Photodiode is a type of light detector capable of converting light energy into either current or voltage, depending upon the mode of operation. Photodiodes are similar to typical semiconductor diodes except that they may be either exposed or packaged with a window to allow light to reach the sensitive part of the device.

Now you can protect your HiFi amplifier from damage due to excess heat. This circuit will cut off the power to the amplifier board at the moment it senses a high temperature. It automatically connects power when the temperature returns to normal. The circuit uses an NTC Thermister as heat sensor and it also has audible warning and reset provisions

The circuit uses the popular timer IC NE 555 as a temperature controlled switch. Its trigger pin 2 is connected to a potential divider comprising VR and the NTC Thermister. Thus the voltage level at pin 2 depends on the resistance of Thermister and the setting of VR. NTC (Negative Temperature Coefficient) Thermister has high resistance in normal temperature and the resistance decreases when the temperature increases. The Threshold pin 6 of IC is used to reset the IC if needed. When the pin 6 gets higher voltage than pin 2 IC will reset and output becomes low.

Power to the amplifier board is given through the Normally Connected (NC) contacts of the relay and the switch S2. So that power to the amplifier board will be available through the NC contacts of the relay and S2 when the relay is in the de-energized state. Resistance of the Thermister is set by VR so as to keep the trigger pin 2 of IC high at normal temperature. When the temperature inside the amplifier cabinet increases, resistance of Thermister decreases so it becomes conducting. This makes the trigger pin 2 of IC low and its output becomes high. T1 conducts, so that the relay energizes and buzzer beeps. This will break the power supply to the amplifier board. The relay will de-energize automatically when the temperature returns to normal

Keep Thermister near the Heat sink of the amplifier IC. Adjust VR for the particular temperature level at which relay energize.

Here is an AC triggered Multi tune door bell. It gives eight melodious tunes for 2 minutes. On each press it will give a new tone which will continue till the melody is over. The circuit is too simple and battery operated It uses the poular melody generator IC UM3481

          

The circuit uses the popular melody generator IC UM3481. It generates  music tones like Jingle bell, Santa clause is coming, Silent night, Holy night, Joy to the world, Rudolph, Red nosed reindeer, Merry Christmas, O come, All Ye faithful, Hark and the herald angel sing.UM 3482 generates 12 music tones. Capacitor C1 and resistor R2 are the oscillation components.Pin1 can be triggered either by DC or AC. Here low voltage AC is applied to the trigger pin 1 through R1 and D1. The circuit works off two pen cells which last for more than one year

Caution: The circuit is connected to high volt AC. Do not touch any parts when it is powered to avoid lethal shock

This circuit generates a sweet melody for two minutes when it is triggered. It uses the ROM IC UM66 to produce the melody. The music stops automatically.

When the push switch is pressed momentarily, NPN transistor T1 conducts and pull the base of T2 and it also conducts. When T2 conducts, C1 charges and provides power to IC UM66. Zener diode ZD is used to regulate the supply voltage to UM66 to 3 volts. When UM 66 gets 3 volts supply, it oscillates and the music tone from it will be amplified by T3 which can be heard through the speaker.

Note

If component removal is required for re assembling, do not bend the excess leads of components before soldering. Cut the excess lead close to the PCB and apply some flux. Apply little solder at the contact point. Do not apply, excess solder or over heat the solder points. PCB tracks will detach. Clean the surface with kerosene, or PCB cleaning fluid to remove flux and dirt.

This water level indicator can intimate the low and high water levels in the overhead tank through audible beeps.

Dual operational amplifier has two Op-Amps inside which can be controlled independently. Both the Op-Amps have inverting and non-inverting inputs and separate outputs. The inverting input of first Op-Amp is pin2 and that of the second is pin6. The non-inverting input of first Op-Amp is pin3 and that of the second is pin5.When the non-inverting inputs 3 and 5 gets higher voltage than the inverting inputs, the outputs 1 and 7 becomes high.

The inverting inputs(pin2) of Op-Amp 1 is connected to a potential divider R3 and R4 while its non- inverting input(pin3) is connected to a probe B ( metal rod) which is placed in contact with the upper water level. The non- inverting input (pin5) of second Op-Amp is connected to another potential divider R1 and R2 while its inverting input (pin6) is connected to another probe(C) which makes contact with the lower water level in the bottom of tank. A common probe A is also placed in the middle water level and it provides current through the probes. When the water level in the tank reduces below the probe C, the electrical contact between probes A and C brakes and the non- inverting input 5 gets more current than the inverting input6. As result output pin7 becomes high and buzzer beeps and Red LED lights indicating that the tank is going to be empty. This is the time for pumping water. When the water level rises and makes contact with probe C, buzzer stops beeping and Red LED turns off. When the water tank is almost full, probe B makes contact with water. At this time the non- inverting input 3 gets more current than inverting input2. As a result output 1 becomes high and Green LED lights indicating that tank is full. This is the time to switch off pump. The green LED remains on until the water level goes below probe B.

