VISUAL AC MAINS VOLTAGE INDICATOR

You should not be surprised if someone tells you that the mains voltage fluctuation could be anywhere from 160 volts to 270 volts. Although majority of our electrical and electronics appliances have some kind of voltage stabilisation internally built-in, more than 90 per cent of the faults in these appliances occur due to these power fluctuations.

This simple test gadget gives visual indication of AC mains voltage from 160 volts to 270 volts in steps of 10 volts. There are twelve LEDs numbered LED1 to LED12 to indicate the voltage level. For input AC mains voltage of less than 160 volts, all the LEDs remain off. LED1 glows when the voltage reaches 160 volts, LED2 glows when the voltage reaches 170 volts and so on. The number of LEDs that glow keeps increasing with every additional 10 volts. When the input voltage reaches 270 volts, all the LEDs glow.

The circuit basically comprises three LM339 comparators (IC1, IC2 and IC3) and a 12V regulator (IC4). It is powered by regulated 12V DC. For power supply, mains 230V AC is stepped down to 15V AC by step-down transformer X1, rectified by a bridge rectifier comprising diodes D1 through D4, filtered by capacitor C4 and regulated by IC4. The input voltage of the regulator is also fed to the inverting inputs of gates N1 through N12 for controlling the level of the AC.

The LED-based display circuit is built around quad op-amp comparators IC1 through IC3. The inverting input of all the comparators is fed with the unregulated DC voltage, which is proportional to mains input, whereas the non-inverting inputs are derived from regulated output of IC4 through a series network of precision resistors to serve as reference DC voltages.

Resistors R13 to R25 are chosen such that the reference voltage at points 1 to 12 is 0.93V, 1.87V, 2.80V, 3.73V, 4.67V, 5.60V, 6.53V, 7.46V, 8.40V, 9.33V, 10.27V and 11.20V, respectively. When the input voltage varies from 160V AC to 270V AC, the DC voltage at the anode of ZD1 also varies accordingly. With input voltage varying from 160V to 270V, the output across filter capacitors C1 and C2 varies from 14.3V to 24.1V approximately. Zener ZD1 is used to drop fixed 12V and apply proportional voltages to all comparator stages (inverting pins). Whenever the voltage at the non-inverting input of the comparators goes high, the LED connected at the output glows.

Assemble the circuit on a general-purpose PCB such that all the LEDs make a bargraph. In the bargraph, mark LED1 for minimum level of 160V, then LED2 for 170V and so on. Finally, mark LED12 for maximum level of 270V.

Now your test gadget is ready to use. For measuring the AC voltage, simply plug the gadget into the mains AC measuring point, press switch S1 and observe the bargraph built around LEDs. Let’s assume that LED1 through LED6 glow. The measured voltage in this case is 220V. Similarly, if all the LEDs glow, it means that the voltage is more than 270V.

Touch Alarm

This is a new type of touch alarm that uses an RF oscillator at its input. One special feature of this touch alarm is that it can use a big-size touch plate. Also, no shielded wire is required between the touch plate and the circuit.

Circuit and working
Fig. 1 shows the circuit of the touch alarm, which is built around CD4060 (IC1), timer 555 (IC2) and a few discrete components. IC1 contains an easy-to-use astable or oscillator stage, followed by a 14-stage binary counter/divider. 

Fig. 1: Circuit of the touch alarm

Pulses from the oscillator stage are fed to the 14-bit binary counter stage. Not all the outputs of the binary counter are available externally. The frequency at pin 7 (Q3) of IC1 is actually divided by 24=16 compared to the initial astable frequency. Similarly, Q4 will have half the frequency of Q3. Reset pin (pin 12) is connected to ground to enable the pulses.
IC1 generates an astable frequency of around 4 MHz using low-cost inductor L1. The frequency available at pin 1 (Q11) of IC1 is around 1 kHz, which is actually astable frequency divided by 2¹². This frequency keeps transistor T2 conducting. Pin 4 of IC2 is held low as long as transistor T2 conducts.

