direct current (DC) signal to be analyzed.  The circuit should be capable of discerning or visually displaying the voltage changes caused by correlated changes in blood resistivity.

As an added feature, this circuit may contain an automatic reset function capable of adjusting one of the differential amplifier inputs to that of the output from the data acquisition (finger holder) device.

This will allow the device to easily accommodate fingers having different electrical resistances and will prevent having to manually adjust voltages using a potentiometer to match independences with each new test subject or finger position.

DOWNLOAD THE COMPLETE PROJECT REPORT FROM HERE

LINK 1

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HOW TO CHECK AMOUNT OF SALT IN LIQUID?-BIOMEDICAL PROJECTS

PURPOSE OF DEVICE

This circuit was designed to detect the approximate percentage of salt contained in a liquid. After careful setting it can be useful to persons needing a quick, rough indication of the salt content in liquid foods for diet purposes etc.

DOWNLOAD THE CIRCUIT DIAGRAM

WORKING OF CIRCUIT

IC1A op-amp is wired as a DC differential amplifier and its output voltage increases as the DC resistance measured across the probes decreases. In fact, fresh water has a relatively high DC resistance value that will decrease proportionally as an increasing amount of salt is added.
IC1B, IC1C and IC1D are wired as comparators and drive D5, D4 and D3 in turn, as the voltage at their inverting inputs increases. Therefore, no LED will be on when the salt content of the liquid under test is very low, yellow LED D5 will illuminate when the salt content is low, green LED D4 will illuminate if the salt content is normal and red LED D3 will illuminate if the salt content is high.
D1 and D2 are always on, as their purpose is to provide two reference voltages, thus improving circuit precision. At D2 anode a stable 3.2V supply feeds the non-inverting inputs of the comparators by means of the reference resistor chain R8, R9 and R10. The 1.6V reference voltage available at D1 anode feeds the probes and the set-up trimmer R4.
One of these two red LEDs may be used as a pilot light to show when the device is on.

HOW TO MAKE PROBES?

It was found by experiment that a good and cheap probe can be made using a 6.3mm. mono jack plug. The two plug leads are connected to the circuit input by means of a two-wire cable (a piece of screened cable works fine).
The metal body of the jack is formed by two parts of different length, separated by a black plastic ring. You should try to cover the longest part with insulating tape in order to obtain an exposed metal surface of the same length of the tip part, i.e. about 8 to 10mm. starting from the black plastic ring.
In the prototype, three tablespoons of liquid were poured into a cylindrical plastic cap of 55mm. height and 27mm. diameter, then the metal part of the jack probe was immersed in the liquid.

NOTES

• Wait at least 30 seconds to obtain a reliable reading.


• Wash and wipe carefully the probe after each test.


• To setup the circuit and to obtain a more precise reading, you can use a DC voltmeter in the 10V range connected across pin #1 of IC1A and negative supply.


• Set R4 to obtain a zero reading on the voltmeter when the probe is immersed in fresh water.


• You may change at will the threshold voltage levels at which the LEDs illuminate by trimming R4. Vary R8 value to change D4 range and R9 value to change D5 range.


• P1 pushbutton can be substituted by a common SPST switch.

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