Showing posts with label FOR. Show all posts
Showing posts with label FOR. Show all posts
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
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.
Simple Music On Hold for Telephones
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Here is a simple circuit for music-on-hold with automatic shut off facility. During telephone conversation if you are reminded of some urgent work, momentarily push switch S1 until red LED1 glows, keep the telephone handset on the cradle, and attend to the work on hand. A soft music is generated and passed into the telephone lines while the other-end subscriber holds.
When you return, you can simply pick up the handset again and continue with the conversation. The glowing of LED1, while the music is generated, indicates that the telephone is in hold position. As soon as the handset is picked up, LED1 is turned off and the music stops.
When you return, you can simply pick up the handset again and continue with the conversation. The glowing of LED1, while the music is generated, indicates that the telephone is in hold position. As soon as the handset is picked up, LED1 is turned off and the music stops.
Music-On-Hold for Telephones Circuit Diagram :
Normally, the voltage across telephone lines is about 50 volts. When we pick up the receiver (handset), it drops to about 9 volts. The minimum voltage required to activate this circuit is about 15 volts. If the voltage is less than 15 volts, the circuit automatically switches off. However, initially both transistors T1 and T2 are cut off. The transistor pair of T1 and T2 performs switching and latching action when switch S1 is momentarily pressed, provided the line voltage is more than 15 volts, i.e. when the handset is placed on the cradle. Once the transistor pair of TI and T2 starts conducting, melody generator IC1 gets the supply and is activated. The mu-sic is coupled to the telephone lines via capacitor C2, resistor R1, and the bridge rectifier.
With the handset off-hook after a ring, momentary depression of switch S1 causes forward biasing of transistor T2. Mean-while, if the handset is placed on the cradle, the current passing through R1 (connected across the emitter and base terminals of pnp transistor T1) develops enough voltage to forward bias transistor T1 and it starts conducting. As a consequence, output voltage at the collector of transistor T1 sustains for-ward biasing of transistor T2, even if switch S1 is released. This latching action keeps both transistors T1 and T2 in conduction as long as the output of the bridge rectifier is greater than 15 volts. If the handset is now lifted off-hook, the rectifier output drops to about 9 volts and hence latching action ceases and the circuit automatically switches off.
EFY lab note. The value of resistor R2 determines the current through resistor R1 to develop adequate voltage (greater than 0.65 volts) for conduction of transistor T1. Hence it may be test selected between 33 kilo-ohms and 100 kilo-ohms to obtain instant latching.) The total cost of this circuit is around Rs 50.
UPS For Telephones
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Cordless telephones are very popular nowadays. But they have a major drawback, i.e. they cannot be operated during power failure. Therefore usually another ordinary telephone is connected in parallel to the cordless telephone. This results in lack of secrecy. UPS is a permanent solution to this problem. Since the UPS is meant only for the cordless telephone, its output power is limited to around 1.5W.
This is sufficient to operate most cordless telephones. as these employ only small capacity adapters (usually 9V/12V, 500mA), to enable the operation of the circuit and to charge the battery present in the handset. The UPS presently designed is of online type.
Here the inverter is ‘on’ throughout, irrespective of the presence of the AC mains.When the AC mains is present, the same is converted into DC and fed to the inverter. A part of the mains rectified output is used to charge the battery. When the mains power fails, the DC supply to the inverter is from the battery and from this is obtained AC at the inverter output. This is shown in fig.1.
Here the inverter is ‘on’ throughout, irrespective of the presence of the AC mains.When the AC mains is present, the same is converted into DC and fed to the inverter. A part of the mains rectified output is used to charge the battery. When the mains power fails, the DC supply to the inverter is from the battery and from this is obtained AC at the inverter output. This is shown in fig.1.
UPS For Telephones Circuit diagram:
The circuit wired around IC CD4047 is an astable multivibrator operating at a frequency of 50 Hz. The Q and Q outputs of this multivibrator directly drive power MOSFETS IRF540. The configuration used is push-pull type. The inverter output is filtered and the spikes are reduced using MOV (metal oxide varistor). The inverter transformer used is an ordinary 9V-0-9V, 1.5A mains transformer readily available in the market.Two LEDS (D6 and D7) indicate the presence of mains/battery. The mains supply (when present) is stepped down, rectified and filtered using diodes D1 through D4 and capacitor C1. A part of this supply is also used to charge the battery.
