LUGGAGE SECURITY SYSTEM

While travelling by a train or bus, we generally lock our luggage using a chain-and-lock arrangement. But, still we are under tension, apprehending that somebody may cut the chain and steal our luggage. Here is a simple circuit to alarm you when somebody tries to cut the chain.

Transistor T1 enables supply to the sound generator chip when the base current starts flowing through it. When the wire (thin enameled copper wire of 30 to 40 SWG, used for  winding  transformers)   loop around the chain is broken by somebody, the base of transistor T1, which was earlier tied to positive rail, gets opened. As a result, transistor T1 gets forward biased to extend the positive supply to the alarm circuit.

In idle mode, the power consumption of the circuit is minimum and thus it can be used for hundreds of travel hours.

To enable generation of different alarm sounds, connections to pin 1 and 6 may be made.

NUMBER GUESSING GAME

 This number guessing game is quite simple. In this game the player thinks of any number between 1 and 99. Then he scans the eight groups of numbers given in the eight boxes in the table. Each group corresponds to a specific switch (indicated on the top of each group) on an 8-way DIP switch. The person scans the numbers in each box and slides the switch corresponding to a box to ‘on’ position if he finds his number in that box. After having scanned all the eight boxes and switching on the relevant DIP switches, he is required to press switch S9 and the number thought of by the person is displayed on the 7-segment displays. After this, all switches on the 8-way DIP switch need to be turned off to try display of another number in a similar fashion.

The circuit comprises two BCDto-7-segment decoder/driver CD4511 ICs (IC1 and IC2). IC1 generates the number for tens position and IC2 generates the number for units position. Input pins 7, 1, 2, and 6 of both the ICs are connected to ground through 1-kilo-ohm resistors. The common cathode terminals of both the displays are connected to push-to-on switch S9.

Suppose you want to display 47. For this, 4 is to be displayed in tens position and 7 in units position. In order to generate 4 (binary 100) on the display (DIS1), switch S2 is to be turned on. To display 7 (binary   111) o n the display (DI S 2), switches S6, S7, and S8 are to be turned on. Thus to generate 47, switches S2, S6, S7, and S8 are to be turned on. The number 47 is placed in groups 6, 7, 8, and 2. So when you spot 47 in these groups, switch on the same combination of switches. On depressing switch S9, 47 appear on the display. Other numbers can be generated using the same procedure.

In order to make the circuit compact, a DIP switch has been used here. As it may be difficult to turn the small switches on and off, you may use SPDT toggle switches in place of the DIP switch. The circuit can be placed inside a plastic case with appropriate cuts made for displays and switches (Fig. 2). A strip of paper containing groups of numbers can be stuck just under the 8-way DIP switch (or under the row of SPDT switches used in place of DIP switch).

This circuit smoothly runs on two pen torch batteries. Thus current-limiting resistors are not necessary for displays. This circuit costs around Rs 200.

WIRELESS SWITCH

Normally, home appliances are controlled by means of switches, sensors, etc. However, physical contact with switches may be dangerous if there is any shorting.

The circuit described here requires no physical contact for operating the appliance. You just need to move your hand between the infrared LED (IR LED1) and the phototransistor (T1). The infrared rays transmitted by IR LED1 is detected by the phototransistor to activate the hidden lock, flush system, hand dryer or else.

This circuit is very stable and sensitive compared to other AC appliance control circuits. It is simple, compact and cheap. Current consumption is low in milliamperes.

The circuit is built around an IC CA3140, IRLED1, phototransistor and other discrete components. When regulated 5V is connected to the circuit, IR LED1 emits infrared rays, which are received by phototransistor T1 if it is properly aligned. The collector of T1 is connected to non-inverting pin 3 of IC1. Inverting pin 2 of IC1 is connected to voltage-divider preset VR1. Using preset VR1 you can vary the reference voltage at pin 2, which also affects sensitivity of the phototransistor.

Op-amp IC1 amplifies the signal received from the phototransistor. Resistor R3 controls the base current of transistor BC548 (T2). The high output of IC1 at pin 6 drives transistor T2 to energize relay RL1 and switch on the appliance, say, hand dryer, through the relay contacts.

The working of the circuit is simple. In order to switch on the appliance, you simply interrupt the infrared rays falling on the phototransistor through your hand. During the interruption, the appliance remains on through the relay. When you remove your hand from the infrared beam, the appliance turns off through the relay.

Assemble the circuit on any general-purpose PCB. Identify the resistors through color coding or using the multimeter. Check the polarity and pin configuration of the IC and mount it using base. After soldering the circuit, connect +5V supply to the circuit.

