Cell Phone Controlled Relay

            Now a days all of them are using various types of cell phone in the world. Imagine that from your cell phone you can control a relay which is connected to you home gate lights or other equipment's in the house. So i build a circuit for this application.While calling to someone, if we press any button in the cell phone a tone produced and the receiver can heard.This tone is called DTMF (Dual Tone Multi Frequency).For more about DTMF, see my previous post.The working of the circuit is based on this principle. 

                                                 Click the circuit for enlarging.

DTMF Decoder

      While calling to someone, if we press any button in the cell phone a tone produced and the receiver can heard.This tone is called DTMF (Dual Tone Multi Frequency). DTMF assigns a specific frequency (consisting of two separate tones) to each key so that it can easily be identified by the electronic circuit. The signal generated by the DTMF encoder is a direct algebraic summation, in real time, of the amplitudes of two sine (cosine) waves of different frequencies, i.e., pressing ‘5’ will send a tone made by adding 1336 Hz and 770 Hz to the other end of the line. The tones and assignments in a DTMF system are shown below.

  

Frequencies	1209 Hz	1336 Hz	1477 Hz	1633 Hz
697 Hz	1	2	3	A
770 Hz	4	5	6	B
852 Hz	7	8	9	C
941 Hz	*	0	#	D

      An MT8870 series DTMF decoder is used here. All types of the MT8870 series use digital counting techniques to detect and decode all the 16 DTMF tone pairs into a 4-bit code output. The built-in dial tone rejection circuit eliminates the need for pre-filtering. When the input signal given at pin 2 (IN-) in single-ended input configuration is recognised to be effective, the correct 4-bit decode signal of the DTMF tone is transferred to Q1 (pin 11) through Q4 (pin 14) outputs.The circuit is shown below.

4 Channel Led Chaser

                 This LED chaser display is build around readily available, low cost components.This circuit is easy for build for a beginner in the electronics. The power supply for the chaser circuit is from a 9v battery.The circuit diagram of the chaser is below.

                                                       Click the circuit for enlarging.

                The NE 555 timer ic and decade counter 4017 is the heart of the circuit. The astable multivibrator built around IC2 produces a clock frequency at its output pin 3. This output is connected to clock pin (pin 14) of the decade counter Cd 4017.The decade counter can count up to 10. But in circuit we only use 4 O\P so the Q4 (pin 10)  is connected to the reset (pin 15) of the CD 4017. The output of IC3 advances by one count every second (depending on the time period of astable multivibrator IC2). The VR1 connected between the Pins of IC2 determine the speed of the glowing of the bulb.