Power Supply Resumption Alarm

Here is an audio annunciator which can be useful in factories, industries, auditoriums, and theatres etc where generator is used when power fails. It produces an alarm for a predetermined time when power resumes.

power supply resumption alarm

The circuit is built around IC555 wired in monistable mode. The surge when power resumes triggers the monostable which switches transistor SL100 on. The relay energized and the bell or buzzer connected to N/O contacts of the relay operates. The preset time depends upon VR1 and C2. The Capacitor C3 connected to pin 2 of 555 should have low leakage.


Resistors (all ¼-watt, ± 5% Carbon)

R1, R2 = 10 KΩ

R3 = 470 Ω



C1 = 1000 µF/25V

C2 = 100 µF/25V

C3 = 1 µF/25V

C4 = 0.01 µF


IC1 = NE555

T1 = SL100

D1 – D3 = 1N4001


X1 = 12V – 0 – 12V 500 mA secondary

F1 = Fuse

RL1 = 12V, 400Ω relay

230V buzzer

Voltage Stepper

In conventional voltage multiplier circuits, AC is used to charge the capacitors network via diodes in one cycle and discharge in the other cycle in a particular combination, which thereby produces multiples of the peak voltage. However, this circuit works on a different principle, and it is DC which is doubled. It can be used to power low current circuits.

IC555 is configured as an astable multivibrator producing rectangular pulses of about 10kHz frequency. Its output is made to drive the transistor pair T1 and T2. Transistor T2 being a pnp type, conduct when its base is negative, i.e. when the output of the IC produces a “low”. This charge C4 via diode D1 and ground (collector of T2 is grounded).

voltage stepper

For the next pulse, i.e. when the output of IC is high, T1 conducts but T2 is cut-off, C4 cannot discharge because of diode D1. So the voltage across C4 and input voltage adds up and charge C5 via D2. Voltage across C5 will equal Vcc pulse voltage across capacitor C4 and Diode D1. Hence the operation.

However, it was found that if current greater then 50 mA and drawn, output voltage, hence regulation, is lost. Any DC voltage between 5V and 18V can be boosted (both voltage being the minimum and maximum range of the IC).

For better results, increase the value of C4 and C5 to 47 µF/40V.


Resistors (all ¼-watt, ± 5% Carbon)

R1 = 220 Ω

R2 = 6.8 KΩ

R3 = 68 Ω


C1, C2 = 0.01 µF

C3 = 0.1 µF

C, C5 = 22 µF/40v


IC1 = NE555

T1 = SL100

T2 = SK100

D1, D2 = 1N4001

Electronics Carbon Gauge

Here is a simple circuit which will help driver, engine mechanics or pollution control officials to determine the amount of carbon (partially oxide fuel) emitting from engine. The basic circuit is so popular that it needs no explanation. The trigger pin 2 of IC1 is connected to negative through 1 M-ohm lin. Pot.

The peripheral device of this circuit is a screen with four aluminum or copper foil pieces connected in two sets A and B. These pieces are fixed on a plastics or laminate sheet in dimensions shown in figure. Otherwise it can be made as a PCB by etching. The two set A and B are connected to X and Y terminals of the main circuit.

To measure the amount of carbon, place the metal foil side of the screen in front of the silencer outlet of the working engine, keeping a fixed distance between them. After a preset period (about 3 minutes) remove the device. Turn VR1 to maximum resistance position (B) and switch on the circuit.

elecronics carbon gauge

The carbon particles sprayed on the metal foil pieces act as resistor between points X and Y. Now rotate VR1 slowly towards minimum resistance position (A). At a critical point, the output of the IC goes high and the LED light up the LED1.


R1 = 47 Ω

R2 = 470 Ω

VR1 = 1 MΩ

IC1 = NC555



Plant Tender

If you are one of those people who would like to spend their well earned summer vacations in the comforts of a hill resort but worried about your potted companions back home, then a plant tender is what you need.

As the soil dries up, its resistance increases causing the voltage at pin 2 of IC555 to fall below 1/3 Vcc. This activates the trigger comparator and the output goes high, switching on the pump. The output remains high for the time interval set by VR1 and is given by

Tminutes  = 1.1 R.C/60*106 where R is in ohms ans C is in microfarad

plant tender

The pump switches on again only soil dries up.

Resistors (all ¼-watt, ± 5% Carbon)

R1 = 1 KΩ

VR1, VR2 = 1 MΩ


C1 = 470 µF/16V

C2 = 0.01 µF


IC1 = NE555

T1 = SL100

D1 = 1N4001


RL1 = 12V, 100Ω Relay

Bicolour LED flasher circuit

Basically this circuit is similar to some flasher circuits already published in www.electronicsproject.org using IC555 as a free running multivibrator. The only difference lies in the way it flash using bi-colour LEDs.