When you touch the touch plate, the oscillator stops working because of loading. No output signals are available at pin 1 of IC1, due to which transistor T2 stops conducting. When transistor T2 is non-conducting, pin 4 of IC2 is high, activating the alarm. The frequency of sound can be changed by changing the value of timing components R6, R7 and C7.
Construction  and testing
An actual-size, single-side PCB for the touch alarm is shown in Fig. 2 and its component layout in Fig. 3. After assembling the circuit on a PCB, enclose it in a suitable case.

Fig. 2: An actual-size, single-side PCB for the touch alarm

Fig. 3: Component layout for the PCB

To test the circuit, check correct power supply at TP1 with respect to TP0. Do not touch the touch plate and verify 1kHz (approx.) frequency at TP2. This frequency will die if you touch the touch plate. At TP3 and TP4, verify voltage levels as indicated in the table.

Liquid-Level Alarm

In water-level controllers for tanks, a DC current is passed through the metallic probes fitted in the water tank to sense the water level. This causes electrolysis and corrosion of probes, inhibiting the conduction of current and degrading its performance. As a consequence, probes have to be replaced regularly to maintain proper current flow.

The liquid-level alarm given here overcomes this problem. A 1kHz AC signal is passed through the probes, so there will be no electrolysis and therefore the probes last longer.

Fig. 1: Block diagram of liquid level alarm

The block diagram for the liquid-level alarm is shown in Fig. 1. The signal generator sends the generated signal to the first metallic probe. The second metallic probe is connected to the sensing circuit followed by the alarm circuit.

The complete circuit for the liquid-level alarm is shown in Fig. 2. The astable multibrator built around IC 555 (IC1) generates 1kHz square wave signal, which is fed to one of the probes via a DC blocking capacitor.

Fig. 2: Liquid level alarm

Fig. 3: Pin configuration of UM66

When the water tank is empty, pnp transistor T1 does not get negative base bias. But as water fills up in the tank, it receives 1kHz signal from IC1 via the probes immersed in water and conducts during the negative half cycle of 1kHz signals. Due to the presence of capacitor C7 (2.2µF), npn transistor T2 continues to get base bias and conducts to provide 3.3V DC to melody generator IC UM66 (IC2). Pin configuration of IC UM66 is shown in Fig. 3. Preset VR1 acts as the output loudness controller. It can be varied to set the alarm sound from the speaker at the desired level.

The circuit works off 12V unregulated power and can be used to detect any conductive liquid.

Battery Charger with Automatic Switch-off

This smart charger automatically switches off when your rechargeable batteries reach the full charge.

The circuit comprises a bistable multivibrator wired around timer IC 555. The bistable output is fed to an ammeter (via diode D1) and potmeter VR1 before it goes to three Ni-Cd batteries that are to be charged.
Normally, the full charge potential of an Ni-Cd cell is 1.2V. Trigger the bistable by pressing switch S1 and adjust potmeter VR1 for 60mA current through the ammeter.

Now remove the ammeter and connect a jumper wire between its points ‘a’ and ‘b.’ Connect the positive output terminal of the batteries to the emitter of pnp transistor T1. The base of transistor T1 is held at 2.9V by adjusting potmeter VR2. The output of transistor T1 is inverted twice by npn transistors T2 and T3.

Thus when the batteries are fully charged to 3×1.2V=3.6V, a voltage higher than this makes transistor T1 to conduct. Transistor T2 also conducts and transistor T3 goes off. The threshold level of timer 555 reaches 6V, which is more than 2/3×VCC = 2/ 3×6=4V, to turn off the timer.

During charging, the threshold level of the timer is held low. The green LED (LED1) glows during charging of the batteries and goes off at the attainment of full charge.

Note that this circuit can be used only for 1.2V, 600mAH Ni-Cd rechargeable batteries that require 60 mA of current for 15 hours to charge fully.

