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First published on: 1-7-2013
Propeller Message Display with Temperature Indicator
Sarthak r. Shah and Abhimanyu k. Varde
Here we describe a microcontroller-based propeller display that displays any message sent to it via hyper-terminal of a personal computer. Moreover, a temperature sensing IC (TMP125) is mounted onto the propeller display to display temperature in real time. It gives a 360-degree view and displays several characters in a revolving circular path using just eight LEDs. It is a cost-effective, attractive way of display that reduces the cost and complexity while making the whole system dynamic and energy-efficient. The main highlight o this project is a vertical facing display with true 360-degree viewing angle.
In this display, the LED strip moves so fast that one is able to see a matrix of LEDs. The time of a single rotation is divided into several shorter time periods during which each individual LED is kept on/off to display different characters. Temperature sensed by the temperature IC is sent to the controller via SPI bus.
Circuit and working
Fig. 3 shows the circuit of the propeller display. The circuit is built around microcontroller P89V51RD2 (IC1), temperature sensor TMP125 (IC2), MAX232 (IC3) and a few discrete components. Port pins P1.0 through P1.7 are connected to CON2, which further needs to be connected to CON3 of the LED strip comprising LED1 through LED8. These LEDs form a circular display when rotated very fast.
Power-on reset is provided by the combination of resistor R9 and capacitor C8. Switch S1 is used for manual reset. An 11.0592MHz crystal (XTAL1) along with two 33pF capacitors (C6 and C7) provides basic clock frequency to the microcontroller.
Interrupt sensor. A sensor is built (see Fig. 4) using IR transmitter (IRTX1) and receiver (IRRX1) to generate interrupts. In each revolution, as the beam is interrupted, the sensor generates a positive pulse, which is inverted with the help of a BC547 npn transistor (T1). The inverted pulse is fed to controller port pin P3.2. The microcontroller executes an interrupt routine when a pulse arrives.
The interrupt routine measures the total time taken for one revolution, divides it by 360 and stores the result in another timer, configured in auto-reload mode. Now the timer overflows after tiny intervals of time. Each time it overflows, the next stored value is called from the lookup table and displayed (refer Fig. 5).
Alternatively, sensor MOC7811 can be used in place of the arrangement that is made using separate IR transmitter and receiver.
Mechanical arrangement is the most critical part of this project. An AC motor capable of around 1400 revolutions per minute (rpm) is used. The PCB on which the components are soldered is cut into a specific shape in order to improve aerodynamics and reduce any vertical movement. Eight bright LEDs are used, which are mounted on another board that is attached to the main PCB at right angle.
The critical part here is to get the power to the rotating board. There are various ways of doing it. One simple way is to use rechargeable batteries, as shown in the circuit diagram, and mount them over the board such that they don’t come out when the motor rotates. The rugged way of doing this is to use an AC motor with a double shaft. A step-down transformer and a bridge rectifieralong with a 7805 IC can be used to get 5 V from the mains supply voltage. Carbon brush, spring and copper wound bush are used to bring the 5V supply on the main revolving PCB as shown in Fig. 6. The motor should be fitted onto a metal frame in order to reduce vibration of the system and get a smooth operation when the motor revolves.
The software is written in ‘C’ language and compiled using Keil compiler. Mentioned below are some important functions and their role:
void serial_int (void) interrupt 4 using 0. Interrupt service routine for serial interrupt. It stores indices of the characters entered, by comparing with the ASCII codes array.
void introutine(void) interrupt 0 using 2. Interrupt service routine for external interrupt 0. It is basically a position decoder, which starts timer interrupt while switching between displaying the message and the temperature.
void timer0(void) interrupt 1 using 3. Interrupt service routine for timer 0 (auto reload mode). It basically displays characters or temperature depending on flag bits, along with blanks in front of the data for scrolling.
Construction and working
An actual-size, single-side PCB for the controller board is shown in Fig. 7 and its component layout in Fig. 8. PCB for the LED strip is shown in Fig. 9 and its component layout in Fig. 10. Suitable connectors are provided on both the PCBs to connect them to each other. To avoid loose connection, it is better that you directly solder the wires at CON2 and CON2 to connect both the PCBs.
There are two critical factors in the hardware assembly of this project: One is the voltage supply that has to be transferred at the top of the board. The other is smooth rotation of the PCB that is mounted on the top of a motor shaft. Any miscalculation of the weight would result in an unbalanced system and the required rpm would not be achieved. Moreover, LEDs have to switch in less than 1/25th of a second, which means that in one whole minute it would be close to 1500 times. Hence this is the minimum effective rpm of the motor that has to be obtained after fiting the PCB and the components on the shaft.
1. Connect the COM port of the computer to the display system (refer Fig. 3). You can also use a USB-to-serial converter
2. Place the interrupt sensor such that the beam is interrupted by the beam interrupter
3. Open any hyper-terminal and type the first letter (all upper case
4. Wait for change in the pattern of glowing LEDs
5. Enter the second letter and wait again for the changing pattern
6. Enter the remaining text simi-larly
7. When the text is complete, enter space and wait for the changing pattern
8. Press ‘Enter.’ All LEDs should glow
9. Wait until the LEDs turn off. Now start the motor. The text should appear in some time on the circular display •
Sarthak R. Shah is working at Cliff Electronics, Vadodara, and Abhimanyu K. Varde is working at QLogic, Pune
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