Friday, January 10, 2014

Video Switch for Intercom System

Nowadays a lot of intercom units are  equipped with video cameras so that you can  see as well as hear who is at the door. Unfortunately, the camera lens is perfectly placed  to serve as a sort of support point for people  during the conversation, with the result that  there’s hardly anything left see in the video  imagery.  One way to solve this problem is to install two cameras on the street side instead only  one, preferably some distance apart. If you  display the imagery from the two cameras  alternately, then at least half of the time you  will be able to see what is happening in front  of the door. Thanks to the video switch module described  here, which should be installed on the street  side not too far away from the two cameras,  you need only one monitor inside the house and you don’t need to install any additional video cables.
Video Switch for Intercom System Circuit diagram:
Video Switch for Intercom System-Circuit-Diagram

Along with a video switch, the circuit includes  a video amplifier that has been used with  good results in many other Elektor projects,  which allows the brightness and the contrast  to be adjusted separately. This amplifier is  included because the distance between the  street and the house may be rather large, so it is helpful to be able to compensate for cable attenuation in this manner.  The switch stage is built around the well  known 4060 IC, in which switches IC2a and  IC2d alternately pass one of the two signals to  the output. They are driven by switches IC2b and IC2c, which generate control signals that  are 180 degrees out of phase. The switching rate for the video signals is  determined by a clock signal from an ‘old  standby’ 555 IC, which causes the signals to  swap every 2 seconds with the speciļ¬ed com ponent values.
Naturally, this circuit can also used in many other situations, such as where two cameras are needed for surveillance but only one video cable is available.
Author :Jacob Gestman Geradts - Copyright : Elektor

Source :

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Motor Speed Control

This circuit will allow you to control the speed of an AC motor, for example an electric drill. The way that this circuit works is as follows. The bridge rectifier produces dc voltage from the 120vac line. A portion on this current passes through the 10K ohm pot. The circuit comprised of the 10k pot, the two 100 ohm resistors and the 50uf capacitors delivers gate drive of the SCR. The diode D1 protects the circuit from reverse voltage spikes. The ratings of the bridge rectifier and the SCR should be 25 amps and PIV 600 volts. The diode D1 should be rated for 2 amps with PIV of 600 volts. The circuit can handle a load up to 10 amps. The SCR should be very well heat sinked.

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Thursday, January 9, 2014

Intelligent Electronic Lock

This intelligent electronic lock circuit is built using transistors only. To open this electronic lock, one has to press tactile switches S1 through S4 sequentially. For deception you may annotate these switches with different numbers on the control panel/keypad. For example, if you want to use ten switches on the keypad marked ‘0’ through ‘9’, use any four arbitrary numbers out of these for switches S1 through S4, and the remaining six numbers may be annotated on the leftover six switches, which may be wired in parallel to disable switch S6 (shown in the figure). 

When four password digits in ‘0’ through ‘9’ are mixed with the remaining six digits connected across disable switch terminals, energisation of relay RL1 by unauthorized person is prevented.For authorized persons, a 4-digit password number is easy to remember. To energies relay RL1, one has to press switches S1 through S4 sequentially within six seconds, making sure that each of the switch is kept depressed for a duration of 0.75 second to 1.25 seconds. The relay will not operate if ‘on’ time duration of each tactile switch (S1 through S4) is less than 0.75 second or more than 1.25 seconds.

This would amount to rejection of the code. A special feature of this circuit is that pressing of any switch wired across disable switch (S6) will lead to disabling of the whole electronic lock circuit for about one minute. Even if one enters the correct 4-digit password number within one minute after a ‘disable’ operation, relay RL1 won’t get energized. So if any unauthorized person keeps trying different permutations of numbers in quick successions for energisation of relay RL1, he is not likely to succeed. To that extent, this electronic lock circuit is fool-proof. This electronic lock circuit comprises disabling, sequential switching, and relay latch-up sections. The disabling section comprises zener diode ZD5 and transistors T1 and T2. Its function is to cut off positive supply to sequential switching and relay latch-up sections for one minute when disable switch S6 (or any other switch shunted across its terminal) is momentarily pressed.

Circuit diagram :
Intelligent Electronic Lock -Circuit-Diagram

Intelligent Electronic Lock Circuit Diagram

During idle state, capacitor C1 is in discharged condition and the voltage across it is less than 4.7 volts. Thus zener diode ZD5 and transistor T1 are in non-conduction state. As a result, the collector voltage of transistor T1 is sufficiently high to forward bias transistor T2. Consequently, +12V is extended to sequential switching and relay latch-up sections. When disable switch is momentarily depressed, capacitor C1 charges up through resistor R1 and the voltage available across C1 becomes greater than 4.7 volts. Thus zener diode ZD5 and transistor T1 start conducting and the collector voltage of transistor T1 is pulled low. As a result, transistor T2 stops conducting and thus cuts off positive supply voltage to sequential switching and relay latch-up sections. Thereafter, capacitor C1 starts discharging slowly through zener diode D1 and transistor T1. It takes approximately one minute to discharge to a sufficiently low level to cut-off transistor T1, and switch on transistor T2, for resuming supply to sequential switching and relay latch-up sections; and until then the circuit does not accept any code.

