A 32 ohms earphone is used in the output unit of this project as recommended by the manufacturers of the TDA2822M IC. According to the ICâ€s datasheet, this 32 ohms earphone will produce an output of about 1.3 watts. In the circuit above, capacitors C11 and C12 are called coupling capacitors. Their functions are to block any DC components in the input and outputs of the pre-amplifier. The pre-amplifier comprises of R5 and capacitor c13 which decouples the power supply of the preamplifier stage, while capacitor C12 and resistors, R2, R3 and R4 with transistor T1 forms a negative feedback amplifier which stabilizes the overall gain (A). Resistor, R4 is known as an emitter swamping resistor which also adds stability to the amplifier. The medium power amplifier amplifies the output of the pre-amplifier to an audible level. It comprises of the TDA2822M IC and those external components needed to make the IC function properly. This other external components are capacitors C14, C15, C16, C17, C18 and resistors R6 and R7. Resistor, R5 and capacitor, C13 form an RC decoupling circuit which are connected across the power supply to smooth out noise. Finally a 32 ohms earphone is used in the output unit.

Sound Amplifier for Ears : The Part List

The components used are: condensed mic, 1=2.2kΩ, 2=330kΩ, 3 = 680kΩ,12 = 33Ω,
5=10kΩ, 6 =220Ω, 7=4.7Ω, 8 =4.7Ω, , 1 =BC547A , 1 =0.01,4=100, 3 = 47, 4 = 10, 5 = 0.01 , 6 = 100, 7 = 0.1, 8 = 0.1, V1=10kΩ, LED- Red, TDA 2822M, switch and battery (3V) and earphone.
The components for the sound amplifier for ears circuit were first assembled on a bread board and tested. After it was found to work as anticipated the components were transferred to a Vero board for the final construction.

Abstract: Hearing aid device is a small electronic gadget that is fit in or behind the ear to improve oneâ€s hearing and consequently communication ability. This research work involves the design and development of a hearing aid device with pre-amplifier; an acoustic signal picked-up using a condenser microphone. TDA 2822M IC is configured to produce an audio amplification which is converted to audio signal through a headphone. Design equations were employed to calculate the physical parameters of the circuit. After the design, the circuit was constructed and tested on 5 people with partial hearing problem. The result showed that there was a significant improvement in the hearing ability of all the patients tested. Recommendations were proposed for further improvement.

Download: Small Hearing Aid Project Document | TDA2822M Datasheet

This is the circuit diagram of TRIAC, SCR and Transistor tester. This is a very simple circuit which can be used for testing of SCRs as well as triacs. The circuit could even be used for checking of PNP and NPN transistors.

The circuit works on 3V DC, derived using a zener diode in conjunction with a step-down transformer and rectifier arrangement, as shown in the figure. Alternatively, one may power the circuit using two pencil cells.

For testing an SCR, insert it in the socket with terminals inserted in proper slots. Slide switch S3 to “on” position (towards “a”) and press switch S1 momentarily. The LED would glow and keep glowing until switch S2 is pressed or mains supply to step-down transformer is interrupted for a short duration using switch S4. This would indicate that the SCR under test is serviceable.

With switch S3 in “off” position (towards “b”), you may connect a milliammeter or a multimeter to monitor the current flowing through the SCR. If the SCR is “no good,” the LED would never glow. If the SCR is faulty (leaky), the LED would glow by itself. In other words, if the LED glows only on pressing switch S1 momentarily and goes off on pressing switch S2, the SCR is good.

For testing a triac, initially connect its MT1 terminal to point A (positive), MT2 to point K (negative), and its gate to point G. Now, on pressing switch S1 momentarily, the LED would glow. On pressing switch S2 momentarily, the LED would go off. Next, on pressing switch S5, the LED will not glow.

Now reverse connections of MT1 and MT2, i.e. connect MT1 to the negative and MT2 to the positive side. For a good working triac, S2 would not initiate conduction in the triac and the LED would remain off. On the other hand, momentary depression of S5 would initiate conduction of the triac and LED1 would glow.

The indication of a leaky triac is similar to that of an SCR. If, during both the above-mentioned tests, the LED lights up, only then the triac is good.

Before connecting any SCR/triac in the circuit, please check its anode/MT1’s connection with the case. (Note: A triac is actually two SCRs connected back to back. The first accepts positive pulse for conduction while the second accepts negative pulse for conduction.)