FM Bug

FM Bug

This FM bug transmitter circuit will let you spy on people. The transmitter can be placed in the desired room and the conversation heard from a place far away just using a regular FM radio set.

The circuit is designed around a single transistor 2N3904 (T1), a custom-made coil (L1), three capacitors (C1 through C3), a trimmer (VC1), two resistors (R1 and R2) and, of course, a condenser microphone (MIC1). The circuit transmits in the frequency range of 88-105 MHz. Transmission range is 100 metres.

Working of the circuit is simple and based on analogue modulation in which a carrier signal is varied corresponding to the message signal.
The microphone picks up the sounds in its vicinity to produce corresponding electrical signal. This is the message signal that needs to be transmitted over FM band. The message signal is fed to the base of transistor T1. The tank circuit made using trimmer VC1 and coil L1 generates the carrier frequency. This frequency can be tuned using the trimmer. Transistor T1 modulates the audio signal from condenser microphone over the carrier signal produced by tank circuit. This modulated signal is transmitted through the antenna (ANT.).

Using trimmer VC1, tune the carrier frequency in FM band and confirm it with an oscilloscope. You will hear the conversation picked up by MIC1 when you tune frequency of the FM radio set to match frequency of the carrier.

Prepare the coil L1 using about 25cm length of 25SWG wire. Wrap the wire around a cylindrical object of 6mm diameter and take it out after eight turns.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. After proper soldering, fix switch S1 on the front side of the cabinet. Ensure that the oscillator is properly tuned. Addition of a dipole antenna will extend range of the FM bug transmitter. The circuit works off a 3V battery.

MEDIUM-POWER FM TRANSMITTER

The range of this FM transmitter is around 100 metres at 9V DC supply. 

Fig. 1: FM transmitter

The circuit comprises three stages. The first stage is a microphone preamplifier built around BC548 transistor. The next stage is a VHF oscillator wired around another BC548. (BC series transistors are generally used in low-frequency stages. But these also work fine in RF stages as oscillator.) The third stage is a class-A tuned amplifier that boosts signals from the oscillator. Use of the additional RF amplifier increases the range of the transmitter.

Coil L1 comprises four turns of 20SWG enamelled copper wire wound to 1.5cm length of a 4mm dia. air core. Coil L2 comprises six turns of 20SWG enamelled copper wire wound on a 4mm dia. air core.

Fig. 2: Pin configurations of transistors BC548 and C2570

Use a 75cm long wire as the antenna. For the maximum range, use a sensitive receiver. VC1 is a frequency-adjusting trimpot. VC2 should be adjusted for the maximum range. The transmitter unit is powered by a 9V PP3 battery. It can be combined with a readily available FM receiver kit to make a walkie-talkie set as shown in Fig. 3

Fig. 3: Walkie-talkie arrangement

Stablised Power Supply for Prototyping

Stablised Power Supply for Prototyping

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This stabalised power supply circuit may be directly connected to 230V AC mains to derive output voltages of 3V to 12V DC for connection to the prototyping board.

230V AC mains input is down-converted to 15V AC by step-down transformer X1, whose secondary winding can support a current of 2 amperes. A bridge rectifier converts the AC to pulsating DC with a peak voltage level of 21V (15x1.4142). LED1 lights up to indicate the availability of output from the rectifier. Resistor R1 (2.2-kilo-ohm) limits the current through LED1 to a safe value below 10 mA. The output from the bridge rectifier is smoothed by 470µF capacitor C1. Capacitor C2 bypasses high-frequency ripple.
An adjustable, LM317T series 3-terminal, positive-voltage regulator is used at the output of the rectifier section for regulation. It is capable of supplying in excess of 1.5A over 1.2V to 37V output voltage range. However, here it has been used to supply discrete voltages in steps of 3V, 5V, 6V, 9V and 12V with the help of 5-way rotary switch S2, which brings in different resistor values between Adj pin of the regulator and ground, while R2 (between Adj pin and output pin) is a fixed resistor of 220 ohms. The output voltage (Vo) is given by the relation:



where ‘Rx’ is the resistance connected between Adj pin of the regulator and ground.

