Fuse-Cum-Power Failure Indicator (New)
This fuse-cum-power failure indicator comprises an LED (LED1), light-dependent resistor (LDR1), inverter circuit and two timer circuits built around IC NE555. LDR1 and preset VR1 form a voltage divider at the input of the cascaded amplifier comprising two BC548 transistors (T1 and T2). The base of transistor T1 is connected to the junction of LDR1 and the preset through resistor R2. The base of transistor T2 is connected to the collector of T1.
Doorbell-Controlled Porchlight (New)
This add-on circuit automatically turns on your porchlight when your doorbell rings, so you can see the person ringing the doorbell at the doorstep. This also helps to guard against burglars, who usually press the doorbell switch to confirm that there is no one at home. By turning on the porchlight, the circuit will trick them into believing that someone is inside the home.
Zener Diode Tester (New)
This zener diode tester can be used to check zener diodes of 3.3V to 18V. The breakdown voltage of the unknown zener diode is indicated on the precalibrated dial of potmeter VR1. The tester can also identify the polarity of zener diodes.
Variable Power Supply With Digital Control (New)
The most frequently used device in electronic workshops and laboratories is a universal power supply that provides a variable, fluctuation-free output. Here we present a variable power supply with digital control that is simple and easy to construct.
Low-Resistance Continuity Tester (New)
Using this circuit you can check continuity of low-resistance paths such as PCB tracks, small coils, intermediate-frequency transformers (IFTs) and low-resistance transformers. However, you can not check semiconductors with this tester. The tester works off a 9V battery.
Efficient LED Blinking for Embedded Systems
Smart hardware design can simplify the embedded software and make it more reliable. Designers use blinking LED signals to indicate different status and for inbuilt testability. Making an LED array blink requires an individual software loop for each LED or an individual timer and specific software to serve it. This can be an issue in a system that uses low-level MCUs which provide limited resources.
Sensitive Optical Burglar Alarm
This optical burglar alarm uses two 555 timer ICs. Both the ICs are wired as astable multivibrators. The first astable multivibrator built around IC1 produces low frequencies, while the second astable multivibrator built around IC2 produces audio frequencies.
This circuit serves both as an emergency light (using a 6W fluorescent tube) and an ultraviolet light (using a 6W ultraviolet tube).
Clap-Operated Electronic Switch
Here is a simple clap-operated electronic switch. Using this switch, you can turn on any appliance by clapping five times and turn it off by a single clap. The switch activates the appliance only if you apply the right clap code (five claps here) within the preset time (10 seconds). If you apply a wrong clap code (other than five claps) or you are unable to apply five claps within 10 seconds, the switch does not activate the appliance.
Panel Frequency Meter
Here is a simple panel frequency meter to measure the frequency of 230V AC mains. When you connect it to the 230V AC line, the display shows the line frequency. Generally, the line frequency is 50 Hz, which may vary from 48 Hz to 52 Hz. Beyond this frequency range, sensitive equipment may start malfunctioning.
Frequency Divider using 7490 Decade Counter
Here is a low-cost circuit for generating different square-wave signals. The circuit is built around a 10MHz crystal oscillator, hex inverter IC 7404 and seven decade counter ICs 7490.
Temperature Monitor for Electronic Equipment
As most electronics componentsí characteristics vary with temperature, they are selected as per expected operating temperature range of the equipment. So it is important that the equipment stays within the temperature range for which it is designed.
SCR-Controlled EHT Power Supply
Presented here is an extra-high-tension (EHT) power supply capable of delivering variable DC voltage from about 100 V to 5 kV with load current up to 500 mA. One way of obtaining such high DC voltages capable of delivering load currents up to a few amperes is to control the AC input voltage to the primary of a power transformer using high-power SCRs connected back to back. The same principle is used in designing this power supply.
Moisture Monitor for Plants
Over-watering and under-watering both are harmful for plants. Roots need air as well as water. If the soil is constantly saturated, air cannot reach the roots and they suffocate. Also, excess water weakens the plant and makes it susceptible to various diseases, particularly fungal attacks. Under-watering, on the other hand, is equally harmful. Plants not receiving enough water droop from the top down and leaf edges turn brown.
Dome Lamp Dimmer
Areading light inside the car greatly assists passengers during night, but often the interior dome lamp is too bright and distracting to the driver. A linear regulator such as a rheostat can be used to control the brightness of the dome lamp but it consumes a lot of power.
CELL-PHONE-CONTROLLED AUDIO/ VIDEO MUTE SWITCH
This cell-phone-controlled audio/video mute switch is highly useful in automobiles. The circuit automatically disconnects power supply to the audio/video system whenever the mobile handset is lifted off the holder for making or receiving a call. You can use any readily available cell-phone holder with some minor alterations or fabricate it yourself as shown in Fig. 1.
Light-Operated Internal Door Latch
Using this light-operated circuit, you can close or open the door of your room remotely from your bed. You just have to focus the torchlight on the light-dependent resistor of the circuit, which you can install inside your room at a suitable position.
Simple Key-Operated Gate Locking System
This simple key-operated gate locking system allows only those persons who know the preset code to open the gate. The code is to be entered from the keypad within the preset time to operate the motor fitted in the gate. If anyone trying to open the gate presses a wrong key in the keypad, the system is disabled and, at the same time, sounds an alarm to alert you of an unauthorised entry.
Mains Box Heat Monitor
This simple circuit monitors the mains distribution box constantly and sounds an alarm when it senses a high temperature due to overheating, helping to prevent disasters caused by any sparking in the mains box due to short circuits. It also automatically switches on a bright white LED when the power fails. The LED gives ample light to check the mains box wiring or fuses in darkness. The circuit beeps once when power fails and again when power resumes.