Showing posts with label alarm. Show all posts
Showing posts with label alarm. Show all posts
Thursday, December 26, 2013
Simple Alarm System
The circuit presented here is a very simple and yet highly effective alarm system for protecting an object. The circuit requires no special devices and can be built using components that you will no doubt be able to find in the junk box. The alarm-triggering element is a simple reed switch. To generate the alarm signal itself any optical or acoustic device that operates on 12 V can be used: for example a revolving light, a siren, or even both. In the quiescent state the reed switch is closed. As soon as the reed switch opens, the input to IC1.B will go low (previously the potential divider formed by R2 and R3 held the input at 5.17 V, a logic high level). A turn-on delay of between 0 and approximately 90 s can be set using P1, and a turn-off delay of between 0 and approximately 20 s can be set using P2. When the system is turned on (using S1), the turn-on delay is activated, giving the user of the system at most 90 s to leave the object alone before the system goes into the armed state, and the object is then protected.
Project image :
Simple Alarm System Project image
Once the reed switch opens the turn-off delay of at most 20 s starts: this allows the rightful owner of the object to turn the system off before the alarm is triggered. If some unauthorised per-son causes the reed switch to open, the alarm will be triggered after the turn-off delay. Also, even if the reed switch is briefly opened and then closed again, the alarm will still be triggered. Once the alarm is triggered, T3 will conduct for about 45 s (because of R8 and C5). The turning off of the alarm is necessary to avoid the nuisance caused by a permanently sounding alarm system. The system then returns to the armed state, which means that the next time the reed switch is opened the alarm will trigger again. If it is not desired that the duration of the alarm be limited, for example if a visual indication is used, D5 should not be fitted. The system can be extended by fitting multiple reed switches in series. As soon as any one is opened, the alarm is triggered.
Circuit diagram:
Simple Alarm System Circuit Diagram
When S1 is closed C3 charges via P1. Depending on the potentiometer setting, it takes between 0 and 90 s to reach the input threshold voltage of IC1.A. The output of IC1.A then goes low and D3 stops conducting. Assuming the reed switch is closed, the inputs of IC1.B stay high and the output therefore low. If the reed switch is opened after the turn-on delay expires the output of the gate will change state and turn on T1. This ensures that the output of the gate remains high even after the reed switch is closed again. C4 now starts charging via P2, reaching the input threshold voltage of IC1.C after between 0 and 20 s, again according to the potentiometer setting. The output of IC1.C goes low, and T2 and T3 are turned on — and the siren sounds. Any Darlington transistor can be used for T3. At the same time, C5 charges via R8, reaching the input threshold of IC1.D in about 45 s. When the output of IC1.D swings low, it pulls the inputs of IC1.A low via diode D5: the siren stops and the system returns to the armed state.
If the potentiometers P1 and P2 are replaced by fixed resistors it is possible to build the circuit small enough to fit in a match-box, without the need to resort to SMD components. This is ideal if the circuit is to be used to protect a motorbike. If the alarm system is to be used in a car, an existing door switch contact can be used instead of the reed switch. In this case an RC combination needs to be added to prevent false triggering. Use a 10 µF/25 V electrolytic for C6, a 100 kΩ resistor for R9 and a 1N4001 for D7. It is again possible to wire multiple door switch contacts in parallel: as soon as one contact closes, IC1.B will be triggered.
Source:http://www.ecircuitslab.com/2012/02/simple-alarm-system.html
Thursday, April 11, 2013
Fire alarm using thermistor NE555
Many fire alarm circuits are presented
here,but this time a new circuit using a thermistor and a timer to do
the trick. The circuit is as simple and straight forward so that, it
can be easily implemented. The thermistor offers a low resistance at
high temperature and high resistance at low temperature. This
phenomenon is employed here for sensing the fire.
here,but this time a new circuit using a thermistor and a timer to do
the trick. The circuit is as simple and straight forward so that, it
can be easily implemented. The thermistor offers a low resistance at
high temperature and high resistance at low temperature. This
phenomenon is employed here for sensing the fire.
The
IC1 (NE555) is configured as a free running oscillator at audio
frequency. The transistors T1 and T2 drive IC1. The output (pin 3) of
IC1 is couples to base of transistor T3 (SL100), which drives the
speaker to generate alarm sound. The frequency of NE555 depends on the
values of resistances R5 and R6 and capacitance C2. When thermistor
becomes hot, it gives a low-resistance path for the positive voltage to
the base of transistor T1 through diode D1 and resistance R2.
Capacitor C1 charges up to the positive supply voltage and increases
the the time for which the alarm is ON. The larger the value of C1, the
larger the positive bias applied to the base of transistor T1 (BC548).
As the collector of T1 is coupled to the base of transistor T2, the
transistor T2 provides a positive voltage to pin 4 (reset) of IC1
(NE555). Resistor R4 is selected s0 that NE555 keeps inactive in the
absence of the positive voltage. Diode D1 stops discharging of
capacitor C1 when the thermistor is in connection with the positive
supply voltage cools out and provides a high resistance path. It also
inhibits the forward biasing of transistor T1.