Transistor Tester Usage

You can also check transistors with this circuit by introducing a resistor (about 1 kilo-ohm) between the junction of switches S1 and S5 and point G. The collector of NPN or emitter of PNP transistor is to be connected to positive (point A), while emitter of an NPN and collector of a PNP transistor is to be connected to negative (point K). The base in both cases is to be connected to point G.

Above figure indicates the conventional current direction and forward biasing condition for PNP and NPN transistors. If the transistor under test is of NPN type, on pressing S1, the LED glows, and on releasing or lifting the finger, it goes off, indicating that the transistor is good. For PNP transistor, the LED glows on pressing switch S5 and goes off when it is released. This indicates that the transistor under test is good. A leaky or short-circuited SCR or transistor would be indicated by a permanent glow of the LED by itself, i.e. without pressing switch S1 or S5.

Triac, SCR, transistor tester circuit come from EFYmag 2001

Here is the simple and low cost mosquito repellent circuit design. The circuit serves to keep the mosquitoes out of the room or the location where the device is installed. According to certain publications, the frequency emitted by the male mosquitoes is said to be around 20??”25 kHz, and so within the realm of ultrasound. But according to others, it is in the region of 5??”7 kHz instead; frequencies that a human ear, even an elderly one, can still hear very well. Rather than spending lots of money buying such a device, which moreover generally have a fixed frequency, weâ€re suggesting building one yourself , especially since the circuit proposed is very simple and cheap to build.

Mosquito Repellent Works

The low cost mosquito repellent circuit uses just a single IC, a CMOS type 4047. This very multi-purpose IC can be wired in very many operating modes, including that of the multivibrator or astable used here. The operating frequency is set by the external components C1, R1, and P1. The latter makes it possible to slightly adjust the frequency, given the uncertainty that exists over the most efective value. To best reproduce the high frequencies produced by the generator, the output transducer used is a simple tweeter, but it must be a piezo one. Such a tweeter behaves in fact much like a capacitor, and so doesnâ€t overload the CMOS IC outputs that are incapable of supplying a substantial current.

To obtain an output signal of sufficient amplitude while being powered from a single 9 V battery. The tweeter is connected between the 4047â€s Q and Q outputs. With this condition, it possible to apply complementary (antiphase) signals to the tweeter so it ‘sees†an alternating voltage of double the supply voltage. In purely theoretical terms, this quadruples the output power available. In practice, itâ€s better to regard it as tripling it, but the beneft achieved by doing it this way is nonetheless very real. All that remains is for you to place the project in the middle of the patio table or beside your lounger in order to get a taste of the calm of a summerâ€s evening without mosquitoes bothering you acoustically or worse, biting. At any rate, thatâ€s what we wish for you.

Source: eeweb

This is the circuit design of the protector for electronic appliance with three-phase power supply. Many of electronic appliances need three-phase AC supply to work. If there is any failure of any of the phases, then it will make the appliance prone to erratic functioning and may even lead to failure. Hence it is of paramount importance to monitor the availability of the three-phase supply and switch off the electronic appliance if found any failure on one or two phases. The power to the appliance should resume with the availability of all phases of the supply with certain time delay in order to avoid surges and momentary fluctuations.

The electronic appliance protector needs three-phase AC power, three 12V relays and a timer IC NE555 along with 230V coil contactor having four poles to switch on and switch off the appliance.

How the protector circuit works

Relays RL1 and RL2 act as a sensing devices for phases Y (Yellow) and B (Blue), respectively. These relays are connected such that each acts as an enabling device for the subsequent relay. Therefore the combination of the relays forms a logical AND gate connected serially.

The availability of phase R (Red) energises relay RL1 and its normally opened (N/O) contacts close to connect phase Y to the input of transformer X2. The availability of phase Y energises relay RL2 and its N/O contacts close to connect phase B to the input of transformer X3, thus applying a triggering input to timer IC NE555 (IC1).

Therefore the delay timer built around NE555 triggers only when all the phases (R, Y and B) are available. It provides a delay of approximately four seconds, which energises relay RL3 and its N/O contact closes to connect the line to the energising coil of four-pole contactor relay RL4. Contactor RL4 closes to ensure the availability of the three-phase power supply to the appliance.