When output at pin 3 of IC 555 goes high it operates on group of LEDs. By inverting the IC’s low output by pnp transistor BC558 the other group of LEDs is made to flash.

AS shown in  circuit diagram the LEDs are arranged in alternately reversed order so that a “twinkle twinkle little stars” effect is produced. The 100K preset VR1 sets the blinking rate.

The circuit diagram for the project is:

bicolor led flasher

To maintain simplicity I omitted the current limiting resistor. So, the red and green LEDs may not flash with equal intensity (as they require different threshold voltage).


Resistors (all ¼-watt, ± 5% Carbon)

R1, R2 = 1.5 KΩ

R3 = 330 Ω

VR1 = 100 KΩ


C1 = 10 µF/10V


IC1 = NE555

T1 = BC558

LED1 – LED5 = Bi-colour LEDs


Electronics Portable Organ

Most of the mechanical device is replaced by electronics circuit due to its portability, low cost, stable performance etc.  Here is a simple project “Portable Electronics Organ”  from single chip IC555  in order to replace the mechanical one. The circuit produces simple tones when the keys are pressed. 

Circuit Description

The entire circuit of “Portable Electronics Organ” is build around Timer IC NE555 which is used in oscillator mode. The output is given to loudspeaker LS1 through DC blocking capacitor C1 from pin 2 (tone generator output). Resistor R2  through R6 and capacitors C2 determines the working frequency of the circuit. By selecting switch various tones can be generated. The preset VR1 is tuned to different notes and correct pitch by aural means, where reference notes is provides by pitch pipes or a tuned music instrument.

electronics portable organ schematic

electronics portable organ schematic


Resistors (all ¼-watt, ± 5% Carbon)

R1 = 1 KΩ

R2 = 1 KΩ

R3 = 1 KΩ

R4 = 1 KΩ

R5 = 1 KΩ

R6 = 1 KΩ

R7 = 100 Ω

VR1 = 20 KΩ variable type



C1 = 10 µF/15V

C2 = 0.1 µF or 100n Ceramic type

C3 =  4700 µF , 15 V


IC1 = LM7809 (9V Regulator IC)

IC2 = NE555 

D1 – D4 = 1N4002



Fuse and Power Failure Indicator

Fuse blowing is common problem, but is so tedious to go and check because every time it may not blown. Here is a simple circuit of fuse and power failure indictor,for indicate the power failure by giving alarm. The logic of this circuit is so simple and it also overcome the problem of checking every time.

Circuit Description of fuse and power failure indicator

The entire circuit of fuse and power failure indicator is build and fabricated around LDR, timer IC and LED. Here LED get power-supply from AC source which is stepped down by transformer X1 and further rectified by diode and changed to regulated 6V D.C. with the help of regulator IC 7806 (IC1). Here LED1 and LDR1 are kept in a cabinet together. Transistor T1 and T2 is used as cascade amplifier which amplify the divided voltage from LDR1. Trigger is given to pin no 2 of IC2 from the collector of T2. Here timer IC (IC2) configured as monostable multivibrator and its output is given to reset pin 4 of IC3 which is configured as astable multivibrator. The high output is given for about 7 second from IC2 to IC3. IC3 generate audio frequency and connected to loudspeaker. An alarm sound is produced from fuse and power failure indicator, indicating the power failure.


Resistors (all ¼-watt, ± 5% Carbon)

R1 = 100 Ω

R2, R4, R6 = 10 KΩ

R3, R5 = 1.5 KΩ

R7 R8 = 1 KΩ

VR1 = 10 KΩ

VR2 = 100 KΩ

VR3 = 1 KΩ


C1 = 1000 µF/25V

C2, C4, C6 = 0.01 µF

C3 = 100 µF/16V

C5 = 0.47 µF

C7 = 33 µF/16V

C8 = 0.47 µF


IC1 = 7806 (regulator IC)

IC­2, IC3 = NE555 (Timer IC)

D1 = 1N4001


X1 = 230V AC primary to 9V-0-9V, 250mA secondary transformer

LED1 = White

F1 = Fuse 500 mA

S1 = ON/OFF Switch

S2 = Push-to-on switch

Batt1 = 6V Battery

LS1 = 8Ω, 0.5W Speaker


Fire Alarm Using Thermistor

Many fire alarm circuit is published in different website. But, here in this website is a simple and inexpensive project of fire alarm using thermistor. where thermistor is used as temperature sensor of fire alarm.Working principle of thermistor is same as LDR (change their resistance with change in heat where LDR change their resistance with change in light fall on it).

Circuit Description of fire alarm using thermistor

The whole circuit of fire alarm using thermistor is build and fabricated around thermistor (TH1) and timer IC (IC1) with its driver transistor. The timer IC (IC1) used in this circuit is as astable multivibrator oscillator used to oscillate in audio frequency band. The two transistor T1 and T2 used to drive the timer IC (IC1). The output from pin 3 of IC1 is fed to loudspeaker through transistor T3 to generate sound. The value of resistor (R5 and R6) and capacitor (C2) determines the frequency of IC2.

