The robot consisted of a number of sensors and actuators, involved in various different tasks.
*For collision avoidance, two medium range IR sensors (crossed) and one short range IR sensor were used. Additionally, to act as a 'back up' should something hit the robot, a bumper with a microswitch was placed on the front.
*For the firing mechanism, the IR emmiter gun was placed on a turret at the front, which consists of a cog connected to a servo by a chain. The IR emmiter was controlled by an infra-red controller circuit, which was essentially just a remote-control. The controller has a permanently depressed button, and the IntelliBrain would supply power to it when the signal needed to be sent. The ammunition lights were connected to and controlled by the IntelliBrain.
*For the colour blob tracking, the CMUCam2 was mounted at the front, slightly higher than the turret and other obstacles, to give it a clear view.
Infrared gun and receiver
Our infrared firing mechanism consisted of a modified remote control and a custom built receiver. The circuit diagrams and operational information for both parts are given below.
Theory of Operation - Gun
Since the remote control switches under the physical buttons are carbon contacts it is impractical to activate the remote control by electronically “pressing” a button. As such the remote control has one button permanently pressed by taping a carbon contact over a button. Using the IntelliBrain as a power source for the remote control is not possible as the output voltage of the IntelliBrain is approximately 5V and the maximum rating of the IR encoder on the remote control is 3.3V [3]. Also, the current provided by the IntelliBrain would not be high enough to operate the infra-red emitter of the remote control. As such, the firing is controlled by switching on and off the battery power supply to the remote control. When the IntelliBrain digital I/O port is set high, the base (b) current of the BC108 NPN transistor is enough to switch on current flow between the collector (c) and emitter (e) , saturating the transistor. 3V is then supplied to the remote control by connecting the positive supply terminal to the emitter and an IR pulse train is fired. The resistors in the circuit are for current limitation purposes to protect the transistor.
Two modifications were made to the remote control. Firstly, the infrared diode was desoldered from the circuit board and connected to long wires to allow aiming of the emitter. Secondly, the onboard power supply decoupling electrolytic capacitor had a value of 47µF. This capacitor suppresses voltage fluctuations in the power lines of the whole remote however it prevented rapid switching of the power supply to the remote control by the IntelliBrain. A new capacitor of value 1µF was wired in place to allow power supply switching whilst also smoothing the power supply.
Theory of Operation – Receiver
The TSOP1738 is an integrated IR receiver, decoder, noise filter and preamplifier that responds ideally to IR coded signals transmitted on a 38kHz carrier wave. As an active low device, the quiescent output voltage is the positive supply, in this case 4.5V. When a signal is received by the TSOP1738, the output drops to 0V and spikes either side of 0V to provide the decoded IR signal. In this application, the decoded output is not important, however negative voltage spikes are very dangerous for CMOS based logic devices and would destroy the chip that follows in the circuit. The diode at the output of the TSOP1738 prevents the output dropping below 0V, and, in conjunction with the 1µF capacitor and resistors, suppresses the decoded output spikes. This part of the circuit provides an output of 4.5V when no signal is detected and slightly above 0V when an IR signal is received. The NOT gate (part of the CMOS logic chip mentioned previously) then inverts the signal so that when a signal is received, 4.5V is presented at the logic output. Low power CMOS chips cannot provide the necessary current to drive an LED so this output is connected to the base of an NPN transistor. The transistor acts as a current amplifier and lights the two LEDs whenever a signal is received, with the 330Ω resistor operating in a protective capacity.
Alternative Solution
The original receiver design included a hit counter with seven-segment display. This worked by using the output of the TSOP1738 and associated circuitry, as shown above, as the input to a BCD counter. Standard counter and display circuitry was used from that point as can be found easily online. Unfortunately, in practice, the circuit did not work when connected to the battery pack despite being quite reliable on prototype board when connected to a power pack. The exact reason for this is unclear but several factors may be involved. The CMOS logic components used were chosen for their low power consumption and fast switching speeds to make them compatible with the output of the IR receiver. CMOS chips are sensitive to unstable ground connections which may be experienced when using battery supplies. Secondly, any IR noise that was not filtered by the TSOP1738 would cause spurious counting. Finally, long wires were used to connect two parts of the circuit together due to the initial constraints of working with the laser-tag drone. The robot lab is subject to lots of interference from mains and network cables in the walls and the wires would act as an aerial. In low power devices, this interference may be enough to cause unusual behaviours.
the receiver
from :www.cs.bham.ac.uk
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