C E R B E R E S
Small 3-wheel ROBOT with PIC16F84 brain & InfraRed eyes.
C E R B E R E S
Small 3-wheel ROBOT with PIC16F84
brain & InfraRed eyes.
General description
Cerberes is a small three wheel robot that avoids obstacles which it senses with its active infrared sensors.
Each rear wheel has its own motor, the single front wheel is not powered. The robot steers "like a tank" by rotating the rear wheels in opposite direction.
The sensors are built with infrared LEDs which are running at 36kHz and two 36
kHz remote control receiver modules.
When the 36 kHz infrared light from the LEDs is reflected by an object, one of
the receiver modules will be triggered, and
the PIC16F84 µController will steer the ROBOT away from the objects by reversing
one of the motors.
Mechanics
Two electric toy ducks were sacrificed for this project.
The motors, axes and axe-holders from the toy ducks
are glued on a piece of
circuit board which acts as a base plate.
The rotation speed of the motors is reduced by the "worm-wheel" of the motor
axes which drive the cogs of the wheel axes.
The circuit is built on another piece of circuit board which is identical to
the base plate.
The circuit is placed on top of the base plate.
The 3.6 Volt battery pack (from a cordless phone) is also glued to the base
plate.
Circuit description
The circuit behind CERBERES can be divided in 3 parts:
CERBERES schematic: (note that +5V is +3.6 Volt )
printable (pdf) schematic
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:
Infrared sensor setup:
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.
The PIC16F84 brain
The PIC 16F84 that is used is a very basic& cheap FLASH µController from MICROCHIP.
It has 13 digital I/O's of which just 5 are used here. The PIC16F84 has no
analog inputs or outputs.
The controller is ideal for experimenting because it can be erased and
re-programmed 1000 times!
In the software RB5 and RB6 are programmed to be inputs; thet check if the IR
sensors detect any objects.
RB0, RB1, RB2 and RB3 are outputs which drive the motors forward and backward to
avoid objects.
The µController uses an RC oscillator as clock (R5 C3).
The software is written in the original MICROCHIP assemby language.
The prgram can be assembled with MPASM which is part of MPLAB.
(MPLAB is free and can be downloaded at www.microchip.com)
Software description
The functionality of the software can be described in a few lines:
1-Start moving forward (both motors forward)
2-If the LEFT sensor is triggered, reverse both motors shortly , then reverse the RIGHT motor
shortly:
(the robot will move backward shortly and then turns right)
3-If the RIGHT sensor is triggered, reverse both motors shortly, then reverse the LEFT motor
shortly:
(the robot will move backward shortly and then turns left)
4-Go back to moving forward.
Does not look very smart, but it is incredibly effective!
The program "V105.asm" is written in the MICROCHIP assembly language, with the comments it will explain itself.
Download the program HERE.
If you want to learn more about the MICROCHIP assembly language:
→ Download specifications of key components at the Component specs page