Specifications

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Figure 4.11: 555 Timer Pulsing Sensor Light
Initial calculated results differed substantially from the measured performance. It was
necessary to connect a Cathode Ray Oscilloscope (CRO) to accurately measure the
waveform output. Slight tuning of resistor values provided an accurate 38 KHz square
wave with 50% Duty Cycle.
The transistor is required on the output of the 555 timer to source the current needed to
supply the three IR LED’s. Each LED can handle 50mA constantly running through
them, however due to the 50% duty cycle; the LED will only be on 50% of the time. It
can then be safely assumed that the LED’s can actually handle 100mA as long as they
are only run through the 555 timers pulse.
To contain the IR LED and project the light in the desired direction, it was decided to
modify and use some discarded LED keychain torches. After removing the batteries,
switch, resistor and white light LED; an IR LED along with the 50 ohm resistor were
soldered on to the Printed Circuit Board (PCB). From the PCB, two wires were run out
the back of the torch to the 555 timer circuit and then the unit sealed to prevent any IR
light escaping which would create undesirable results. Figure 4.12 and 4.13 show the
torch modified into an infrared emitter.