Wednesday, September 1, 2010




1.1 Background and Objectives

Remote controls are being applied to a variety of devices nowadays. From television remote to remote controlled robots, we can see their applications on the rise. Due to the increasing complexity of lifestyle in this generation, people are looking for products that are accurate, reliable and easy to use. They also want control of lots of devices with some simple systems. In such cases, remote controlling provides the answer.

A variety of remote control configurations are available for controlling devices. Some of them include the IR remote and radio signal based remote. Besides being able to control other devices, people also want to control their household loads using simple systems. In such situations, tele-remote systems can be used. Telephone circuits always gain interest, because telephones are everywhere and quite often there are old telephone sets lying around somewhere. Those telephones can be used for many interesting experiments including the tele-remote.

Tele-remote control, like any other remote control systems, is used for controlling loads and devices with a simple control setup. But unlike other remote control systems which have a limited range space of functionality, tele-remote can be used from anywhere. It overcomes the problem of range and thus, a user of this system can control the loads from any place provided with a telephone connection

1.2 Basic Concept of Tele-remote

The basis of tele-remote system is the DTMF signals of telephone which are used as control signals to control loads. DTMF, better known as touch-tone, is a system of signal tones used in telecommunications. DTMF is actually the signal to the phone company that we generate when we press an ordinary telephone's touch keys.

Different types of signals are available on the telephone terminals based on the mode of operation. When the telephone is in the idle state (on hook), certain level of DC voltage is present. When the telephone is ringing, about 80V rms AC voltage appears on the line. When conversing over the telephone (off hook), a lower level of DC voltage with a small current (0.1mA) is present. And finally, when the keys are pressed from the telephone keypad, the internal circuitry of the telephone generates DTMF signals. These DTMF signals are basically superposed sine waves of different types, each corresponding to the different keys of the telephone keypad. They are the unique tones we hear when we press any telephone keys.

Each signal being unique to the key associated with it is used in the tele-remote system to control and identify the load associated with it. Thus, in the tele-remote system, whenever a key is pressed, the load is controlled based on the DTMF signal of the key transmitted to the receiver end. With DTMF, each key we press on our phone generates two tones of specific frequencies. So that a voice can't imitate the tones, one tone is generated from a high-frequency group of tones and the other from a low frequency group.


The following are the objectives of this project:

1. To get acquainted with the various telephone parameters

2. To develop a tele-remote circuit capable of controlling loads

3. To get basic knowledge of the digital circuit used for remote controlling.

4. To reinforce the theoretical understanding and gain experience with circuit devices and construction.

Wednesday, March 24, 2010

Sawtooth wave generator

Sawtooth wave generators using opamp are very common. But the disadvantage is that it requires a bipolar power supply.

A sawtooth wave generator can be built using a simple 555 timer IC and a transistor as shown in the circuit diagram.

The working of the circuit can be explained as follows:
The part of the circuit consisting of the capacitor C, transistor,zener diode and the resistors form a constant current source to charge the capacitor. Initially assume the capacitor is fully discharged. The voltage across it is zero and hence the internal comparators inside the 555 connected to pin 2 causes the 555's output to go high and the internal transistor of 555 shorting the capacitor C to ground opens and the capacitor starts charging to the supply voltage. As it charges, when its voltage increases above 2/3rd the supply voltage, the 555's output goes low, and shorts the C to ground, thus discharging it. Again the 555's output goes high when the voltage across C decreases below 1/3rd supply. Hence the capacitor charges and discharges between 2/3rd and 1/3rd supply.

Note that the output is taken across the capacitor. The 1N4001 diode makes the voltage across the capacitor go to ground level (almost).

The frequency of the circuit is given by:

f = (Vcc-2.7)/(R*C*Vpp)


Vcc= Supply voltage.
Vpp= Peak to peak voltage of the output required.

Choose proper R,C,Vpp and Vcc values to get the required 'f' value.