0 Light Alarm Circuit

When there is no light, the light dependent resistor or LDR have a very high resistance, several Mega-ohms. When there is illumination in the surrounding, the LDR resistance drops to several kilo-ohms, this time it will activate transistor Q1. When Q1 is on, the musical UM66T IC will turn on and will generate sounds to the speaker via transistor Q2. You can use 3V to 3.6V battery as a power supply.

light activated alarm schematic

Part List:

R1 - 1 kilo ohms 1/4W

R2 - 120 kilo ohms 1/4W

R3 - 470 ohms 1/4W

POT - 100 kilo ohms potentiometer

ldr - any light dependent resistor

Q1, Q2 - 9013, 2N2222 or similar NPN transistor

SPK - 8 to 16 ohms speaker

UM66T - any Melody generator IC 

UM66T pinout

9013, 2N2222 pinout

2 Touch Alarm Circuit ver 3.0

Another simple touch alarm that uses 555 timer IC.

schematic diagram

555 IC pinout

Part List:
R1 - 5.6 Mega ohms 1/4W
R2 - 47 kilo ohms 1/4W
R3 - 1 kilo ohms 1/4W
C1 - 22uF electrolytic capacitor rated 16V
LED - red or any LED (see led info to know more)
BUZZER - any 5v to 12v buzzer
555 IC - LM555 or any timer IC

Other versions:
Simple Touch alarm
Touch switch using Transistor
Touch Alarm ver 1.1.0
Touch switch (ON and OFF separated)

1 Refrigerator door alarm circuit

One way of conserving energy in refrigerator unit is making sure that that its door is always closed when not in use. Sometimes we forgot or we left the door open for several hours or even days, as a result is an increase in electricity consumption and bills. Using this simple circuit, it will remind us always to close the fridge door.
In the circuit is Light Dependent Resistor (LDR) that senses light. When the door is open, the light inside is up thus making the resistance of LDR very low. When this condition continues for several seconds (around 20 sec), the buzzer will sound.

fridge alarm schematic

Part List:

R1 - 12 kilo ohms 1/4W

R2 - 33 kilo ohms 1/4W

R3 - 100 kilo ohms 1/4W

LDR - any LDR

D1, D2 - 1N4148, 1N4001 or  similar diode

C1 - 10nF capacitor

C2 - 47uF electrolytic capacitor rated 16V

Q1 - 2N2222, CS9013 or similar NPN transistor

IC - HEF4060B or any 14-stage counter/divider and oscillator

Bz - piezoelectric buzzer

Bat - 3V to 6V battery 

4060 pins

2N2222 pins

0 Simple Touch alarm and sensor

Touch indicator
When the plate A is touched (plate b is grounded), the LED will light up to 3.3 seconds then it turns off.

Touch indicator schematic

Part List:
R1 - 30 kilo-ohms 1/4W
R2 - 5 kilo-ohms 1/4W
R3 - 220 ohms 1/4W
R4 - 100 kilo-ohms potentiometer
C1 - 100uF electrolytic capacitor rated 10V or higher
C2 - 100nF Mylar, ceramic or any type of capacitor
Q1 - 2N3906, 2N4403, or similar PNP transistor
LED - red, yellow or any 3mm to 5mm LED
Touch plate -any conductor, a piece of wire or all plate
555 timer IC
DC - 6V to 9V battery

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Touch Alarm

The circuit above can be converted into an alarm circuit by just replacing or adding buzzer and doing some modifications. See circuit below. At the moment plate A is touched (grounded plate b), the buzzer will sound for 3.3 sec.

alarm circuit

Part List:
R1 - 30 kilo-ohms 1/4W
R2 - 5 kilo-ohms 1/4W
R3 - 49 kilo-ohms 1/4W (buzzer sound control)
R4 - 100 kilo-ohms potentiometer
C1 - 100uF electrolytic capacitor rated 10V or higher
C2 - 100nF mylar, ceramic or any ype of capacitor
Q1 - 2N3906, 2N4403, or similar PNP transistor
Q2 - 2N2222, 9013, or similar NPN transistor
Buzzer - 6V to 12V buzzer with min 20mA current rating
Touch plate -any conductor, a piece of wire or all plate
555 timer IC
DC - 6V to 9V battery

Diagrams of part list: Click to enlarge.

555 pins

2N222 and 9013  pins

LED pins

Buzzer

Circuit operation:
The heart of the circuit is the 555 timer IC operated in monostable mode. When the voltage across pin 2 is less than 1/3 of supply voltage, the timer will output a pulse in pin 3 with the duration equal to 1.1*R1*C1. Meaning you can increase or decrease the 3.3 sec time duration by just replacing the values of R1 and C1. You can also use the 555 timer calculator to test and check the time constant or duration. The use of R4 is for adjusting the sensitivity of the circuit. Increasing the value increases the sensitivity.

2 Smoke Alarm Circuit

This circuit  alarms you when smoke is detected. If used well, this can help you prevent fire or prevent fire from spreading and causing further damage.

smoke alarm schematic

Circuit operation
The LED will light up the light dependent resistor (LDR ) and making its resistance very low (as low as 500 ohms). In this case,  transistor Q1 and Q2 are off. When there is smoke in the area, light from LED will be blocked resulting to an increase in LDR resistance. In this case, both transistor are on, and 555 timer starts to generate sound to the speaker.

LDR

Part List:
R1-300 ohms
LED-white or any high  intensity LED
LDR
R3-100k potentiometer
R2-10 kilo-ohms
Q1-2N222,cs9013, or  similar NPN transistor
R4-1.2 kilo-ohms
R5-1.2 kilo-ohms
Q2-cs9012, any general purpose PNP transistor
C2-10nF
c3-2.2uF electrolytic capacitor
R6-1.2 kilo-ohms
R7-5 kilo-ohms
C1-100nF
555 IC
9V battery or DC supply
speaker (8-ohms)

Before you finalized the installation, first turn on the circuit and test it. While LED is on, adjust the value of potentiometer R3 until no sound is heard in the speaker.Then try to block the light from LED to LDR with a piece of paper to make sure if it is working, if not try adjusting R3 again.

0 Rain alarm circuit

Alarm schematic

materials:
555 timer IC
speaker - 8 ohms
10nF capacitor
aluminum foil touch plate (see above diagram)
2.2uF electrolytic capacitor
R1-3.9k ohms
R2-15k ohms
C-22nF or 0.022uF electrolytic capacitor

When it is raining the aluminum plate becomes  wet, at that instant the 555 IC will start to to produces  pulses (around 2kHz) to the speaker as an alarm.
You can visit links about 555 timer basics or 555 timer calculator for more info.

4 touch alarm circuit using 555 IC

This touch alarm circuit will sound when somebody touches the trigger plate. This plate can be place in any metal such our doors or in metal knobs. It can be use as your simple burglar alarm or security circuit that can place in your doors.

touch alarm schematic

Touch alarm circuit operation
The heart of this alarm circuit is the 555 timer  that is operated in monostable mode. The timer produces an output sound to buzzer with the time delay constant equal to 1.1RC.When the voltage at the trigger pin of 555 IC (pin 2) reaches below 1/3 of the supply voltage, the output will be high with the duration of 1.1RC. For 3 seconds duration, R is 12K resistor while 22k for 5 seconds. You can use 555 timer calculator for your delay options. The circuit is very similar to Simple Touch alarm and sensor.

Touch alarm materials:

Part list