BATTERY-LEVEL INDICATOR

Normally, in mobile phones, the battery level is shown in dot or bar form. This lets you easily recognize the battery level. Here is a circuit that lets you know the battery level of a device from the number of LEDs that are glowing. It uses ten LEDs in all. So if three LEDs glow, it indicates battery capacity of 30 per cent. Unlike in mobile phones where the battery-level indicator function is integrated with other functions, here only one comparator IC (LM3914) does it all.

The LM3914 uses ten comparators, which are internally assembled in the voltage divider network based on the current-division rule. So it divides the battery level into ten parts. The circuit derives the power supply for its operation from the battery of the device itself. It uses ten LEDs wired in a 10-dot mode. The use of different colored LEDs makes it easier to recognize the voltage level on the basis of the calibration made. Red LEDs (LED1 through LED3) indicate battery capacity of less than 40 per cent. Orange LEDs (LED4 through LED6) indicate battery capacity of 40 to less than 70 per cent and green LEDs (LED7 through LED10) indicate battery capacity of 70 to fewer than 100 per cent. The brightness of the LEDs can be adjusted by varying the value of preset VR2 between pins 6 and 7.

Diode D1 prevents the circuit from reverse-polarity battery connection. The tenth LED glows only when the battery capacity is full, i.e., the battery is fully charged. When the battery is fully charged, relay-driver transistor T1 conducts to energize relay RL1. This stops the charging through normally-open (N/O) contacts of relay RL1.

 For calibration, connect 15V variable, regulated power supply and initially set it at 3V. Slowly adjust VR1 until LED1 glows.

Now, increase the input voltage to 15V in steps of 1.2V until the corresponding LED (LED2 through LED10) lights up. Now the circuit is ready to show any voltage value with respect to the maximum voltage. As the number of LEDs is ten, we can easily consider one LED for 10 per cent of the maximum voltage.

Connect the voltage from any battery to be tested at the input probes of the circuit. By examining the number of LEDs glowing you can easily know the status of the battery. Suppose five LEDs are glowing. In this case, the battery capacity is 50 to 59 per cent of its maximum value.

Assemble the circuit on a general purpose PCB. Calibrate it and then enclose in a box.

LAPTOP PROTECTOR

Protect your valuable laptop against theft using this miniature alarm generator. Fixed inside the laptop case, it will sound a loud alarm when someone tries to take the laptop. This highly sensitive circuit uses a homemade tilt switch to activate the alarm through tilting of the laptop case. 

                                                    

The circuit uses readily available components and can be assembled on a small piece of Vero board or a general purpose PCB. It is powered by a 12V miniature battery used in remote control devices.

IC TLO71 (IC1) is used as a voltage comparator with a potential divider comprising R2 and R3 providing half supply voltage at the non-inverting input (pin 3) of IC1. The inverting input receives a higher voltage through a water-activated tilt switch only when the probes in the tilt switch make contact with water. When the tilt switch is kept in the horizontal position, the inverting input of IC1 gets a higher voltage than its non-inverting input and the output remains low.

IC CD4 5 3 8   (IC2)   is    u s e d   as   a monostable with timing elements R5 and C1. With the shown values, the output of IC2 remains low for a period of three minutes. CD4538 is a precision monostable multivibrator free from false triggering and is more reliable than the popular timer IC 555. Its output becomes high when power is switched on and it becomes low when the trigger input (pin 5) gets a low-to high transition pulse. The unit is fixed inside the laptop case in horizontal position. In this position, water inside the tilt switch effectively shorts the contacts, so the output of IC1 remains low. The alarm generator remains silent in the standby mode as trigger pin 5 of IC2 is low. When someone tries to take the laptop case, the unit takes the vertical position and the tilt switch breaks the electrical contact between the probes. Immediately the output of IC1 becomes high and monostable IC2 is triggered. The low output from IC2 triggers the pnp transistor (T1) and the buzzer starts beeping. 

Assemble the circuit as compactly as possible so as to make the unit matchbox size. Make the tilt switch using a small (2.5cm long and 1cm wide) plastic bottle with two stainless pins as contacts. Fill two-third of the bottle with water such that the contacts never make electrical path when the tilt switch is in vertical position. Make the bottle leak proof with adhesive or wax. Fix the tilt switch inside   the   enclosure of the circuit in horizontal position. Fit the unit inside   the   laptop case in horizontal position using adhesive. Use   a miniature buzzer and a micro switch (S1) to make the gadget compact. Keep the laptop case in horizontal position and switch on the unit. Your laptop is now protected.