This circuit can stop the working of water pump when the overhead tank is full. Relay connected to the circuit triggers and breaks the power to the water pump.

The circuit uses the CMOS IC CD 4001 to drive the relay. When the probe A connected to the  inputs of gate 1 of IC is floating, output of IC1 remains low and the transistor T1 remains non conducting. Water pump then gets power through the common and NC contacts of the relay. When the water level rises, probe A and B gets electrical continuity and the output of IC1 turns high and activates the relay. The breaks the power to the pump.

Note : When the probes are free, circuit becomes active. LED and relay turns on. When water makes contat with the probes, relay turns off.

Relay is not included in the kit.LED indication can be used to confirm the the On/Off action of  the relay.

Note

If component removal is required for re assembling, do not bend the excess leads of components before soldering. Cut the excess lead close to the PCB and apply some flux. Apply little solder at the contact point. Do not apply, excess solder or over heat the solder points. PCB tracks will detach. Clean the surface with kerosene, or PCB cleaning fluid to remove flux and dirt.

Light your table top with this cool White LED Lamp. It is powered from the USB port and is ideal to take notes while browsing internet. The USB port can provide 5 volts and 100 mA current which is sufficient to light the LED lamp

Power to the circuit is derived from the USB port using the standard A type plug. To give continuous regulated power, Zener regulation is employed. Resistor R1 restricts base current to T1 so that output will be current regulated. Since White LED requires minimum 3.6 volts, 4.7 volt Zener is used to provide constant 4.7 volts to LEDs. Resistor R2 restricts LED current to around 20 mA, so that they will not be damaged. Connect the power lines to USB plug observing correct polarity. Use a reflector for LEDs to get maximum illumination.

Here is a simple solution to protect TV from voltage surge through cable during lightning. It gives protection, if the induced current is not too high. The circuit uses only two Safety capacitors to limit the voltage entering into the antenna input of the TV. This can be also used in Modem cables.

The circuit does not require a PCB or soldering. The safety capacitors can be serially connected to the core and shield of coaxial cable as shown in the circuit diagram.

Safety Capacitor

Safety Capacitors are designed to withstand high impulse voltages in applications where human beings might be exposed to voltage surges. These capacitors will shunt the energy from the impulse to ground, protecting the circuit from the surge. Safety capacitors are available in ceramic disc and metalized film or paper. The film type are made of self-healing metalized polyester, polypropylene or paper and usually come in a “box” style casing as the capacitor is encased is a flame retardant or flame-proof case.

There are two types of Safety Capacitors

X Capacitor: Capacitor for use in situations where failure would not lead to danger of electric shock. This is also known as a line to line application.

Y Capacitor: Capacitor for use in situations where failure could lead to danger of electric shock. This is also known as a line to ground application.

X rated capacitors are of two types

X1  Type

Rated voltage 250 V AC

Withstanding voltage 1500V AC 60 Hz

Impulse Voltage 4000V

X2 Type

Rated voltage 250 V AC

Withstanding voltage 1500V AC 60 Hz

Impulse Voltage 2500V

The purpose of these capacitors is to reduce radio frequency interference and to ensure safety from shock and fire. These are generally used in circuits where alternating, pulsating, intermittent and high steady voltage exists.

Keep Mosquitoes and Insect pests away from your premises using this Ultrasonic Pest Chaser. Its frequency can be varied from 10 KHz to 90 KHz so that most of the insect populations can be repelled.

Most of the animals including insects can hear ultrasound which is annoying to them. So they avoid such situations. This Ultrasound generator uses the popular CMOS IC CD 4047 to generate Ultrasound. This IC is the precision Monostable / Astable IC and its oscillating frequency can be changed using C2 and VR1. In this design it is used as an Astable Multivibrator. With the given values of C2 and VR1, the output frequency can be adjusted from 10 KHz to 90 KHz. Most of the animals respond to 22 KHz to 60 KHz. Mosquitoes respond to 38 KHz. The ultrasound vibrations from the outputs of IC will be amplified by a set of Push-Pull amplifier T1 through T4.A Piezo tweeter is used as sound generator because the piezo element is designed to generate KHz signals. The circuit requires 9 to 12 volt supply which can be obtained from a PP3 battery.

Note:           Response to the circuit should be tested through observation. Response depends on the number of insects, their behavior etc.  Try different frequencies and find the suitable frequency of a particular insect group.

This circuit turns on a White LED and its brightness can be controlled through the Pot VR1.