Circuit diagram with Parts list. IC1 (NE555) is configured as a free running oscillator at audio
frequency. The transistors T1 and T2 drive IC1. The output (pin 3) of
IC1 is couples to base of transistor T3 (SL100), which drives the
speaker to generate alarm sound. The frequency of NE555 depends on the
values of resistances R5 and R6 and capacitance C2. When thermistor
becomes hot, it gives a low-resistance path for the positive voltage to
the base of transistor T1 through diode D1 and resistance R2.
Capacitor C1 charges up to the positive supply voltage and increases
the the time for which the alarm is ON. The larger the value of C1, the
larger the positive bias applied to the base of transistor T1 (BC548).
As the collector of T1 is coupled to the base of transistor T2, the
transistor T2 provides a positive voltage to pin 4 (reset) of IC1
(NE555). Resistor R4 is selected s0 that NE555 keeps inactive in the
absence of the positive voltage. Diode D1 stops discharging of
capacitor C1 when the thermistor is in connection with the positive
supply voltage cools out and provides a high resistance path. It also
inhibits the forward biasing of transistor T1.
Notes.
- The circuit can be powered from a 6V battery or a 6V power supply.
- Click Here ! for the circuit diagram of a power supply circuit for this project.
- The thermistor can be mounted on a heat resistant material like mica to prevent it from damage due to excessive heat.
- The LED acts as an indication when the power supply is switched ON.
Wednesday, April 10, 2013
Pc Temperature Alarm
If your PC overheats, it could damage its expensive components. Here’s a circuit that warns you of your PC getting heated. Today’s computers contain most of the circuitry on just a few chips and reduced power consumption is a byproduct of this LSI and VSLI approach. Some PCs still have power supplies that are capable of supplying around 200W, but few PCs actually consume power to this extent.
On the other hand, apart from some portable and small desktop computers that use the latest micro-power components, most PCs still consume significant amount of power and generate certain amount of heat. The temperature inside the aver-age PC starts to rise well above the ambient temperature soon after it is switched on. Some of the larger integrated circuits become quite hot and if the temperature inside the PC rises 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. Various means to combat overheating are available, ranging from simple temperature alarms to devices like temperature-activated fans to keep the microprocessor cool.
Here is a temperature alarm that activates an audio ‘beeper’ if the temperature inside the PC exceeds a preset threshold. This temperature is user-adjustable and can be anywhere between 0°C and 100°C. The unit is in the form of a small PC expansion card, which you simply need to plug into any avail-able slot of the host PC. It is powered from the PC and consumes only about 12 mA. The sensor (LM35) used here pro-vides a substantial amount of on-chip signal conditioning, including amplification, level shifting and phase in-version. As a result, it provides an out-put of 10 mV per degree centigrade rise in temperature. It caters to a temperature measurement range of 0°C to 100°C, which corresponds to 0V to 1V of voltage.
Pc Temperature Alarm Circuit Diagram
Fig. 1 shows the circuit of the PC temperature alarm and Fig. 2 shows the pin configuration of sensor LM35. IC LM35 (IC1) is an easy-to-use temperature sensor. It is basically a three-terminal device (two supply leads plus the output) that operates over a wide supply range of 4 to 20V. It consumes only 56 µA at 5V and generates insignificant heat.
IC2 is an operational amplifier used here as a voltage comparator. VR1 pro- vides a reference voltage that can be set anywhere from 0V to approximately 1V, which matches the output voltage range of IC1. This reference voltage is applied to the inverting in- put of IC2 and the output of IC1 is coupled to the non-inverting input. Consequently, the output of IC2 is low if the output of IC1 is below the reference voltage, or high if the output of IC1 exceeds the reference voltage.
Pin details of LM35
The low-frequency oscillator IC3 is a standard 555 astable multivibrator circuit. It is gated via the reset input at pin 4, which holds output pin 3 low when IC3 is gated ‘off’ (when the out-put of IC2 is low). This prevents IC4 from oscillating. IC4 is another 555 astable multivibrator circuit, gated via its reset input. It has an operating frequency of approximately 2.5 kHz. When IC3 is activated, its output pro-vides a square wave of 1 Hz. This is used to trigger IC4, which gives an audio output of 2.5 kHz in bursts. It is connected to loudspeaker LS1 to generate alarm.
The alarm circuit can be fitted into any spare expansion slot of the PC, but be careful to fit it the right way round. Before setting VR1 to a suitable thresh-old temperature, decide what that temperature should be. The technical specification in your computer’s manual might be of help here. If we assume that the room temperature will not normally exceed 25oC, the temperature of the interior of the computer would be up to 35oC. Unless you have good reason to use a different threshold temperature, VR1 should be set for a wiper potential of 350 mV.
Trial-and-error method can be used in the absence of test equipment to enable VR1, but it would be a bit time-consuming. There is a slight complication in that the computer’s outer casing must be at least partially removed to provide access to VR1. Once VR1 has been adjusted, the outer casing must be put back into place so that the interior of the computer can warm up in the normal way. You must therefore al-low time for the temperature inside the computer to rise back to its nor-mal operating level each time VR1 is readjusted.
