SOLAR BUG

Solar Bug
This is a fun circuit, hide this solar-powered circuit suitably and see the reaction of your friends to the chirpy sound produced by it every few minutes. In all probability, it will coax them to find out where the sound is coming from. The circuit runs off a miniature solar power panel, which can be taken out from an old calculator such as Citizen CT-500. A panel giving 1.5V to 2.5V is required. Note that the circuit can work properly from a panel as small as 3 cm(square).

Working
If a digital voltmeter is connected across capacitor C2, a slow build-up of voltage can be observed when the panel is exposed to light. Transistors T1 and T2 form a relaxation oscillator.When C1 charges to 0.6V, transistor T1 conducts and the charge built up in C2 is discharged through the
piezobuzzer to produce a short beep. While testing the circuit, the value of resistor R1 can be reduced to, say, 1 kilo-ohm. Use a good-quality buzzer to ensure that the sound produced is
loud enough.

Circuit Diagram


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FORMULAE

ELECTRICAL CIRCUITS

ELECTRICITY

TIME CONSTANT CALCULATION

NOTATION CALCULATION

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Laser Activated Switch

Laser Activated Switch

This circuit is built around a 555 timer using very few components. Since the circuit is very simple, even a novice can easily build it and use it as a controlling device. A laser pointer, now easily available in the market, can be used to operate this device. This circuit has been tested in operational conditions from a distance of 500 metres and was found to work satisfactorily, though it can be controlled from still longer distances. Aiming the laser beam exactly on to the LDR is a practical problem. The circuit is very useful in switching on/off a fan at night without getting off the bed. It can also be used for controlling a variety of other devices like radio or music system. The limitation is that the circuit is operational only in dark or dull-lit environments. By focusing the laser beam on LDR1 the connected gadget can be activated through the relay, whereas by focusing laser beam on LDR2 we can switch off the gadget. The timer is configured to operate in bistable mode. The laser pointers are available for less than Rs 30 in the market. The cost of the actual circuit is less than Rs 25.

Circuit Diagram


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Mobile Cellphone Charger

MOBILE CELLPHONE CHARGER

Charging of the cellphone battery is a big problem while travelling as power supply source is not generally accessible. If you keep your cellphone switched on continuously, its battery will go flat within five to six hours, making the cellphone useless. A fully charged battery becomes necessary especially when your distance from the nearest relay station increases. Here’s a simple charger that replenishes the cellphone battery within two to three hours. Basically, the charger is a current-limited voltage source. Generally, cellphone battery packs require 3.6-6V DC and 180-200mA current for charging. These usually contain three NiCd cells, each having 1.2V rating. Current of 100mA is sufficient for charging the cellphone battery at a slow rate. A 12V battery containing eight pen gives sufficient current (1.8A) to charge the battery connected
across the output terminals. The circuit also monitors the voltage level of the battery. It automatically cuts off the charging process when its output terminal voltage increases above the
predetermined voltage level.

Working
Timer IC NE555 is used to charge and monitor the voltage level in the battery. Control voltage pin 5 of IC1 is provided with a reference voltage of 5.6V by zener
diode ZD1. Threshold pin 6 is supplied with a voltage set by VR1 and trigger pin 2 is supplied with a voltage set by VR2. When the discharged cellphone battery is connected to the circuit, the voltage given to trigger pin 2 of IC1 is below 1/3Vcc and hence the flip-flop in the IC is switched on to take output pin 3 high. When the battery is fully charged, the output terminal voltage
increases the voltage at pin 2 of IC1 above the trigger point threshold. This switches off
the flip-flop and the output goes low to terminate the charging process. Threshold pin 6 of IC1 is referenced at 2/3Vcc set by VR1. Transistor T1 is used to enhance the charging current. Value of R3 is critical in providing the required current for charging. With the given value of 39-ohm the charging current is around 180 mA. The circuit can be constructed on a small general-purpose PCB. For calibration of cut-off voltage level, use a variable DC power source. Connect the output terminals of the circuit to the variable power supply set at 7V. Adjust VR1 in the middle
position and slowly adjust VR2 until LED1 goes off, indicating low output. LED1 should turn on when the voltage of the variable power supply reduces below 5V. Enclose the circuit in a small plastic case and use suitable connector for connecting to the cellphone battery.

