Door Bell Controlled Security Camera

One way to check whether anybody is at home is ring the doorbell.Burglars too employ this very technique to check whether youare at home!

The circuit described here comes handy in such a situation.It is a simple doorbell-controlled multipurpose electronic switch that instantly powers up a connected security device, for instance, a night-vision door camera,for functioning.

The circuit works off a 9V DC supply.The input of the circuit is connected in parallel with the 230V AC electric doorbell.An electromagnetic relay is used at the output of the circuit to activate the security device connected to it.Note that the output socket (J1) works as a power switch and is designed for activating the 9V security device (night-vision door camera).Similarly, you can use this 9V supply to energise another switching relay in an existing home security system.The circuit can be reset by pressing switch S1.

Courtesy: www.efymag.com

Portable Mobile Sniffer

This handy, pocket-size mobile transmission detector can sense the presence of an activated mobile phone from a distance of oneand-a-half metres.So it can be used toprevent use of mobile phones in examination halls,confidential rooms,etc.It is also useful for detecting the use of mobile phone for spying and unauthorised signal transmission.

Here the circuit uses a 0.22μF ceramic disk capacitor (C3) to capture the RF signals from the mobile phone.The lead length of the capacitor is fixed as 18 mm with a spacing of 8 mm between the leads to get the desired frequency.The disk capacitor along with the leads acts as a small gigahertz loop antenna to collect the RF signals from the mobile phone.

When the mobile phone signal is detected by C3, the output of Op-ampIC1 becomes high and low alternately according to the frequency of the signal as indicated by LED1. This triggers monostable timer IC2 through capacitor C7.Capacitor C6 maintains the base bias of transistor T1 for fast switching action.The low-value timing components R6 and C9 produce very short time delay to avoid audio nuisance.

Courtesy: D.Mohankumar

LT3466 IC Based 10+10 White LED Driver

Linear Technology’s LT®3466 is a dual step-up converter capable of driving up to 20 white LEDs with a programmable constant current.The device is also capable of driving asymmetric LED strings with independent dimming and shutdown control for multipanel LCD backlight applications.

The LT3466 IC  uses a constant-current, step-up architecture to directly regulate LED current, providing consistent light intensity and color across all LEDs regardless of variations in their forward voltage drop. Important features including soft-start,open LED protection, adjustable switching frequency and DC dimming control are integrated into the part, making the LT3466 LED driver an ideal solution for space-constrained portable devices such as cellular phones, PDAs and digital cameras.

Circuit  described here is capable of driving two sets of ten,in-series LEDs with 12mA of constant current per converter.IC LT3466 is a dual channel white LED driver capable of driving up to 20 white LEDs from a single cell Li-Ion (3-5 VDC) input. The device features 44V internal power switches,internal Schottky diodes, adjustable switching frequency, DC dimming control, automatic soft-start, open LED protection and optimized internal compensation.As shown,the circuit works from single Li-Ion (3V) battery or 5V wall adapter.Inrush current during start-up is well controlled due to the internal soft-start circuitry of LT3466.The circuit is also protected with an internal 39V output clamp when the LEDs are disconnected or fail open.Efficiency is above 80% over a wide LED current range of 5mA to 12mA, reaching 83% at 12mA.In this ciruit,the operating frequency was set to 400kHz using a 147k resistor at RT terminal.

LT3466 features single pin shutdown and dimming control for each converter.The LED current in the two drivers can be set independently by modulating the CTRL1 and CTRL2 pins.There are two different types of control methods - DC voltage dimming and filtered PWM signal dimming.Note that,the LED currents are proportional to the voltages at the CTRL1 and CTRL2 pins.DC voltage dimming is achieved by reducing the voltage on the CTRL pin.As the CTRL voltage decreases, the feedback voltage across the LED current programming resistor decreases from 200mV to 0V,thus reducing the LED current from ILED to 0A.PWM dimming works similar to DC voltage dimming except that the DC voltage input to the CTRL pins comes from an RC-filtered PWM signal.The corner frequency of RC components should be much lower than the frequency of the PWM signal for proper filtering.

Courtesy: Linear Technology® http://www.linear.com

Night Vision Enhancer

Here is a simple green LED flashlight built around 555 timer IC (IC1) and powered from a 3V battery pack.Green LED based flashlights are widely used by hunters and others in similar activities to light their way and alert other hunters of their presence. Green light does not interface with the hunt,as animals are blind or indifferent to green light.Green is also a good night-vision retention colour that allows one to distinguish colours such as yellow and red on maps and charts.

Key component of this flashlight is a 555 timer (IC1) that is configured as an astable multivibrator generating a 50 per cent duty cycle square wave at approximately 6Hz frequency.Three 5mm green LEDs (LED1 through LED3) are connected at output pin 3 of IC1.Resistors R1 through R3 limit the current through the LEDs to prevent them from burning up. Switch S1 is the main on/off switch.In normal mode,IC1 is disabled by normally-closed (N/C) push-to- off switch S1 keeping its output low.As a result,all LEDs (LED1–LED3) glow continuously.On the other hand,when S2 is depressed,IC1 is enabled and all LEDs start flashing at approximately 6Hz frequency.

You can also use this flashlight like a signal lamp.Flashing frequency can be varied by changing the value of capacitor C2.The entire circuit can be easily assembled on a general-purpose PCB and enclosed in a small plastic box. Drill suitable holes for LEDs and switches.Finally, attach the battery holder using a double-sided glue tape.

Courtesy:T.K.Hareendran/efy/ http://www.efymag.com/