Infrared is light that is beyond red in the spectrum. While it�s not visible to the human eye, infrared transmitters and receivers can send and receive infrared signals. The infrared beam has large frequency and therefore it is measured in terms of its wavelength. Infrared signals only work over a short distance.
IC555 is connected as shown in the figure it will trigger itself and free run as a multi-vibrator. The external capacitor charges through Ra and Rb and discharges through Rb only. Thus the duty-cycle may be set precisely by the ratio of 7these two resistors. The capacitor charges and discharges between 1/3 Vcc and 2/3 Vcc. The Charge and discharge are independent of the supply voltage.
The charge time (output high) is given by :
t1 = 0.693 (Ra + Rb) Ct
Construction & Working :
In this experiment Ic555 is used to produce the required frequency. The 1st and 8th pin of the Ic555 is the ground and Vcc respectively. 2nd is the trigger, 3rd is the output of the chip. The trigger is fed back to the pin 6th which make the system unstable. Here Ic555 acts a Astable vibrator or a square wave generator.
Infra red waves are the waves with large frequencies. High frequency could be produced by IC 555 which is Astable multivibrator so it can create any frequency. Normally the infrared frequency is around 40Hz. The signal with such a high frequency is allowed to enter the IR transmitting diode. Relating to the project requirements three IR diodes are connected in parallel to each other at the output. However care is to be taken as to cover the sides of the diodes with the silver paper so as to let the IR waves escape from its front cone only. This may reduce the path angle but make it more sensitive and increase its range.
fig
2.8
The resistors are selectively chosen as to get the required frequency. While adding 10W resistance, which will increase current in the sensor. The signal on pin 3 of the 555 is 30Hz and has a duty cycle of 50% . Due to the very small pull-up resistor the 555 sinks about 109mA. The specs say the 555 can sink up to 225mA so it's well below the danger level. We can measure the frequency by an oscilloscope.
So here � = 30 Hz;
We choose the value of the capacitance. Let Ct = 0.1 m F
� = 1.44 / R Ct
30 = 1.44 / R (0.1m )
R = 1.44 / (30) (0.1m ) = 480 KW
We know that R = (Ra + 2Rb)
We adjust the values of the Ra and Rb so as to set time for charging and discharging of the capacitor.
