Shadow detector

                         
                                            This is the circuit of a simple burglar alarm that produces a loud beep when somebody crosses a protected area or door. The circuit is highly sensitive and can detect the shadow of the moving person from a distance of 1 meter.We can adjust the sensitivity of the circuit by the help of the variable resistor VR1.

                                         

Working Principle: Op Amp IC UA 741 is used as a voltage comparator. Its inverting input pin2 receives half supply voltage through the potential resistors R1 and R2. The non inverting input pin3 gets a variable voltage through LDR and VR. Normally when the LDR gets light, its resistance will be low and it conducts and provide a high voltage to the non inverting input of IC. This makes the output of IC high. The high output from IC is given to the base of T1 through a current limiting resistor R3. T1 is PNP transistor and it conducts only when its base becomes negative. Here normally the base of T1 will be high due to the high output from IC. So T1 remains off and Buzzer and LED connected to its emitter remains off. When a person passes in front of LDR, the shadow of the person reduces the resistance of LDR and the voltage at the non inverting input of IC decreases. This makes the output of IC low. Immediately T1 conducts activating Buzzer and LED indicating the entry of a person.

Applications :- Burglar alarm ,Light senser etc..

Voltage Multiplier

                                We require high voltage at certain applications ,we see mosquito bats to kill mosquito , in that we produce a high voltage from a battery . we can use this circuit for making high voltage from a low voltage source         

This circuit produces a very high voltage and care must be used to prevent getting a nasty shock.  The transformer can produce over 1,000v and the 8-stage multiplier can produce up to 20,000v 

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circuit diagram

MOBILE DETECTOR

                        This circuit will help to detect the presence of an activated mobile phone or an unauthorized transmition in a protected room  to prevent from spy works  .The circuit can detect both the incoming and outgoing calls, SMS and video transmission even if the mobile phone is kept in the silent mode     The transmission frequency of mobile phones ranges from 0.9 to 3 GHz with a wavelength of 3.3 to 10 cm. So a circuit detecting gigahertz signals,The moment the ckt detects RF transmission signal from an activated mobile phone, it starts sounding a beep alarm and the LED blinks. The alarm continues until the signal transmission ceases.                      Assemble the circuit on a general purpose PCB as compact as possible and enclose in a small box like junk mobile case. As mentioned earlier, capacitor C3 should have a lead length of 18 mm with lead spacing of 8 mm. Carefully solder the capacitor in standing position with equal spacing of the leads. The response can be optimised by trimming the lead length of C3 for the desired frequency. You may use a short telescopic type antenna.

AM TRANSMITTER USING 555

                                   This circuit can transmit AM radio wave in MW frequency of around 600 Kilo hertz.The Timer IC 555 is used a free running multi vibrator which have been adjusted in frequency of 540 Kilo hertz.
                                 The sound signal is inputted to pin number 5 of IC from the condenser mic. And The IC converts the signal to a Amplitude modulated frequency and outputs from pin number 3 from which an Antenna of length of around 2-3 meters converts the frequency to radio waves which can be received by any Medium Wave radio receiver by tuning it to around 600 Kilo Hertz from a distance of up to 25-30 meters.

Parts list:

Resistance:                         Value:
R1                                         380 ohms.
R2                                          1 Kilo ohms.

Condensers:                        Value
C1                                         0.001 Micro farad
C2                                         1000 micro farad /   25 volt.

Semiconductors:
 IC1                                      
Timer IC 555

Others:
Condenser Mic.
Battery

Electronics Assistant

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Tools for Beginners

LED Tools

                 Here are two simple tools for the Hobbyists to check their Prototype boards. These use LEDs as visual indicators for the quick check 

Continuity Tester

               This can be used to check the continuity of PCB tracks, wires, transformer windings, fuse, diodes, resistors etc. It uses a Red LED to give the positive indication of continuity.

 Polarity Indicator

                 This simple LED tool indicates the polarity of connection. It uses a bicolour LED to give two indications. If the polarity is correct, Green LED lights, and if polarity is reversed, Red LED lights.