Source: http://www.ecircuitslab.com/2012/05/pc-temperature-alarm.html
Wednesday, March 27, 2013
Four in One Burglar Alarm
In this circuit, the alarm will be switched on under the following four different conditions: 1. When light falls on LDR1 (at the entry to the premises). 2. When light falling on LDR2 is obstructed. 3. When door switches are opened or a wire is broken. 4. When a handle is touched. The light dependent resistor LDR1 should be placed in darkness near the door lock or handle etc. If an intruder flashes his torch, its light will fall on LDR1, reducing the voltage drop across it and so also the voltage applied to trigger 1 (pin 6) of IC1. Thus transistor T2 will get forward biased and relay RL1 energise and operate the alarm.
Sensitivity of LDR1 can be adjusted by varying preset VR1. LDR2 may be placed on one side of a corridor such that the beam of light from a light source always falls on it. When an intruder passes through the corridor, his shadow falls on LDR2. As a result voltage drop across LDR2 increases and pin 8 of IC1 goes low while output pin 9 of IC1 goes high. Transistor T2 gets switched on and the relay operates to set the alarm.
Four-in-One Burglar Alarm Circuit Diagram
The sensitivity of LDR2 can be adjusted by varying potentiometer VR2. A long but very thin wire may be connected between the points A and B or C and D across a window or a door. This long wire may even be used to lock or tie something. If anyone cuts or breaks this wire, the alarm will be switched on as pin 8 or 6 will go low. In place of the wire between points A and B or C and D door switches can be connected. These switches should be fixed on the door in such a way that when the door is closed the switch gets closed and when the door is open the switch remains open. If the switches or wire, are not used between these points, the points should be shorted. With the help of a wire, connect the touch point (P) with the handle of a door or some other suitable object made of conducting material. When one touches this handle or the other connected object, pin 6 of IC1 goes ‘low’.
So the alarm and the relay gets switched on. Remember that the object connected to this touch point should be well insulated from ground. For good touch action, potentiometer VR3 should be properly adjusted. If potentiometer VR3 tapping is held more towards ground, the alarm will get switched on even without touching. In such a situation, the tapping should be raised. But the tapping point should not be raised too much as the touch action would then vanish. When you vary potentiometer VR1, re-adjust the sensitivity of the touch point with the help of potentiometer VR3 properly. If the alarm has a voltage rating of other than 6V (more than 6V), or if it draws a high current (more than 150 mA), connect it through the relay points as shown by the dotted lines. As a burglar alarm, battery backup is necessary for this circuit. Note: Electric sparking in the vicinity of this circuit may cause false triggering of the circuit. To avoid this adjust potentiometer VR3 properly.
Source : http://www.ecircuitslab.com/2012/01/four-in-one-burglar-alarm.html
Sunday, March 24, 2013
Fire Alarm Using Thermistor
Small and simple unit, Can be used for Home-Security purpose
In this fire alarm circuit, a Thermistor works as the heat sensor. When temperature increases, its resistance decreases, and vice versa. At normal temperature, the resistance of the Thermistor (TH1) is approximately 10 kilo-ohms, which reduces to a few ohms as the temperature increases beyond 100 C. The circuit uses readily available components and can be easily constructed on any general-purpose PCB.
Circuit Diagram:
Fire Alarm Using Thermistor Circuit Diagram| Parts | Description |
| R1 | 470R |
| R2 | 470R |
| R3 | 33K |
| R4 | 560R |
| R5 | 470R |
| R6 | 47K |
| R7 | 2.2K |
| R8 | 470R |
| C1 | 10uF-16V |
| C2 | 0.04uF-63V |
| C3 | 0.01uF-63V |
| Q1 | BC548 |
| Q2 | BC558 |
| Q3 | SL100B |
| D1 | Red Led |
| D2 | 1N4001 |
| IC1 | NE555 |
| SPKR | 1W-8R |
| TH1 | Thermistor-10K |
Timer IC NE555 (IC1) is wired as an astable multivibrator oscillating in audio frequency band. Switching transistors Q1 and Q2 drive multivibrator IC1. The output of IC1 is connected to NPN transistor Q3, which drives the loudspeaker (SPKR) to generate sound. The frequency of IC1 depends on the values of resistors R6, R7 and capacitor C2. When Thermistor TH1 becomes hot, it provides a low-resistance path to extend positive voltage to the base of transistor Q1 via diode D2 and resistor R3. Capacitor C1 charges up to the positive voltage and increases the ‘on’ time of alarm. The higher the value of capacitor C1, the higher the forward voltage applied to the base of transistor Q1. Since the collector of transistor Q1 is connected to the base of transistor Q2, transistor Q2 provides positive voltage to reset pin 4 of IC1. R5 is used such that IC1 remains inactive in the absence of positive voltage. D2 stops discharging of capacitor C1 when the Thermistor connected to the positive supply cools down and provides a high-resistance (10k) path. It also stops the conduction of Q1. To prevent the Thermistor from melting, wrap it up in mica tape. The circuit works off a 6V-12V regulated power supply. D1 is used to indicate that power to the circuit is switched on.
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