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.

Standard 5v Regulated Power Supply

             For the proper working of every muc based projects a regulated 5v power supply is required. The standard voltage for all the muc( Atmel,AVR,Pic ) are 5v. So i prepared the circuit for the power supply.The circuit for the power supply is below.

Click the circuit for enlarging