In 12V position (‘off’ position of the switch), the value of Rx is R3+R4=1900 ohms, while in various other positions it is the series equivalent of 1900 ohms in shunt with another resistance selected by the rotary switch. The table shows the equivalent series resistance in various positions of the rotary switch.

Fig. 1: Adjustable power supply

Note: X1 rating in the circuit diagram is wrongly printed. That is, 15V-0-15V should be read as 0-15V.

Discrete resistor (with 1% tolerance) switching is preferred to employment of a variable resistor because the wiper contact becomes erratic after some use and the tolerance (variation with temperature) of a variable resistor is also much higher.

The LM317T regulator is to be fitted with a heat-sink between the regulator and PCB to provide the best heat transfer. Note that the higher the load current or the lower the voltage across the load, the higher will be the heat dissipation at the regulator. Assuming that you adjust the output to 3V and the load draws a current of 1.5A, there is a voltage drop of approximately 10 volts across IC1. The power dissipation at IC1 is 10×1.5=15 watts. To dissipate this heat, you must use a heat-sink of 4×10cm size or so. A 3mm aluminium plate of the mentioned size screwed to the regulator will work efficiently. A minimum voltage differential of 3 to 4V between the input and output voltages is essential for proper regulation.

Fig. 2: Pin configuration of LM317

Switch S1, transformer X1, LED1, fuse F1 and rotary switch S2 are preferably mounted suitably in a metallic box. The heat-sink (aluminium sheet) is to be inserted flat between regulator and the PCB and secured using a nut and bolt after applying some heat-sink paste on the metal portion of LM317T. Use a rotary switch mounted on the box and extend the connections from the PCB to the rotary switch position with common connection going to the pole of the rotary switch. As LM317T has built-in short-circuit protection, no fuse at its output is necessary. The circuit should be wired using a proper PCB.

Cat and Dog Repellent Circuit Diagram

august 23, 2012 by team elprocus 

The electronic dog repellent circuit diagram below is a high output ultrasonic transmitter which is primarily intended to act as a dog and cat repellent. The ultrasonic dog repellant uses a standard 555 timer IC1 set up as an oscillator using a single RC network to give a 40 kHz square wave with equal mark/space ratio. This frequency is above the hearing threshold for humans but is known to be irritating frequency for dog and cats.

Since the maximum current that a 555 timer can supply is 200mA an amplifier stage was required so a high-power H-bridge network was devised, formed by 4 transistors TR1 to TR4. A second timer IC2 forms a buffer amplifier that feeds one input of the H-bridge driver, with an inverted waveform to that of IC1 output being fed to the opposite input of the H-bridge which can be seen at A & B in an oscilloscope.

Get more information on various electrical projects, embedded projects, electronics projects, communication projects by visiting this blog regularly.

With this cool cat/dog repeller circuit you could chase the cats  off  from any where you want. In fact, I designed this circuit to chase my cat from my computer table. Most of the animals like cats respond violently to ultrasonic sound and in fact it’s the best way to chase them off. This principle is employed in this circuit.

The circuit here is nothing but an astable multivibrator wired around NE555 (IC1). The cat repellent circuit produces an ultrasonic sound in the range 15-20Khz. The NE 555 is enough to drive a small piezo buzzer and no amplification stages are needed. The POT R2 can be used to adjust the frequency of sound.

Cat Repeller Circuit diagram with Parts list. 

Notes.

• The circuit can be powered from a 9V battery or the power can be tapped from your computer SMPS.
• The circuit can be assembled  on a general purpose PCB.

Authors view. 