The sequential switching section comprises transistors T3 through T5, zener diodes ZD1 through ZD3, tactile switches S1 through S4, and timing capacitors C2 through C4. In this three-stage electronic switch, the three transistors are connected in series to extend positive voltage available at the emitter of transistor T2 to the relay latch-up circuit for energising relay RL1.  When tactile switches S1 through S3 are activated, timing capacitors C2, C3, and C4 are charged through resistors R3, R5, and R7, respectively. Timing capacitor C2 is discharged through resistor R4, zener diode ZD1, and transistor T3; timing capacitor C3 through resistor R6, zener diode ZD2, and transistor T4; and timing capacitor C4 through zener diode ZD3 and transistor T5 only. The individual timing capacitors are chosen in such a way that the time taken to discharge capacitor C2 below 4.7 volts is 6 seconds, 3 seconds for C3, and 1.5 seconds for C4. Thus while activating tactile switches S1 through S3 sequentially, transistor T3 will be in conduction for 6 seconds, transistor T4 for 3 seconds, and transistor T5 for 1.5 seconds.

The positive voltage from the emitter of transistor T2 is extended to tactile switch S4 only for 1.5 seconds. Thus one has to activate S4 tactile switch within 1.5 seconds to energise relay RL1. The minimum time required to keep switch S4 depressed is around 1 second. For sequential switching transistors T3 through T5, the minimum time for which the corresponding switches (S1 through S3) are to be kept depressed is 0.75 seconds to 1.25 seconds. If one operates these switches for less than 0.75 seconds, timing capacitors C2 through C4 may not get charged sufficiently. As a consequence, these capacitors will discharge earlier and any one of transistors T3 through T5 may fail to conduct before activating tactile switch S4.  Thus sequential switching of the three transistors will not be achieved and hence it will not be possible to energise relay RL1 in such a situation. A similar situation arises if one keeps each of the mentioned tactile switches de-pressed for more than 1.5 seconds.

When the total time taken to activate switches S1 through S4 is greater than six seconds, transistor T3 stops conducting due to time lapse. Sequential switching is thus not achieved and it is not possible to energise relay RL1. The latch-up relay circuit is built around transistors T6 through T8, zener diode ZD4, and capacitor C5. In idle state, with relay RL1 in de-energised condition, capacitor C5 is in discharged condition and zener diode ZD4 and transistors T7, T8, and T6 in non-conduction state. However, on correct operation of sequential switches S1 through S4, capacitor C5 is charged through resistor R9 and the voltage across it rises above 4.7 volts. Now zener diode ZD4 as well as transistors T7, T8, and T6 start conducting and relay RL1 is energised. Due to conduction of transistor T6, capacitor C5 remains in charged condition and the relay is in continuously energised condition. Now if you activate reset switch S5 momentarily, capacitor C5 is immediately discharged through resistor R8 and the voltage across it falls below 4.7 volts. Thus zener diode ZD4 and transistors T7, T8, and T6 stop conducting again and relay RL1 de-energises.

Source :
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Simple Bilateral Current Source Circuit Diagram

Hi Friends ! I am sorry for dont update i post because i am busy few days. OK to day share with you Simple Bilateral Current Source Circuit Diagram This circuit uses a CA3193 precision op amp to deliver a current independent of variations in RL. 

With RI set equal to R3, and R2 approximately equal to R4 + R5, the output current, h. is: VlN (R4)/(R3) (R5). 500-I`A load current is constant for load values from 0 to 3ohm.

Simple Bilateral Current Source Circuit Diagram

Simple Bilateral Current Source Circuit Diagram

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Friday, December 27, 2013

Long Range FM Transmitter

The power output of many transmitter circuits are very low because no power amplifier stages are incorporated. The transmitter circuit described here has an extra RF power amplifier stage, after the oscillator stage, to raise the power output to 200-250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element Yagi antenna, this transmitter can provide reasonably good signal strength up to a distance of about 2 kilometres.

Long Range FM Transmitter Circuit diagram :

Simple Long Range FM Transmitter-Circuit diagram

The circuit built around transistor T1 (BF494) is a basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts. Transistor T2 (2N3866) forms a VHF-class A power amplifier. It boosts the oscillator signal power four to five times. Thus, 200-250 milliwatts of power is generated at the collector of transistor T2.

