Sajee tronics

This circuit provides secrecy when two or more telephones are connected in parallel to a telephone line. The circuit also prevents incoming calls to as well as outgoing calls from other telephones connected in parallel, except from the one lifted first. When someone picks up the handset of the telephone connected in parallel to the original (master) phone for making an outgoing call, no dial tone is heard and the phone appears to be dead. But when a call comes, the ring signal switches the SCRs ‘on’ and conversation can be carried out. As soon as the handset is kept on the hook, the SCR goes off and the telephone can again only receive incoming calls. When a call comes, conversation can be made only from the telephone which is lifted up first. To carry out conversation from the other telephone, the handset of the telephone that was lifted up first has to be placed on the hook and then the push-to-on switch of the associated circuit of the other telephone has to be pressed after li...

Check the Phone Line with a Multimeter      Before you try to tap into the electricity in the phone line, you should check it with a multimeter to see what you are working with. Start by cutting open a phone cord and separating the internal wires. In most cases you will have one red wire and one green wire. Strip the insulation off the ends. Then plug the cord into a phone jack and use a multimeter to measure the output voltage. Then I hooked up various resistors to see what the output would be with different loads. I determined that the supply voltage isn't regulated. This means that the voltage changes depending on the resistance of the circuit that it is powering. After some calculating, I worked out that the base signal coming out of my phone jack pretty closely resembles a 52 Volt DC source with a 628 ohm internal resistance. Basically this means that I can run a 12V circuit at 64mA, a 9V circuit at 68mA, or a 5V ci...

Next, upload the following sketch to your Arduino and open the serial monitor window in the IDE: // -------------------------------------------------------- #include <SoftwareSerial.h> SoftwareSerial RFID(2, 3); // RX and TX int i; void setup() { RFID.begin(9600); // start serial to RFID reader Serial.begin(9600); // start serial to PC  } void loop() { if (RFID.available() > 0)  { i = RFID.read(); Serial.print(i, DEC); Serial.print(" "); } } // -------------------------------------------------------- Reading and recognising RFID cards To do anything with the card data, we need to create some functions to retrieve the card number when it is read and place in an array for comparison against existing card data (e.g. a list of accepted cards) so your systems will know who to accept and who to deny. Using those functions, you can then make your own access system, time-logging device and so on. Let’s demonstrate an example of this. It will check ...

The listings at Amazon and other online vendors show this inexpensive membrane keypad as  "4x4 Universial 16 Key Switch Keypad Keyboard For Arduino." There was no documentation  for the product, nor were there any links from Amazon. Here are my notes on how to connect  and test this keypad. The arrangement of the keys is 1 2 3 A  4 5 6 B  7 8 9 C  * 0 # D There is a ribbon with 8 wires running from the bottom of the keypad. With the keypad face up,  the wires connect in sequence from left to right to Arduino digital pins 2 - 9. Don't use digital  pins 0 and 1 on the Arduino Uno, since they are used for serial communication. The Arduino Keypad library is available from the  Arduino Playground .  The following code will allow you to test the keypad. As each key is pressed, the corresponding  character should appear on a separate line in the Arduino IDE's serial console. #include <Keypad.h> const byte ROWS = 4; ...

Digital wall clocks, table clocks and desk clocks with pointer or LCD display are readily available in the market. Here we present a clock that can be built in a small budget using AT89C2051 microcontroller. Additional feature of the clock is that the time display is visible even in the dark. Circuit description Fig. 1 shows the circuit of the microcontroller-based clock. It comprises microcontroller AT89C2051 (IC1), inverting buffer ULN2003 (IC2), real-time clock (RTC) DS1307 (IC3), regulator 7805 (IC4), non-inverting source driver UDN2982 (IC5) and a few discrete components. Microcontoller AT89C2051 is the heart of the clock. It is an 8-bit microcontroller with 2kB Flash programmable and erasable read-only memory (PEROM), 128 bytes of RAM, 15 input/output (I/O) lines, two 16-bit timers/counters, a five-vector two-level interrupt architecture, a full-duplex serial port, a precision analogue comparator, on-chip oscillator and clock circuitry. Port pins P1.7 down through P1.1 of the mic...