The rating of contactor RL4 can be selected according to the full-load current rating of the appliances. Here the contact current rating of the four-pole contactor is up to 32A. The availability of phases R, Y and B is monitored by appropriate LEDs connected across the secondary windings of transformers X1, X2 and X3, respectively. Hence this circuit does not require a separate indicator lamp for monitoring the availability of the three phases. When phase R is available, LED1 glows. When phase Y is available, LED2 glows. When phase B is available, LED3 glows.

The main advantage of this protector circuit is that it protects three-phase appliances from failure of any of the mounted on the backside of cabinet. Connect the appliance through external wires.

WARNING: The circuit contains mains high voltage. Make sure the AC mains is disconnected during assembly of the circuit and double check everything before connecting your circuit to the AC mains.

The current through the LED is not limited to the resistor, but the capacitance of the capacitor. The capacitance can be selected so that the current passing through it could regulate directly after power LED. Since the current leads the voltage by 90?, does not arise (ideally) no power loss of the capacitor and the capacitor is not heated. Reactance capacitor can be easily calculated with the following formula:

where Xc is capacitive reactance in Ohm, pi (3.14), f frequency in Hertz and C the capacitance in Farad.
A capacitor with a capacity of 100nF will have for grid frequency reactance:
1 / (2x3.14x50x10-7) = 31831 Ohm

If you connect this capacitor voltage network, it will be the power flow 230/31831 = 0.0072 A. If we connect in series with the capacitor bridge rectifier with LED, the current has substantially altered.

Resistor R1 has only one task – during off indicator light capacitor discharge and prevent unpleasant “kick” while handling the device off.

Warning: The entire circuit is connected to the high voltage electrical network. Therefore, it is necessary to work at maintaining the necessary caution.

source:http://www.belza.cz/ac-led/kontrol.htm

Initially, when the water level is below the L1 strip, supplying electrical oscillation frequency is not transferred to the diode D1. Thus the low output and LED1 does not light. Also, because the base voltage of the transistor T1 is low, it is in a state of cut-off and the collector voltage is high, which enables to produce melody IC1 (UM66) and the alarm is sounded.

When the water is just touching the L1 level detector strip, the oscillation frequency of the supply transferred to the diode D1. This straightening supply voltage and positive DC voltage developing capacitor C1, which is lit LED1. At the same time the base voltage of the transistor T1 becomes high, which makes forward bias and collector voltage falls to near ground potential. Disabling IC1 (UM66) and the alarm is inhibited.

Depending on the quantity of water present in the tank, which shows the level of the corresponding LED lights up. It thus showing medium level of water in the tank with a bar-chart style.

When the water in the tank just touching the highest level detector lines L12, DC voltage developed in capacitor C2. This makes it possible to produce a melody IC1 (UM66) and the alarm sounds again.

This is indicator circuit to indicate that there is blown fuse, it will show the condition of fuse through LEDs. This circuit works for fuse 220V mains. This compact circuit is very useful and reliable. It is a low cost circuit which use very few components. Generally, when an electronic device indicates no power, the potential cause may be just a blown fuse.

Under normal conditions (when fuse is alright), voltage drop in first arm is 2V + (2 x 0.7V) = 3.4V, whereas in second arm it is only 2V. So current flows through the second arm, i.e. through the green LED, causing it to glow; whereas the red LED remains off.

When the fuse blows off, the supply to green LED gets blocked, and because only one LED is in the circuit, the red LED glows. In case of power failure, both LEDs remain “off”.

LED Indicators:

• RED LED : the fuse blows
• GREEN LED : normal condition, the fuse is OK
• Both LED Off : power failure, the mains is off or the circuit is broken.

This circuit can be easily modified by adding alarm/buzzer circuit to produce a siren in fuse-blown condition (see Fig. 2). An optocoupler is used to trigger the siren. When the fuse blows, red LED glows. Simultaneously it switches “on” the siren.

In place of a bicolour LED, two LEDs of red and green colour can be used. Similarly, only one diode in place of D1 and D2 may be used. Two diodes are used to increase the voltage drop, since the two LEDs may produce different voltage drops.

This is the circuit diagram of electronic quiz button table. This circuit is simple, easy to built and of course it is not expensive. This quiz circuit used for four game participants. It determines the contestant who first presses the switch (S1 through S4) to answer a question and locks out the remaining three entries so the others contestant’s buttons won’t work. Simultaneously, the respective audio alarm sounds and the bulb glows. The quiz table can be used for more number of contestants simply by adding buzzers, bulbs, MOSFETs and diodes.