For better results, assemble the circuit on a good-quality glass epoxy board and house the transmitter inside an aluminium case. Shield the oscillator stage using an aluminium sheet. Coil winding details are given below:
  • L1 - 4 turns of 20 SWG wire close wound over 8mm diameter plastic former.
  • L2 - 2 turns of 24 SWG wire near top end of L1. 
    (Note: No core (i.e. air core) is used for the above coils)
  • L3 - 7 turns of 24 SWG wire close wound with 4mm diameter air core.
  • L4 - 7 turns of 24 SWG wire-wound on a ferrite bead (as choke)
Potentiometer VR1 is used to vary the fundamental frequency whereas potentiometer VR2 is used as power control. For hum-free operation, operate the transmitter on a 12V rechargeable battery pack of 10 x 1.2-volt Ni-Cd cells. Transistor T2 must be mounted on a heat sink. Do not switch on the transmitter without a matching antenna. Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the fundamental frequency near 100 MHz.

This transmitter should only be used for educational purposes. Regular transmission using such a transmitter without a license is illegal in India.

WARNING: Transmitting on the UK Commercial FM band is also illegal in the UK, please see the general disclaimer. This circuit is shown for educational purposes only.

Source :
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Thursday, December 26, 2013

Simple Gated Alarm

Sometimes the need arises for a simple, gated, pulsed alarm. The circuit shown here employs just four components and a piezo sounder and is unlikely to be out-done for simplicity. While it does not offer the most powerful output, it is likely to be adequate for many applications.

Circuit diagram :
Simple Gated Alarm-Circuit Diagram
Simple Gated Alarm Circuit Diagram
A dual CMOS timer IC type 7556 is used for the purpose, with each of its two halves being wired as a simple astable oscillator (a standard 556 IC will not work in this circuit, nor will two standard 555’s). Note that the CMOS7556 is supplied by many different manufacturers, each using their own type code prefix and suffix. The relevant Texas Instruments product, for instance, will be marked ‘TLC556CN’. The circuit configuration used here is seldom seen, due probably to the inability of this oscillator to be more than lightly loaded without disturbing the timing. However, it is particularly useful for high impedance logic inputs, since it provides a simple means of obtaining a square wave with 1:1 mark-space ratio, which the ‘orthodox’ configuration does not so easily provide.

IC1.A is a slow oscillator which is enabled when reset pin 4 is taken High, and inhibited when it is taken Low. Out-put pin 5 of IC1.A pulses audio oscillator IC1.B, which is similarly enabled when reset pin 10 is taken High, and inhibited when it is taken Low.

In order to simplify oscillator IC1.B, piezo sounder X1 doubles as both timing capacitor and sounder. This is possible because a passive piezo sounder typically has a capacitance of a few tens of nanofarads, although this may vary greatly. As the capacitor-sounder charges and discharges, so a tone is emitted. The value of resistor R2 needs to be selected so as to find the resonant frequency of the piezo sounder, and with this its maximum volume. The circuit will operate off any sup-ply voltage between 2 V and 18 V. A satisfactory output will be obtained at relatively high supply voltages, but do not exceed 18 V.

Author :Rev. Thomas Scarborough –Copyright : Elektor
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Wednesday, December 25, 2013

The mutual inspection of cell phone jammer should also be performed

The mutual inspection of cell phone jammer should also be performed.
In addition, the rural market is a new market segment, a large number of competitors have not yet entered the competition is not intense, very favorable for Amagatarai phone. Monopolistic tendencies of foreign brands on the channel, so for phones Amagatarai, the key point is to find a breakthrough, in order to build a massive distribution network, the best way is the coexistence of a variety of sales channels. Day language mobile phone retailers, specialty retail, home appliance chain stores, specialized chains and integrated supermarkets, such as shopping malls, supermarkets, etc. should be selected. Different buying habits of consumers, to choose a different retailer, to expand the sales network, to ensure that each class of customers with access to day language phone products. Workshop management of cell phone jammer plays an important role in manufacturing cell phone jammer .cell phone jammer can create enough interference to jam all cell phone signals
To meet the economic characteristics of the contemporary Amagatarai phone channels, the channel length should not be too long, If it is too long, then one for each level distributors to charge some of the profits to the final price of natural variability, is not conducive to the promotion of sales. Flat channel is more conducive to day language is more direct and fast communication with consumers, receive timely feedback information, accurate information on market trends. But the need for a clear channel flat in real terms is the abatement of long and useless links and improve operational efficiency. Constructed between the distributors and consumers of Amagatarai into a complete, organic, and efficient network system. If the worker has any suggestion about workshop management of cell phone jammer , heshe can write a letter to the director.
If the flattening channels of day language to be unrestricted, would like the cost investment in the channel is larger, to centralize power in the distributors, and makes Amagatarai manufacturers in a passive situation. The present tendency of the mobile phone market, the mobile phone hypermarkets position in the market has become increasingly prominent, not only as the Dixon type of specialized cell phone store, and supermarket chains like Gome, Suning, Five Star appliances have also joined. It is due to join supermarkets in the distribution process, the language needs to put in more effort and resources in promoting store sales above. Amagatarai phone should make full use of the advantage of localization and mobile connectivity and other operators to launch a customized mobile phone, meet the diverse needs of consumers. cell phone jammer can create enough interference to jam all cell phone signals.
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