Small 3-wheel ROBOT with PIC16F84 brain & InfraRed
eyes, By: TJACO. Page: 1-2-3-4-Home
IR Receiver
The Motor Drive circuit
Both motors are driven by an identical full bridge
circuit. The circuit of Motor1 will be used to explain the functionality.
Motor1 is driven by a full bridge built with transistors Q1, Q2, Q3
and Q4.
This full bridge is able to drive the DC motor forward backward and
full stop by driving the base
connections of Q1/Q4 and Q2/Q3 as in the table below:
base Q1/Q4 base Q2/Q3 Motor1 action
0 Volt 0 Volt STOP
0 Volt 5 Volt REVERSE
5 Volt 0 Volt FORWARD
5 Volt 5 Volt STOP
The transistors are switched as "emitter followers"
which results in a voltage loss of 2 x 0.6 Volts. In this case the
voltage drop was very useful because the the motors from the toy ducks
were much too fast at
Cerberes' battery voltage of 3.6Volts.
The base drive of the transistors limits the output
current to about 1-2 Amps typical at 3.6Volts:
The maximum current from the PIC µController is 20mA at 5Volt
, the typical Hfe of the BD43 /436 is 130;
this results in a typical maximum current of 20mA x 130 = 2.6 Amps at
5 Volt supply.
When the low operating voltage of 3.6 V is taken into account the max
PIC current is even lower: about 1-2 Ampere.
When using other transistors, take into account that
a Hfe of 100 or more is required.
The motors from the toy ducks use about 1 Ampere at 2.4 Volts The battery
pack has a capacity of about 300mAh
resulting in a typical operating time of about 9 minutes!
The Infrared Sensors
In the schematic above, LM393 comparator U1A is used
as a 36 kHz oscillator. The frequency is adjusted to exactly 36 kHz
with (multiturn) potentiometer P1.
U1B is a buffer which picks up he oscillator signal without influencung
the frequency setting.
The signal is passed on to Q9 which switches two Infrared Leds (e.g.
from a remote control) at 36 kHz.
The LM393 is used because it can operate at very low voltages down to
2 Volt , a standard opamp can not be used here!
U3 & U4 are standard SFH506 36kHz infrared receivers
which are normally used yo pick up IR signals from
an IR remote control. Note: the newer SFH-5110-36 can also be used here.
For Cerberes the receivers are used to pick up the
36 kHz IR light from the IR LED's which is reflected by objects
close to the robot (see drawing below). When the sensor picks up a 36kHz
IR signal, the output will become low.
The LED's and the receiver point in the same direction. To prevent false
detections, the receivers must be
shielded from the LEDS by a small aluminum plate which is glued in between
as in the drawing below:
The 100µF capacitor and the 47 ohm resistors
are necessary to prevent electric interference from the 36 kHz oscillator
to reach the receiver which is extremely sensitive to this frequency.
When the frequncy is tuned right, the sensors are able
to detect large objects (like a wall) at a distance of 50 cm. Smaller
objects must be closer to be detected.
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