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					  CIRCUIT
    IDEAS

SMART CELLPHONE HOLDER                                                                                  S.C. DW
                                                                                                               IVEDI




  T.K. HAREENDRAN                                                                                          holder will stop
                                                                                                           flashing to indicate


T
         his smart cellphone holder                                                                        that the mobile
         makes sure that you don’t                                                                         phone is not in the
         forget to carry your mobile                                                                       cell holder of the
phone. Fitted in the car, it keeps                                                                         case. Resistor R1
searching for the mobile phone                                                                             limits the current
within the holder using infrared                                                                           flowing through
(IR) rays and alerts you through a                                                                         IR LED1 and resis-
flashing LED when it doesn’t find one.                                                                     tor R6 limits the
You can attach the circuit to your ex-                                                                     operating current
isting cellphone holder or, with a little                                                                  and hence lumi-
skill, construct one as per your require-                                                                  nance of LED2.
ment.                                                                                                      Variable resistor
    The circuit, wired around IC                                                                           VR1 determines
LM555 (IC1), derives power from the                                                                        the detection sen-
12V DC automobile battery. Diode D1                                                                        sitivity          of
is an accidental wrong-polarity input                                                                      phototransistor T1.
guard. Resistor R7 limits the inrush                                                                       The blinking rate
current to IC1.                             Fig. 1: Circuit of smart cellphone holder                      of LED2 can be
    When power is applied to the cir-                                                                          changed by
cuit, the low-frequency astable                                        from IR LED1                            changing the
multivibrator built around IC1 is acti-                                fall        on                          value of ca-
vated and LED2 at its output pin 3                                     phototransistor                         pacitor C1 (or
flashes briefly.                                                       T1 and it con-                          R3-R4 resistor
    When ignition switch S2 is flipped                                 ducts to pull                           combination).
to ‘on’ position, the +12V DC from the                                 the base of                                 P     i    n
car’s battery disables the astable                                     LED driver                              configurations
multivibrator via diode D2 and LED2         Fig. 2: Pin configurations transistor T2     Fig. 3: Proposed      of BC547 and
turns off.                                  of BC547 and 2N5777        t o w a r d s     cellphone holder      phototransistor
    When the ignition is turned off and                                ground to dis-                          2 N 5 7 7 7 ,
the mobile phone is in its holder, LED2     able the visual indicator (LED2).            and the proposed cellphone
again starts blinking. In case the               If you’ve forgotten to carry your       holder are shown in Figs 2 and 3, re-
cellphone holder is empty, IR rays          cellphone, LED2 fitted in the cellphone      spectively.




94 • JULY 2005 • ELECTRONICS FOR YOU                                                                       WWW.EFYMAG.COM

				
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Description: mini project for engineering students