Useful for vehicle owners, this gadget automatically turns on indoor/outdoor garage lights and raises an alert when an automobile enters the garage. Assume switch S2 is in ‘on’ (closed) state. When power switch S1 is turned on, the complete circuit is energised by the 12V DC supply. LED3 lights up to provide power-on indication. Simultaneously, IC3 (CD4017B) is instantly reset by the power-on-reset circuit formed by the combination of capacitor C4 and resistor R5, and green LED2 lights up as a standby indicator. As per the physical arrangement, IR rays from IR-LED fall on phototransistor T1 and it conducts to pull up the inputs of NAND gate N1 (used here as an inverter) to logic 1. As a result, the output of gate N2 goes high to make the monostable built around IC2 inactive. Now, when a vehicle moves through the door, the IR beam is interrupted and the output state of gate N2 changes from high to low state, which triggers the monostable and red LED1 (Rx on) lights up briefly. The o...

T his circuit lets you locate wire breaks, so you can cut the wire  insulation at precise points and repair the breaks. The working of the circuit is based on capacitive effects developed in a tube detector. Before detecting breaks, it is essential to know which wire has a break. This can be easily detected using a continuity tester. Once the wire having a break is detected, signals from the col lectors of transistors T2 and T3 (points A and B) have to be applied to the two ends of that wire and a tube has to be run along the wire for detecting the break point. This tube actually forms a capacitor with the test wire. When the tube crosses the break point in the wire, there is a flip in the LED glow indicating wire break.                    The circuit is built around a 555 timer IC (IC1), CMOS NAND gate CD4011 (IC2), CMOS NOR gate CD4001 (IC3) and a few discrete components. IC 555 is wired as an astable multivibrator....

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. Figs 1 and 2 show the block and circuit diagrams of the key-operated code locking system, respectively. Connect points A, B, C, D, E, F and ground of the circuit to the respective points of the keypad. Keys S7, S16, S14 and S3 are used here for code entry, and the remaining keys are used for disabling the system. It is very important to press the keys in that order to form the code. To start the motor of the gate, press switches S7, S16, S14 and S3 sequentially. If the keys are pressed in a different order from the preset order, the system will lock automatically and the motor will not start. Fig. 1: ...

Normally, sound intensity up to 30 dB is pleasant. Above 80 dB, it becomes annoying. And if it goes beyond 100 dB, it may affect your psychomotor performance, detracting your attention and causing stress. Noise pollution may also affect your hearing ability.

Description: The ADXL345 is a small, thin, low power, 3-axis accelerometer with high resolution (13-bit) measurement at up to ±16 g. Digital output data is formatted as 16-bit twos complement and is accessible through either a SPI (3- or 4-wire) or I2C digital interface. The ADXL345 is well suited to measure the static acceleration of gravity in tilt-sensing applications, as well as dynamic acceleration resulting from motion or shock. Its high resolution (4 mg/LSB) enables measurement of inclination changes less than 1.0°. Several special sensing functions are provided. Activity and inactivity sensing detect the presence or lack of motion and if the acceleration on any axis exceeds a user-set level. Tap sensing detects single and double taps. Free-fall sensing detects if the device is falling. These functions can be mapped to one of two interrupt output pins. An integrated, patent pending 32-level first in, first out (FIFO) buffer can be used to store data to minimize host processor...

Usually, low-priced home stereo power amplifiers don’t have output level indicators. An output power level indicator can be added to each channel of these stereo power amplifiers. As low levels of the output power are not disturbing and damaging to the people, there is no need to add a preamplifier and low-level detector before IC LM3915. But you should know when the output power becomes considerably high. Here we present a very simple, low-cost stereo-level indicator circuit for home power amplifiers with power rating of around 0.5W. The circuit is built around two LM3915 dot/bar display driver ICs (IC1 and IC2). LM3915 senses analogue voltage levels to drive ten LEDs, providing a logarithmic 3dB/step analogue display. The voltage levels below 1V are not important because these correspond to a low level of the audio signal. Similarly, input voltage levels above 30V correspond to too high levels of the output power, which are not applicable for home power amplifiers. So the voltage...

While driving on highways, motorists should not exceed the maximum speed limit permitted for their vehicle. However, accidents keep occurring due to speed violations since the drivers tend to ignore their speedometers. This speed checker will come handy for the highway traffic police as it will not only provide a digital display in accordance with a vehicle’s speed but also sound an alarm if the vehicle exceeds the permissible speed for the highway. The system basically comprises two laser transmitter-LDR sensor pairs, which are installed on the highway 100 metres apart, with the transmitter and the LDR sensor of each pair on the opposite sides of the road. The installation of lasers and LDRs is shown in Fig. 1. The system displays the time taken by the vehicle in crossing this 100m distance from one pair to the other with a resolution of 0.01 second, from which the speed of the vehicle can be calculated as follows: As per the above equation, for a speed of 40 kmph the displa...