Circuit Diagram


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Secret Bell

Secret Bell
Many people move through the corridors and steps in multi storied buildings. As most of them are strangers for the inhabitants of the flats, it becomes necessary to verify the identity of the visitor before opening the door as he can be a burglar. This circuit helps you identify the members of your family. It is basically a switch less musical bell that activates with a single puff of breath. The condenser mic fitted inside the existing door-bell switch box will trigger the bell on detecting air-pressure changes following the breath. As only the members of your family know the secret of the bell and hence puff out before the hole for the switch box, the door can
be opened without fear.

Working
The front end of the circuit is a condenser mic amplifier with fixed sensitivity. Transistor T1 amplifies the signal received from the condenser mic through capacitor C1. When transistor T1 conducts, a short negative pulse triggers the monostable wired around IC1. The monostable time is decided by resistor R7 and capacitor C5. Reset pin 4 of IC1 is made stable by R6 and C3. Resistor R5 acts as a pull-up resistor for trigger pin 2 of IC1 to keep the trigger pin high in the standby mode. The high output from IC1 is used to power IC UM66 (IC2). IC2 generates a soft melody on receiving 3 volts at pin 2. Transistor T2 amplifies the music
notes. A zener diode maintains the power for IC2 at a safer level of 3 volts. The condenser mic
should be connected to the circuit using a single-core shielded wire to reduce noise interference

Circuit Diagram


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Abbreviations from N-Zt

Abbreviations used in Electronics N to TWT

N

Number of turns in an inductor

N

Revolutions per minute

n

Nano (10^-9)

N

Negative

nA

Nanoampere

NC

Normally closed

NC

No connection

NEG, neg

Negative

nF

Nanofarad

nH

Nanohenry

nm

Nanometer

NO

Normally open

NPN

Negative-positive-negative

ns

Nanosecond

nW

Nanowatt

OP AMP

Operational amplifier

P

Pico (10^-12)

P

Power

p

Instantaneous power

P

Positive, also peak

PA

Public address or power amplifier

pA

Picoampere

PAL

Programmable Array Logic

PAM, pam

Pulse amplitude modulation

P_ap

Apparent power

P_av

Average power

PCB

Printed circuit board

PCM, pcm

Pulse-code modulation

PDM

Pulse-duration modulation

pF

Picofarad

PLD

Programmable Logic Device

PLL

Phase locked loop

PM

Phase modulation, also Permanent magnet

PNP

Positive-negative-positive

POT, pot

Potentiometer

P-P

Peak to peak

PPM

Pulse-position modulation

PRF

Pulse repetition frequency

PRT

Pulse repetition time

pw

Pulse width

PWM, pwm

Pulse width modulation

Q

Charge, also quality

q

Instantaneous charge

R

Potentiometer

R

Resistance

RAM

Random access memory

RC

Resistance-capacitance, also Radio controlled

rcvr

Receiver

rect

Rectifier

ref

Reference

rf

Radio frequencies

RF

Radio frequencies

RFI

Radio frequency interference

R_L

Load resistor

RLC

Resistance-capacitance-inductance

RMS, rms

Root mean square

ROM

Read only memory

rpm

Revolutions per minute

SCR

Silicon controlled rectifier

SHF

Super high frequency

SIP

Single in-line package

SNR

Signal-to-noise ratio

SPDT

Single pole double throw

sq cm

Square centimeter

SSB

Single sideband

SW

Short wave

SWR

Standing-wave ratio

SYNC, sync

Synchronous

T

Tera (10¹²)

T

Torque

T

Transformer

t

Time in seconds

TC

Time constant, also temperature coefficient

TE

Transverse electric

temp

Temperature

THz

Terahertz

TM

Transverse magnetic

TR

Transmit-receive

TTL

Transistor-transistor logic

TV

Television

TWT

Travelling wave tube

UHF

Ultra high frequency

UHV

Ultra high voltage

UJT

Unijunction transistor

UV

Ultraviolet

V

Vacuum tube

V, v

Volt

v

Instantaneous voltage

VA

Volt ampere

V_av

Voltage (average value)

V_BE

DC voltage base to emitter

V_c

Capacitive voltage

V_CE

DC voltage collector to emitter

VCO

Voltage controlled oscillator

VHF

Very high frequency

V_in

Input voltage

V_L

Inductive voltage

VLF

Very low frequency

V_m, V_max

Maximum voltage

VOM

Volt ohm milliameter

V_out

Output voltage

V_p

Primary voltage

V_S

Source voltage

VSWR

Voltage standing wave ratio

V_T

Total voltage

W

Watt

X_C

Capacitive reactance

X_L

Inductive reactance

Y

Admittance

Z

Impedance

Z_in

Input impedance

Z_o

Output impedance

Z_p

Primary impedance

Z_s

Secondary impedance

Z_T

Total impedance

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