The circuit uses the Timer IC 555 in the Astable multivibrator mode with the timing components R1, VR1 (50K or 100K), R2 and C1 (0.1). In the Astable mode, the output of IC1 turns high and low continuously depending on the output frequency. The output frequency is controlled by the values of the timing components. When the Pot VR1 is turned maximum, brightness of White LED decreases and when the Pot is turned to minimum resistance, the brightness of LED increases.

Note

This circuit prevents overheating of soldering iron bit and subsequent damage. It keeps the soldering iron suffiently warm but not hot during the interval between soldering works. The circuit also saves electric energy and protects delicate components through heat controlled soldering

A Triac and a Diac are used to control the AC power through the soldering Iron. Diac adjusts the gate current of Triac and thus its firing angle. As a result, current flows through the soldering iron in a controlled manner. This controls the temperature of the soldering iron element. When the power is switched on, diode D1 forward biases and provides DC to Diac. It conducts only when the voltage across it increases above 34 volts. The conduction of the Diac can be adjusted through the 100K Pot. When Diac conducts, Triac gets gate current and it also conducts. This completes the current path through the soldering iron. The firing angle of the triac determines the rate of current flow through it and thus the heat. Resistor R1 and Diode D2 bypasses the current when Diac turns off and C1 stabilize the pulsing of Diac.

Connect the soldering iron to the AC socket as shown in the diagram and adjust VR for the required heat.

Caution: The circuit uses high voltage AC and there chance of lethal shock if handled carelessly. Do not touch any parts when powered and use a plastic knob for VR. Enclose the circuit in a shock proof case with the AC socket on the top.

Here is a DC adapter to charge mobile phone or play a music system using the power from car battery. The adapter will provide 4.7 volt regulated DC from the 13.5 volt DC power of the car battery

The circuit is basically a DC to DC converter with voltage and current regulation. The 13.5 volt DC from the car battery is given to the collector and base of T1. R1 restricts the base current of T1 to a safer level so that output will be current regulated. Zener diode ZD is an NPN Darlington transistor so that its high gain gives maximum output current. Diode D1 protects the circuit from accidental polarity reversal during connections. By changing the value of ZD, it is possible to get required output voltage. Here the 4.7 volt zener gives regulated 4.7 volts for charging a mobile phone. The adapter can be plugged into the cigarette lighter outlet of the car dash board.

Here is a crude but efficient charger for Lead Acid Battery. It uses a 12 volt car bulb as current regulator and charge status indicator. The brightness of the bulb indicates how much charge is flowing into the battery. When the battery becomes fully charged, lamp turns off. If the lamp is staying on with full brightness for more than 30 minutes, it indicates that the battery is dead and is not accepting charge.

Charging current is obtained from a 15-0-15 volt secondary 2 Ampere step down transformer. Diodes D1 and D2 are rectifiers which can handle 3 ampere current. In order to give “DirtyDC” for charging, a low value filtering capacitor C1 is used. So that the DC voltage will have some ripples which is necessary for better charging of lead acid battery.

The trick of the lamp is interesting. A 12 volt car tail lamp bulb is used in the circuit. It is connected in series with the positive output rail so that current flows through the bulb into the positive terminal of the battery. From the positive terminal, current passes through the battery chemistry into the negative terminal and then returns into the transformer. So the current flowing through the bulb depends on how much charge is using by the battery. When the charger is connected to the battery, the lamp turns on only if the battery requires charging current. OFF state of the bulb indicates that the battery is dead. If the battery holds some charge, bulb will turns on. If the battery is partially discharged and holding 50% charge, bulb will light brightly when the charger turns on. Then the brightness gradually reduces and finally the filament appears as a red hot line. This indicates that the battery is fully charged. The bulb also restricts the flow of current like a resistor.

This circuit works on the toggle mode through finger touch. For each touch on the touch plate, either the buzzer beeps or stops.

IC1 is used as a short duration Monostable using the timing components R1 and C1. The trigger pin 2 of the timer IC555 is connected to the touch plate. So normally, the output of IC1 is low. When the touch plate is touched with the finger, IC1 triggers and gives a short pulse to the input of IC2. CD 4017 is the Johnson decade counter IC with 10 outputs. Each output goes high with a positive pulse in its input pin 14. Here IC2 is wired as a toggle switch by shorting the third output pin 4 to the reset pin 15. So the outputs 2 and 3 of IC2 turns On / Off with each pulse in its input pin 14.

Either LED or Buzzer can be used to demonstrate the toggle function of IC2.

Note

If component removal is required for re assembling, do not bend the excess leads of components before soldering. Cut the excess lead close to the PCB and apply some flux. Apply little solder at the contact point. Do not apply, excess solder or over heat the solder points. PCB tracks will detach. Clean the surface with kerosene, or PCB cleaning fluid to remove flux and dirt.

This temperature meter uses the precision micro power centigrade sensor IC LM35. The output voltage of the IC is linearly equal to +10Mv per degree centigrade. The temperature level is displayed through LED readout.

