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Friday, December 20, 2013

AC 220V Mains Powered Emergency Light and Alarm

This circuit is permanently plugged into a mains socket and NI-CD batteries are trickle-charged. When a power outage occurs, the lamp automatically illuminates. Instead of illuminating a lamp, an alarm sounder can be chosen. When power supply is restored, the lamp or the alarm is switched-off. A switch provides a "latch-up" function, in order to extend lamp or alarm operation even when power is restored.

Emergency Light and Alarm Circuit Diagram

Parts List:

R1 = 220K
R2 = 470R
R3 = 390R
R4 = 1.5K
R5 = 1R
R6 = 10K
R7 = 330K
R8 = 470R
R9 = 100R
D1 = 1N4007
D2 = 1N4007
D3 = 1N4007
D4 = 1N4007
D5 = 1N4007
D6 = Led
D7 = 1N4148
Q1 = BC547
Q2 = BC327
Q3 = BC547
Q4 = BC547
Q5 = BC327
C1 = 330nF-400V
C2 = 10uF-63V
C3 = 100nF-63V
C4 = 10nF-63V
LP1 = 2.5V-300mA Torch Lamp Bulb
PL1 = Male Mains Plug
SW1 = SPST Switches
SW2 = SPST Switches
SW3 = SPDT Switches
SPKR = 8 Ohms Loudspeaker
B1 = 2.5V Battery (two AA NI-CD rechargeable cells wired in series)

Mains voltage is reduced to about 12V DC at C2s terminals, by means of the reactance of C1 and the diode bridge (D1-D4). This avoids the use of a mains transformer. Trickle-charging current for the battery B1 is provided by the series resistor R3, D5 and the green LED D6 that also monitors the presence of mains supply and correct battery charging.

Q2 & Q3 form a self-latching pair that start operating when a power outage occurs. In this case, Q1 biasing becomes positive, so this transistor turns on the self latching pair. If SW3 is set as shown in the circuit diagram, the lamp illuminates via SW2, which is normally closed; if set the other way, a square wave audio frequency generator formed by Q4, Q5 and related components is activated, driving the loudspeaker.

If SW1 is left open, when mains supply is restored the lamp or the alarm continue to operate. They can be disabled by opening the main on-off switch SW2. If SW1 is closed, restoration of the mains supply terminates lamp or alarm operation, by applying a positive bias to the Base of Q2.

Note:

Close SW2 after the circuit is plugged.

Warning!

The circuit is connected to 230Vac mains, then some parts in the circuit board are subjected to lethal potential!. Avoid touching the circuit when plugged and enclose it in a plastic box.

Source
Red Free Circuit Design

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Sunday, September 29, 2013

Simple Knock Alarm With Piezo Sensor

This circuit uses a thin piezoelectric sensor to sense the vibrations generated by knocking on a surface; eg, a door or table. Basically, it amplifies and processes the signal from the sensor and sounds an alarm for a preset period. In operation, the piezoelectric sensor converts mechanical vibration into an electrical signal. This sensor can be attached to a door, a cash box, cupboard, etc using adhesive. A 1-1.5m long shielded cable can then be connected between the sensor plate and the input of the circuit. The signal generated by the sensor is amplified by transistors Q1-Q3 which are wired as common-emitter amplifiers.

The signal is then rectified by diode D1 and amplified by transistors Q4-Q6. As shown, the output from Q6s collector is fed to pin 4 (reset) of 555 timer IC1. This is wired as an astable multivibrator. Each time Q6 turns on, its collector goes high and IC1 activates and produces an alarm tone in the speaker. The alarm automatically turns off 10s after knocking ceases - ie, the time taken for the 22µF capacitor on Q4s emitter to discharge. Finally, note that it may be necessary to adjust the 470O resistor in Q6s collector circuit to ensure that IC1 remains off in the absence of any perceptible knock. A value somewhere between 220O and 680O should be suitable.

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Monday, August 12, 2013

Drinking Water Alarm

The
State Jal Boards supply water for limited duration in a day. Time of
water supply is decided by the management and the public does not know
the same. In such a situation, this water alarm circuit will save the
people from long wait as it will inform them as soon as the water supply
starts. At the heart of this circuit is a small water sensor. For
fabricating this water sensor, you need two foils—an aluminium foil and a
plastic foil. You can assemble the sensor by rolling aluminium and
plastic foils in the shape of a concentric cylinder. Connect one end of
the insulated flexible wire on the aluminium foil and the other end to
resistor R2. Now mount this sensor inside the water tap such that water
can flow through it uninterrupted. To complete the circuit, connect
another wire from the junction of pins 2 and 6 of IC1 to the water
pipeline or the water tap itself. The working of the circuit is simple.

Drinking Water alarm Circuit Diagram

Timer
555 is wired as an astable multivibrator. The multivibrator will work
only when water flows through the water tap and completes the circuit
connection. It oscillates at about 1 kHz. The output of the timer at pin
3 is connected to loudspeaker LS1 via capacitor C3. As soon as water
starts flowing through the tap, the speaker starts sounding, which
indicates resumption of water supply. It remains ‘on’ until you switch
off the circuit with switch S1 or remove the sensor from the tap. The
circuit works off a 9V battery supply. Assemble the circuit on any
general-purpose PCB and house in a suitable cabinet. The water sensor is
inserted into the water tap. Connect the lead coming out from the
junction of 555 pins 2 and 6 to the body of the water tap. Use on/off
switch S1 to power the circuit with the 9V PP3 battery.

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Saturday, April 6, 2013

Infrared Proximity Detector Alarm

This circuit can be built from readily available low-cost components, some of which may even be hiding in your junkbox! The indicated value of 22 Ω for resistor R1 causes an average current of about 65 mA through infrared emitter D1. Because the IRED is pulsed at a duty factor of about 50% through the action of T1 and IC1, a peak current of 128 mA flows during every half cycle. This may seem a lot but in fact is well within the safe speciﬁcation of the LD274. The LM567 PLL IC is configured to supply a switching frequency of about 20 kHz. When the infrared beam emitted by D1 is reflected by a nearby object, IC1, through receiver diode D2 and transistor T2, receives the recovered 20 kHz signal at its input, pin 3.

Infrared Proximity Detector Alarm Circuit Diagram

Because the ‘567 PLL is then locked, the IC output (pin 8), drops low, triggering the 555 chip in monostable mode (IC2) and so causing acoustic actuator Bz1 to sound. The monostable remains on as long as the reﬂected signal is being received. Because of the presence of T3, capacitor C5 is allowed to charge only when no signal is being received. In that condition, the 555 is turned off automatically after a time determined by R9-C5. Using the component values shown, this will be about 5 seconds. Obviously D1 and D2 should be mounted such that the latter can only pick up reﬂected infrared light.

The choice of the two infrared components used in this circuit will be uncritical but they must be ‘band’ compatible, i.e., generate (D1) and respond to (D2) the same wavelength. The operating point of the receiver input circuit is rather dependent on ambient day-light levels and the value of R4 may need to be adjusted a little to ensure a voltage of between 1.5 V and 4 V on the collector of T1 when no signal is being received. Some dc buzzers cause a lot of back-emf so it may be necessary to insert a diode in series with the output of IC1. If necessary, this diode should preferably be a Schottky type because of the inherent low voltage drop of about 0.4 V as opposed to 0.65 V for a typical small-signal silicon diode.

Source: http://www.ecircuitslab.com/2011/05/infrared-proximity-detector-alarm.html

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