I do not guarantee the full effectiveness of the circuit.You have try a lot of frequency settings to make your cat feel discomfort at last. More over the cat may get accustomed to the sound after some time. For me the circuit was only a partial success  and now my cat feels nothing even if the speaker is placed in it’s ear.You try your luck. Best of luck.

Inverter Hack From UPS

PC Temperature Controller

Here is a simple temperature controller that turns your personnel computer (PC) off when the temperature of the PC increases beyond the optimal temperature value. Some of the larger integrated circuits become quite hot and if the temperature inside the PC becomes too high, these devices may not be able to dissipate heat fast enough. This, in turn, could lead to failure of devices and eventually of the PC.

Let us assume that the maximum working temperature of your PC is 55°C. So for safe working of your PC, this temperature controller uses a temperature sensor (LM35) and a comparator (CA3140) which disconnect the PC from the power supply whenever the temperature of your PC rises above 55°C. This threshold value is user-adjustable and can be set anywhere between 0°C and 100°C.

Fig. 1: Temperature controller circuit

Fig. 1 shows the circuit of the PC temperature controller, while Fig. 2 shows pin configurations of the components used. The circuit works off 9V DC, which is derived from main power as follows: Mains power supply is rectified by a bridge rectifier comprising diodes D1 through D4, divided by a resistor network comprising R1 and R2, and stabilised by zener diode ZD1. Capacitor C1 filters the ripples.

Using preset VR1 you can set the reference voltage. The reference voltage at non-inverting pin 3 of the comparator is set such that the temperature of the PC is 55°C. When the temperature of the PC is below 55°C, the voltage at the inverting input (pin 2) of IC2 is lower than the voltage at the non-inverting input (pin 3). At this stage, the comparator output at pin 6 of IC2 is high. This high output triggers triac 1 (BT136), providing mains power to operate the PC.

Fig. 2: Pin configurations of components

When the temperature of the PC increases above 55°C, the inverting input (pin 2) of IC2 also goes above the non-inverting input (reference voltage) at pin 3 and hence the comparator output goes low. This stops triggering of triac 1 (BT136) preventing mains power supply from reaching the PC.

Thus this arrangement provides mains voltage to the PC at temperature of up to 55°C and stops when the temperature goes above 55°C.

Assemble the circuit on any general-purpose PCB in the form of a PC expansion card, so you can use it as an add-on card to any PC. Plug it in, switch-on the supply and use your computer with safety temperature device.

Power Pulser

The idea behind this multipurpose power pulser is very simple. As shown in the circuit (Fig. 1), it uses a low-frequency oscillator to drive a voltage regulator. Timer chip LM555 (IC1) is wired as an astable multivibrator. Components R1 and R2, VR1 and C1 produce the free-running frequency. You can adjust it to some extent by varying potentiometer VR1. The output of IC1 at pin 3 controls the switching on/off of adjustable voltage regulator LM317T (IC2) through npn transistor SL100B (T1).

Fig. 1: Power pulser circuit

You can use input power supply of 5V-18V, 1.5A and adjust the output to 1.25V-15V, 1.5A. This pulsed output can be used for incandescent lamps, DC motors, electromagnetic relays and LEDs.

After selecting the desired load, power up the unit with switch S1 in ‘on’ condition. Now connect a digital multimeter across the output terminal (pin 2) of IC2 and set the required output voltage using potmeter VR2. The frequency of IC1 can be set through VR1, provided switch S1 is ‘on.’ Note that with 18V DC input, the maximum output voltage is approximately 15V only. The frequency of the astable multivibrator can be selected by using values of components R1, VR1, R2 and C1 according to your requirement.

Fig. 2: Pin configuration of regulator lm317

Fig. 3: Proposed cabinet

Assemble the circuit on any general-purpose PCB and enclose in a cabinet as shown in Fig. 3. Connect switch S1 and LED1 on the side of the cabinet. Fix potmeters VR1 and VR2 at the bottom of the front side. Also fix the input and output terminals on the front side of the cabinet. Using external wires, connect the power supply to the input terminal and the load to the output terminal.