The circuit uses the precision temperature IC LM35. This three pin transistor like IC give output linearly equal to +10mV per degree rise in temperature. It can measure temperature between -4 degree to +110 degree centigrade. Its related type LM34 is Fahrenheit sensor and its output is equal to -10mV per degree Fahrenheit. Output of IC1 is directly given into the input of the display driver IC LM3914. It is a monolithic integrated circuit with 10 active low outputs that can drive 10 LEDs directly without a current limiting resistor. The internal circuitry of the IC adjusts the current passing through the LEDs. The input of LM 3914 is very sensitive and its outputs 18 – 10 sinks current one by one as the input receives an increment of 125 milli volts. Here only 6 outputs are used to drive 6 LEDs. More LEDs can be included in the remaining outputs if required. As the IC LM35 senses temperature rise, LEDs one to six light up. If the sensitivity is not high, VR2 can be omitted. Then output of IC1 should be directly connected to the input of IC2

Calibration

When power is applied, some of the LEDs will glow. Calibrate the circuit by giving different temperature to IC1. For this a thermometer and hot water of different temperature is required. Sock some cotton with warm water of around 37degree (normal room temperature) and gently make contact with IC1. Adjust VR1and VR2, till LED1 glows.

This is the circuit of a very sensitive clap switch. It switches ON/OFF a White LED or electrical appliances through claps. The circuit can sense the sound of claps from a distance of 1-2 meters.

 Condenser Mic picks up sound vibrations caused by the clap. These sound vibrations are given to the inverting input (pin2) of IC1.It amplifies the sound collected by the Mic. Resistor R1, R3 and variable resistor VR1 adjust the sensitivity of the amplifier. Resistor R1 set the sensitivity of Mic. The amplified output pulses from IC1 passes to the input of IC2 (CD 4017).Resistor R4 keeps the input (pin14) of IC2 low so as to prevent false triggering. IC2 is a decade counter IC which is wired as a toggle switch. That its outputs 1 and 2 (pins 2 and 3) becomes high and low when the input pin14 receives pulses. Pin4 (output4) is connected to the reset pin15 so that further counting will be inhibited. The high output from IC2 passes through the current limiter R6 to the base of switching transistor T1. When T1 conducts, White LED (D2) turns on. If a 6V 100 ohms relay is connected to the points marked (A and B), the relay will also energize and the load (bulb or electrical equipments) will be switched on. In the next clap, output pin 2 becomes low and relay and White LED will be switched off. LED D1 (Red LED) indicates the OFF position.

This is the circuit of a simple intercom for personal conversation between two persons. It is a real fun for the children. Condenser Mic is used to pick up sound signals which are amplified by inverting amplifier made of IC 741. The amplified signals are given to a speaker which gives the audible voice. Capacitors C1 and C3 are DC blocking capacitors. Resistor R1 reduces the current to Mic and determines its sensitivity.

Assembling

Assemble the circuit on the PCB. Two units are required to make the Intercom. Observe polarity of capacitors, Mic and battery snapper. Orient IC in the correct direction. Connect the two units using two long wires (Thin wires) as shown in the diagram.

Wiring

Connect the ends of the wires to speaker connection points of PCB1. Connect the other ends to Speaker 2 that is kept in the unit 2. Similarly connect another two wires to the speaker connection points of PCB 2 and connect the other ends to Speaker 1 kept in unit 1.

Test

After assembling the circuit, connect battery. Speak in front of Mic. Your voice will be heard in the speaker of the other unit. The circuit has no volume control. Sound output is very clear and is pleasing.

The circuit uses a high bright White LED and an LDR to detect the arrival of mail in the box. IC NE 555 is designed as a Schmitt trigger by shorting its trigger pin 2 and threshold pin 6. When the light from the LED illuminates the face of LDR, it conducts taking the pin 6 of IC low. Output pin 3 then remains low inhibiting the buzzer. When a mail falls between the LED and LDR, light from LED will be blocked and the LDR becomes non conducting. This makes the threshold pin of IC high and its output goes high. The high output makes buzzer active and music will be heard. The music stops only when the letter is removed from the box. Keep LED and LDR at the opposite sides of the box.

Here is a simple Dancing LED Circuit. The LEDs turns on/off alternately giving a dancing appearance. It is a simple Astable multivibrator using two NPN transistors. It works on the principle of charging and discharging of capacitors C1 and C2. Current from the positive of battery flows through first set of LEDs D1-D3 to the collector of T1 through resistor R1. Resistor R1 limits current through the LEDs to protect them. The current through R1 charges capacitor C2. It then discharges through the base of T2 and resistor R4. This gives base current to T2 and it conducts. As a result second set of LEDs D4-D6 lights as the current flows through T1.Capacitor C2 again charges and the cycle repeats.