This electronic quiz circuit provides an option for varying the time for which an individual buzzer and the corresponding bulb should be “on” after a particular competitor has pressed the push button. These timings can be set by presets VR1 through VR4 as required.

The circuit works off 12V, 1.5A power supply. The current rating of the power supply should be according to the load (wattage of bulbs). For higher-wattage bulbs, use power supply of a higher current rating. LEDs can be an alternative for low current power supply (battery).

This is the figure of how to setup the electronic quiz button on the quiz table:

If participant A presses switch S1, MOSFET T1 is triggered and the corresponding bulb BL1 (connected between drain of the MOSFET and 12V supply) glows and simultaneously piezobuzzer PZ1 connected in parallel to bulb BL1 sounds for the preset time. At the same time, capacitor C1 charges up to 12V, which then discharges through preset VR1. The discharging time of capacitor C1 is decided by preset VR1. For example, if preset VR1 s set for a resistance of 4.7k, it will give a delay of approximately 4 seconds, meaning that buzzer PZ1 and bulb BL1 will be “on” for 4 seconds. It also indicates that participant A is the first to press his switch. Even if any other participant, say, participant B, presses switch S2 after participant A has already pressed switch S1, buzzer PZ2 and bulb BL2 will not function since MOSFET T2 has no gate voltage to trigger because it is grounded through R2 and D1. The same principle applies for other contestants as well.

You can also use a group of LEDs to replace the bulb lamp, so you may create the better decoration for the looks of the lamp.

Circuit Notes:
This simple circuit uses a 741 op-amp in differential mode as a continuity tester.
The voltage difference between the non-inverting and inverting inputs is amplified by the full open loop gain of the op-amp. Ignore the 470k and the 10k control for the moment, and look at the input of the op-amp. If the resistors were perfectly matched, then the voltage difference would be zero and output zero. However the use of the 470k and 10k control allows a small potential difference to be applied across the op-amp inputs and upset the balance of the circuit. This is amplified causing the op-amp output to swing to full supply voltage and light the LED’s.

Setting Up and Testing of the circuit
The probes should first be connected to a resistor of value between 0.22 ohm and 4ohm. The control is adjusted until the LED’s just light with the resistance across the probes. The resistor should then be removed and probes short circuited, the LED’s should go out. As the low resistance value is extremely low, it is important that the probes, (whether crocodile clips or needles etc) be kept clean, otherwise dirt can increase contact resistance and cause the circuit to mis-operate. The circuit should also work with a MOSFET type op-amp such as CA3130, CA3140, and JFET types, e.g. LF351. If the lED’s will not extinguish then a 10k preset should be wired across the offset null terminals, pins 1 and 5, the wiper of the control being connected to the negative battery terminal.

source: http://www.zen22142.zen.co.uk/Circuits/Testgear/connectiontester.htm

This is the circuit diagram of zener diode tester which tests zener diodes with breakdown voltages extending up to 120 volts. This is a handy circuit and will help you to measure the zener diode easily. This diode zener tester can be fitted in a 9V battery box. 1/3 of the box may be used for four 1.5V batteries and the remaining 1/3 is sufficient for accommodating this circuit.

The circuit has main advantage that it works with a voltage as low as 6V DC and consumes less than 8 mA electric current. In this circuit a commonly available center tap transformer with 230V AC primary to secondary 9-0-9V, 500mA secondary is used in reverse to achieve higher AC voltage across 230V AC terminals. Transistor T1 (BC547) is configured as an oscillator and driver to achive required AC voltage across transformer’s 230V AC terminals. This AC voltage is converted to DC by diode D1 and filter capacitor C2 and is used to test the zener diodes. R3 is utilized as a series current limiting resistor.

After assembling the circuit of zener diode tester, check DC voltage across points A and B without connecting any zener diode. Now switch on the S1 switch. The DC voltage across A-B should vary from 10V to 120V by adjusting potmeter VR1 (10k). If every thing is all right, the circuit is ready for use. For testing a zener diode of unknown value, connect it across points A and B with cathode towards A. Adjust the potensiometer of VR1 so as to achive the maximum DC voltage across A and B. Note down this zener value corresponding to DC voltage reading on the digital multimeter. When testing zener diode of value less than 3.3V, the meter shows less voltage instead of the actual zener value. However, correct reading is obtained for zener diodes of value above 5.8V with a tolerance of ? 10%. In case zener diode shorts, the multimeter will show 0 volts.