Sunset Lamp

LDR-based automatic lights flicker due to the change in light intensity at dawn and dusk. So compact fluorescent lamps (CFLs) are unsuitable in such circuits as flickering may damage the electronic circuits within these lamps. The circuit described here can solve the problem and switch on the lamp instantly when the light intensity decreases below a preset level.

The circuit uses popular timer IC NE555 (IC1) as a Schmitt trigger to give the bistable action. The set and reset functions of the comparators within the NE555 are used to give the instantaneous action. The upper threshold comparator of IC1 trips at 2/3Vcc, while the lower trigger comparator trips at 1/3Vcc. The inputs of both the threshold comparator and the trigger comparator of NE555 (pins 6 and 2) are tied together and connected to the voltage divider formed by LDR1 and VR1. The voltage across LDR1 depends on the light intensity.

In daylight, LDR1 has low resistance and the input voltage to the threshold comparator goes above 2/3Vcc and its output becomes zero, which resets the internal flip-flop of IC1. But the input to the trigger comparator is still more than 1/3Vcc, which keeps output pin 3 of IC1 low. Triac BT136 connected to output pin 3 of IC1 remains quiescent due to insufficient value of current for firing it. Thus lamp L1 remains ‘off’ during daytime.
At sunset, the resistance of LDR1 increases, and the voltage at the input of the threshold comparator decreases below 2/3Vcc and that of the trigger comparator goes below 1/3Vcc. As a result, the outputs of threshold and trigger comparators go high, which sets the flip-flop. This changes output pin 3 of IC1 from low to high. Triac1 gets the necessary gate current through resistor R2 and fires. Thus it completes the power supply to the lamp through Triac1. LED1 glows to indicate the high output state of IC1.

Power supply to the circuit is directly derived from the mains through capacitor C4. This capacitor delivers current in the circuit. Diodes D1 and D2 rectify the AC from capacitor C4 and capacitor C3 provides the necessary smoothing. Zener diode ZD1 provides rectified 15V DC for the circuit. Bleeder resistor R4 removes the stored voltage of the capacitor when the circuit is unplugged.

Assemble the circuit on any general-purpose PCB and enclose in a plug-in type adaptor box. Connect the live and neutral points to the pins of the adaptor box. Provide in the box 5mm holes for LDR1 and LED1. Plug the unit at a place where daylight is sufficient to inhibit the circuit operation during daytime. Light from the lamp should not fall on LDR1 at night.

Caution. The circuit carries 230V AC and most of its points are at mains lethal potential. So do not touch any point in the circuit when it is powered and adjust the preset only with a plastic or insulated screwdriver.

Crystal AM Transmitter

Here is the circuit of a medium-power AM transmitter that delivers 100-150 mW of radio frequency (RF) power.

At the heart of the circuit is a crystal oscillator. A 10MHz crystal is used to generate highly stable carrier frequency. Audio signal from the condenser mic is amplified by the amplifier built around transistors T1, T2 and T3. The amplified audio signal modulates the RF carrier generated by the crystal oscillator built around transistor T4. Here modulation is done via the power supply line. The amplitude-modulated (AM) signal is obtained at the collector of oscillator transistor T4.

Fig. 1: Circuit of crystal AM transmitter

Fig. 2: Oscillator coil

Fig. 3: Modulation transformer

By using matching dipole antenna and co-axial cable, the range of signal transmission can be increased. For maximum range, use a sensitive radio with external wire antenna.

The circuit works off a 9V-12V battery. For oscillator coil L1, wind 14 turns of 30SWG wire round an 8mm diameter radio oscillator coil former with a ferrite bead (see Fig. 2). For modulation transformer X1, you can use the audio output transformer of your old transistor radio set. Alternatively, you can make it from E/I section transformer lamination with inner winding having 40 turns of 26SWG wire and the outer winding having 200 turns of 30SWG as shown in Fig 3.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet.