In place of a single 12V, 4Ah battery, one may use two 6V, 4Ah batteries (SUNCA or any other suitable brand). The circuit can be easily assembled on a general-purpose PCB and placed inside a metal box. The two transformers may be mounted on the chassis of the box. Also, the two batteries can be mounted in the box using supporting clamps. The front and back panel designs are shown in the Fig. 3. The same circuit can deliver up to 100W, provided the inverter transformer and charging transformer are replaced with higher current rating transformers, so that the system can be used for some other applications as well.
Schematic diagram for the One Transistor FM Radio with Improved Audio Gain
Schematic diagram for the One Transistor FM Radio with Improved Audio Gain
One Transistor FM Radio with improved audio gain.
Some wiring notes:
Unless you have experience with super-regenerative radios, I highly recommend using the FAR Circuits printed circuit board.
Connect the two sections of the variable capacitor (C3) in series to linearize the tuning somewhat. That is, use the connections on either end of C3 and dont use the middle lead.
L2, the RF choke should not be near a ground. The same is true for L1. Capacitance to ground will disturb the feedback.
The gain is just enough to drive an earphone. If you live too far away from radio stations, you might have trouble hearing one. There is no option here for an external antenna (that would require and extra transistor).
You can drive a speaker if you add an external audio amplifier.
If you want a little more audio gain, or you cannot locate a TL431CLP chip, you can use some other audio amplifier in the circuit where pins 1 and 2 of D1 normally connect. You can use an LM386 or a TDA7052 audio amplifier. Quasar DIY project kit #3027 is a complete TDA7052 audio amplifier kit and it works fine in this application.
Continue reading[...]
Some wiring notes:
Unless you have experience with super-regenerative radios, I highly recommend using the FAR Circuits printed circuit board.
Connect the two sections of the variable capacitor (C3) in series to linearize the tuning somewhat. That is, use the connections on either end of C3 and dont use the middle lead.
L2, the RF choke should not be near a ground. The same is true for L1. Capacitance to ground will disturb the feedback.
The gain is just enough to drive an earphone. If you live too far away from radio stations, you might have trouble hearing one. There is no option here for an external antenna (that would require and extra transistor).
You can drive a speaker if you add an external audio amplifier.
If you want a little more audio gain, or you cannot locate a TL431CLP chip, you can use some other audio amplifier in the circuit where pins 1 and 2 of D1 normally connect. You can use an LM386 or a TDA7052 audio amplifier. Quasar DIY project kit #3027 is a complete TDA7052 audio amplifier kit and it works fine in this application.
VOLTAGE REGULATOR MODULES VRM A SOLUTION FOR CPU CORE VOLTAGE ELECTRONIC CIRCUIT
VOLTAGE REGULATOR MODULES (VRM) A SOLUTION FOR CPU CORE VOLTAGE ELECTRONIC CIRCUIT
For each phase, the components on point (2) above which constitute all the power components are placed on a small plug-in board of 1.15? x 0.85? that delivers 40 Amps and receives the PWM TTL signal from the controller. This module has a footprint of about 0.85? x 0.25? of the motherboard space and may be placed anywhere on the board as close as possible to theCPU reducing the transmission impedance and losses and giving the Motherboard designer the flexibility to optimize the power and PCB space utilization. Each modular board may be fitted individually with its own heat sink.
VOLTAGE REGULATOR MODULES VRM A SOLUTION FOR CPU CORE VOLTAGE ELECTRONIC CIRCUIT
VOLTAGE REGULATOR MODULES (VRM) A SOLUTION FOR CPU CORE VOLTAGE ELECTRONIC CIRCUIT
For each phase, the components on point (2) above which constitute all the power components are placed on a small plug-in board of 1.15? x 0.85? that delivers 40 Amps and receives the PWM TTL signal from the controller. This module has a footprint of about 0.85? x 0.25? of the motherboard space and may be placed anywhere on the board as close as possible to theCPU reducing the transmission impedance and losses and giving the Motherboard designer the flexibility to optimize the power and PCB space utilization. Each modular board may be fitted individually with its own heat sink.
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