Specifications

35
The noise filter, illustrated in figure 4.14, reduced the noise to a negligible level. From
this, the Z1955 receiving chips delivered a different behaviour. First experiments on the
bread board provided three working receivers over a distance of approximately 300-
400mm shown by a 5V drop. As this was suitable performance for the Mobile
Manoeuvring Robot, the circuitry was soldered to a circuit board. Testing at a later date
showed none of the circuits working again. After another long, strenuous testing period
it was discovered that the Z1955 chips were also sensitive to sunlight and incandescent
light, despite the claims of the data sheet. If an IR light source was provided to the
robot, such as an incandescent bulb, the sensors would work successfully to a distance
of approximately 300 mm. Without the additional IR light source the sensors would
sometimes work at a distance of 20 – 30 mm.
To overcome this problem it was finally decided to implement some variable brightness
IR LED’s to supply IR light to the back of the receiving chips. The Z3235 LED can
handle at most 50mA constant current. A 1k variable resistor was implemented in
series with a 100 ohm resistor so to provide a variable current which could not exceed
50 mA. Calculations prove that the current can be varied between 4.5 and 50mA:
(5)
(100 0)
50
MAX
MIN
V
I
R
V
mA
=
=
+Ω
=
(5)
(100 1000)
4.55
MIN
MAX
V
I
R
V
mA
=
=
+Ω
=
Where: I = Current flowing through LED [Amps];
V = voltage drop across LED [Volts];
R = resistance in series with LED [Ohms].
At 4.5 mA the Z3235 LED is hardly on but does produce some IR light. This variable
brightness light provides a method of controlling the sensitivity of the IR receiver and
hence a control of the distance sensed. The IR LED’s provide enough IR to ensure
successful operation of the receiving chips and hence the accomplishment of the
distance sensors.