Hum-Sensitive Touch Alarm

Radiation signals from mains wiring can travel a few metres of distance. These can be induced by the electromagnetic field in the human body also.

This touch-sensitive alarm is based on generation of the AC hum signal. When someone touches the touch plate, low-power AC hum (the same as induced from AC wiring of the house) is generated on the touch plate. This signal is first amplified by the high-gain preamplifier built around IC 741 (IC1) and then fed to another op-amp CA3140 (IC2) that is wired as a voltage comparator.

When IC2 receives the amplified signal at its input pin 3, its output goes high. As a result, transistor T1 conducts to sound the buzzer. At the same time, LED1 glows. For satisfactory working of this unit, power it from mains- derived 12V.
Assemble the circuit on a general-purpose PCB and enclose in a small cabinet. Use a shielded wire to connect the touch plate to the circuit keeping the length of the wire and the size of the touch plate as short as possible.

Hot-Water-Ready Alarm

Electric kettles turn off automatically when water has boiled. What if the boiler beeps to alert you when your water has boiled? The tripping sound of the thermal switch may not register as an alarm in your mind. Here is such an add-on unit that gives intermittent beeps at the end of boiling. It has the advantages of extremely low component count, low cost, small size and light weight. Fig. 1: Hot-water-ready alarm circuit In this circuit, current flowing through the coil provides a magnetic field that actuates a reed switch. The coil is made of twelve turns of 20SWG enamelled copper wire to carry the current (refer Fig. 3). The number of turns will change with the wattage of the kettle and the type of reed switch. Fig. 2: Reed switch Fig. 3: Reed switch with coil When the kettle is switched on, current through the coil creates a magnetic field, the reed contact closes and C2 charges through R1. Although the reed switch periodically opens at AC mains frequency, the time constant of R1 plus R2 and C1 is such that there is no voltage across C1. When the kettle switches off, the reed switch is permanently open and capacitor C1 starts charging through resistor R2. This capacitor C1 voltage makes transistor T1 conduct and the buzzer sounds (as it gets the supply path) to indicate that water has boiled. Fig. 4 shows a piezobuzzer. Fig. 4: Piezobuzzer Assemble the circuit on a common PCB and enclose in a plastic case. Connect mains input power supply to the circuit and 230V AC output to the kettle. Fix the unit near the kettle. Connect reed switch and its coil as shown in the circuit.

1W LED For Automotive Applications

1W LED For Automotive Applications

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This simple circuit lets you run a 1W LED from the battery of your car. IC MC34063 is used here as a buck converter. It is a monolithic switching regulator sub-system intended for use as a DC-DC converter. The device consists of an internal temperature-compensated reference, a comparator, a controlled duty-cycle oscillator with an active current-limit circuit, a driver and a high-current output switch. These functions are contained in an 8-pin dual in-line package. Another major advantage of the switching regulator is that it allows increased application flexibility of the output voltage.

Fig. 1: 1W white LED for automotive

Fig. 2: Internal diagram of MC34063

Fig. 1 shows the circuit of 1W LED for automotive applications. Fig. 2 shows the internal diagram of IC MC34063, while Fig. 3 shows a 1W LED. The non-linear voltage-to-current relationship and variation in forward voltage with temperature necessitate the use of a 320mA constant-current power source as provided by this supply. The current through LED1 is sensed by a 15-ohm resistor (R4) and the sensed voltage is fed to the controller to maintain a constant current. Resistor R1 limits the peak current given to IC1. With capacitor C2 (470pF), the oscillator runs at about 100 kHz.

Once the circuit is ready, do not connect the LED directly: First, use a 10-ohm, 5W resistor as the dummy load and verify the current and voltage across the dummy. Run the dummy for a few minutes. If the result is satisfactory, replace the dummy with the actual LED1. Inductor L1 used here is a salvaged drum core of size 8×10mm2 with 70 turns of 33SWG enameled copper wire, giving an inductance of about 220 µH.