The low resistance path of extend positive voltage to the base of transistor is provided when the thermistor TH1 become hot. Further collector of transistor T1 is connected to base of transistor T2 provides positive voltage to reset pin 4 of IC1 for reset. Fire alarm using thermistor circuit works on wide range of input power supply voltage i.e. 6v to 12V.


Resistors (all ¼-watt, ± 5% Carbon)

VR1 = 10 Kohms Variable Resistor for changing the sensitivity of the circuit.

R3, R7, R8 = 470 Ω

R2 = 33 K Ω

R4 = 560 Ω

R5 = 47 KΩ

R6 = 2.2 KΩ


C1 = 10 µF/16V

C2 = 0.04 µF

C3 = 0.01 µF


IC1 = NE555 (timer IC)

T1 = BC548

T2 = BC558

T3 = SL100B or any Medium power general purpose NPN transistor like: 2N4922 , 2N4921,2N4238, FCX1053A

D1 = 1N4001


TH1 = Thermistor 10 KΩ

LS1 = 8 Ω, 1W speaker

Doorbell-Controlled Porch light

Here is a simple circuit doorbell control porch light using timer IC (NE555),can be used in doorbell in order to get indication of porch light with door alarm if any one rings the door bell.

Circuit Description of door bell control porch light

The circuit of doorbell control porch light is build around timer IC NE555 (IC2) and is here used as astable multivibrator. The output at pin 3 goes high of IC1 when trigger is given to pin 2 from power supply. Here transistor T1 is used as relay driver transistor which energized relay when output from pin 3 of IC1 is high. After relay energized AC current flow via N/O terminal and switch on the porch light.

Triac1 with Diac1 hear is used for switch in order to glow porch light during night and switch it off at day with the help of preset VR1 and LDR1.

Note:- Here the power source of 230V AC is replaced by 110V with replacing transformer having primary 110V and secondary 12V. Rest of the circuit  of door bell controll porch light is same.


Resistors (all ¼-watt, ± 5% Carbon)

R1 = 1 MΩ

R2 = 10 KΩ

R3 = 470 Ω

VR1 = 100 KΩ


C1 = 1000 µF/25V

C2 = 22 µF/25V


IC1 = NE555 (Timer IC)

T1 = BC548

DIAC1 = DB-3

TRIAC1 = BT136

D1, D2 = 1N4001


SW1 PUSH-TO-ON DPST (Double Pole Single Switch) Switch

B1 = Porch Light Bulb 100W

X1 = 230V AC primary to secondary 0-12V, 250MA secondary transformer

RL1­ = 12V, 200 Ω 1C/O relay


Here is a simple, expensive and easy to use electronics street light switch using LDR and NE555. The working of this circuit is truly based on light sensing, i.e. automatic turn it on in night (no sunlight available) and turn It off at sunshine (sunlight available).

Circuit Description of Electronics street light switch

As electronics street light switch is a switching circuit so, for more detail we can divide this circuit into two section i.e. power supply and switching circuit.

In this power supply section the work of step-down transformer is done by register R1 and further rectification to change into 9.1V dc is by diode D1 and zener diode ZD1. The output voltage across zener diode is further filtered by capacitor C1 and C2.

The another section  of street light is switching section built around light-dependent register LDR1 with the help of transistor T1 through T3 and timer IC NE555 (IC1), where LDR1 is used as sensor of this switching circuit.As in day time the resistance of LDR1 remain low but it is reverse in night time i.e. high resistance is offered by LDR1. For this property of LDR1 the timer IC used in this circuit is as inverter. So, high input at pin 3 is provided by low input at pin 2 and vice-versa. Lastly, this inverter is used to turn street bulb B1 on with the help of triac (triac is activated).

The transistor T1 and T2  is remain cut-off to make pin 4 and pin 8 of IC1 low due to light fall on LDR1 during day time. Due to this transistor T3 is also cut-off and trigger voltage is not received by IC1 through pin 2. As a result the output voltage at pin 3 is low which does not activate triac and the street bulb does not glow.

Automatic street light switch


Resistors (all ¼-watt, ± 5% Carbon)

R1 = 10 KΩ/10-watt

R2 = 33 KΩ

R3 = 39 KΩ

R4, R6, R7 = 10 KΩ

R5 = 100 Ω


C1, C5 = 0.1 µF

C2 = 1000 µF/25V

C3 = 10 µF/25V

C4 = 0.01 µF


IC1 = NE555 timer IC

T1, T3 = BC548

T2 = 2N2222

ZD1 = 9.1V/0.5V

D1 = 1N4001

Triac1 = BT136

LED1 = RED color


LDR1 = light-dependent resistor

F1 = Fuse, 5A

B1 = 100W/ 230V AC

SW1 = On/off Switch


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