Fig. 1: 1W LED

Assemble the circuit on a general-purpose PCB and enclose in a suitably small cabinet. Solder the IC base for easy troubleshooting. Fix the inductor and the LED1 (using a suitable heat-sink) in the PCB. Like all light bulbs, LED lighting is not water-proof. If used outdoors, it must be mounted in a sealed enclosure. Thermal management is very important for the power LED. Otherwise, a high temperature will shorten its life.

EFY note. During testing at EFY Lab, we used an LED from Kwality Photonics whose part number is KLHP3433.

water tank water level controller

FM bug

mobile sniffer

This circuit can detect the use of a GSM mobile in mobile-phone-restricted areas such as examination halls and other ‘do not disturb’ areas. It can detect the activity of the phone from a distance of eight metres or more. The sniffer keeps monitoring the RF level in the area and gives warning indication if the RF level increases due to mobile phone activity. If two identical units of this sniffer are placed in the room, the range can be extended to a radius of 15-16 metres. The circuit can detect all forms of mobile phone activity even in the silent mode.

The circuit is designed as a sensitive RF detector. RF signal diode 1N34 forms the major element. Along with resistor R1 and capacitor C2, the diode picks up RF energy in the area. In the standby mode, the output from the diode is around 0.6 millivolt, which rises to 60 millivolts when it receives the high energy radiation from the mobile phone. Since the voltage level from the sensor diode is too weak, three-stage amplification is provided to give the warning indication through the speaker.

Output pulses from the sensor diode (1N34) are preamplified by transistor BFR96 (T1). It is an RF/microwave low power transistor with high current gain and bandwidth. It has a high power gain of 14.5 dB at 0.5 GHz. Resistor R2 maintains the feedback and capacitor C4 keeps the collector voltage of T1 steady for maintaining the amplification.
The preamplified signals are fed to the second amplifier stage built around IC TL071 (IC1). It is a low-noise, JFET-input op-amp with low input bias and offset current. The BiFET technology provides fast slew rates to IC1. Here IC1 is designed as an inverting amplifier with resistor combination of R4 and R5 as potential divider to set half sup-ply voltage to its non-inverting input. The inverting input of IC1 receives the preamplified signals from T1. Variable resistor VR1 adjusts the feedback of the inverting amplifier and hence its gain.

The amplified signals from IC1 pass through capacitor C5 and diode D2 into volume control VR2. It also receives the signals from unit 2 identical to unit 1 through capacitor C6 and diode D3. From volume control VR2, power amplifier IC2 gets the amplified signals. IC LA4440 (IC2) is a two-channel audio power amplifier with inbuilt dual channels for stereo and bridge amplifier applications. In dual mode it gives 6W, and in bridge mode the output is 19W. It has good ripple rejection of 46 dB, small residual noise, built-in over-voltage and surge-voltage protection, and pin-to-pin protection. Here IC2 is wired in bridge configuration using only one input.

Normally, a feeble hissing noise is heard from the speaker, indicating that the sniffer is active. The hissing noise is due to the detection of RF in the area. Its loudness can be adjusted using VR2. When a mobile phone is activated within the range of eight metres, a loud motor-boating sound is heard through the speaker. This is due to a very high RF activity during the activation of the mobile phone. The sound is louder if the mobile phone is within a radius of two metres.

Power to the circuit is derived from a 12V, 4.5Ah rechargeable battery, as AC power supply may generate audible disturbances in the circuit. A plug-in charger can be used to recharge the battery. Only one power supply with power amplifier is sufficient and the two units can be connected to the power amplifier. Use a good-quality 8-ohm, 6W speaker for LS1. RF reception and performance of the circuit depend on many factors, such as output power of the mobile phone, its orientation and position.

For a Nokia handset, the circuit receives RF signals from a distance of 8 metres and the speaker produces a loud enough warning signal.