May 30th, 2009

Digital Thermometer

Digital Thermometer

Here is a very good looking thermometer project showing the reading as a discrete value and as a bar-graph on a LCD. The idea is simple, we have a sensor, the LM35, who outputs a voltage depending on the ambient temperature, LM35’s rating is 10mV/ degree Celsius. Since the sensor’s output is analog we need to convert it to digital in order to display the value on LCD. The ATmega8 microcontroller will handle the A/D conversion, with a 4.8mV resolution it is within the accuracy range of the sensor.

Once the LM35’s output has been converted to digital, the microcontroller will make the calculations necessary to determine the temperature and display that value. The LCD is a 204 type so the bar-graph will show 20 levels. ATmega8’s port C0  reads the analog data from the sensor and port D controls the LCD. The code was written in BASCOM AVR which has some built-in functions to handle the A/D converter of the microcontroller. For Fahrenheit measurement use LM34 sensor.

All in all it is easy to build, as a school project perhaps,  with possibility for upgrade, for example to be able to change the bar-graph’s scale.

Digital Thermometer: [Link]

Seven segment display

In this article I intend to explain the functionality and usage of the seven segment display, probably you have seen many projects with these type of displays, however the price drop of LCD’s tend to overtake the market, there are a still few applications for which these devices are more suited. For large numeric displays, like clock’s, railway station displays, low-cost measuring devices or very stressing environments the led based displays are better, and cheaper.  The most simple led display available is the seven segment display, it consists of 7 led stripes arranged forming the number 8, because of its simple construction it is very robust and can function in very low or high temperatures, can withstand vibrations,  mechanical shocks without problems, for what the LCD would fail to work or even get permanently damaged.

single sevene segment

If we look at one digit we can see 10pins each segment and the small dot are LED’s, each of them has one terminal connected to a common pin, from this comes the name common anode or common cathode, and the other terminal is connected to a standalone pin, since the common pin is doubled, we have the 10 pins for each digit.

seven segment config

Lets take as example the common anode type, to light the segments we need to connect the positive supply rail to the common pin, and pull to ground the segments, each segment depending on its size can handle a few miliamps, after all we are talking about LED’s not bulbs. That’s fine if you need just one number to display, but how can these digits be connected to form a multi-digit display? The first approach is to connect each segment to a micro-controller pin, this way for each digit you need 8 pins and isn’t elegant at all.

The other solution is to connect each corresponding segment from the digits to a common bus, and power the digits one at a time, thus multiplexing the data.

seven segment schematic

This multiplexing probably sounds more complicated than it really is, look at the next picture:

illustrated seven segment operation

Since the digits share the same data bus, each digit will have the same number displayed, like the wheel on the picture, to change the number the “data guy” rotates the wheel. So how can we display 1234 you might ask, well wee need another guy, the selector, which will leave only one digit to be seen, all the others are shut off, by synchronizing the “data guy” and the “selector guy ” so they operate at the same time, when the wheel is at the 1111 position, the selector opens the first window, when at 2222 it opens the second and so on. By changing the data and selecting the digits at many times per second the human eye will see a steady image with 1234, the display refresh rate should be above 50 times in 1 second, otherwise the image may flicker.

In the schematic the “selector guy” is implemented by the PNP(BC327) transistors and the “data guy” is the data bus (PORTB0..7). For the practical demonstration I used my own avr development board with ATmega88, four HD1131R type seven segment digits which I rescued from an old TV a few years ago, the connections are made on a prototyping pcb with scrap wires. The data port is PORTB, for selection the PD4..PD7 pins where used.

Like I mentioned earlier we need a periodic data update with digit selection to have a steady image, and since the entire display refresh should be above 50Hz and we have four digits we need to change the digits at frequencies greater than 200Hz. Since the amount of time which one digit is turned on is 1/digit count, as the display has more digits the light intensity gets weaker, in our case the HD1131R has 1.6V voltage drop, by powering from 5V and trough 330Ohm results 10mA trough each segment, but since we have 4 digits the average current received by each segment is 10/4=2.5mA, this will result in a very fade light. For the demo application I used 220Ohm values, with the on board 100Ohm resistors the average current is 2.5mA/segment, although I could have used only the on board resistors, thus having 8.5mA average current, but the peak would be 35mA, and the micro-controller can handle at maximum 25mA/pin, off course the pin won’t get blowed right away from 35-50mA, but you should be careful when designing similar circuits.

That’s enough about the hardware, let’s see the software, for the periodic refresh I used a timer interrupt with 244Hz, resulting in 61Hz refresh rate, in Europe we have 50Hz AC power, so that’s fine, but in the case of 60Hz AC power the refresh rate should be at 70Hz to avoid the stroboscopic effect.

For the display data I used 4 byte buffer from which the interrupt reads and sends to PORTB, the buffer index can be used also for digit selection(see void DisplayData(uint8_t* Display) ).

Because of the seven segment data format by directly writing 5 to PORTB, won’t result in displaying 5, so the numeric data must be converted first, because there isn’t any easy algorithm to mathematically make this conversion, we must use another method, for a one digit display you could make if-else or switch statements, but with 2 digits you already need 100 lines of code, for 3 digits 1000 lines and so on. A very handy solution is a look-up table:

const uint8_t DigitCharTable[] = { DigImage0, DigImage1, DigImage2, DigImage3, DigImage4,\
DigImage5, DigImage6, DigImage7, DigImage8, DigImage9, };

By indexing the table with the numeric value 0-9 we get the needed data format, example PORTB = DigitCharTable[5]; results in displaying 5;

The void NumToDispValue(int16_t Number, uint8_t* DataBuffer) function uses this method to convert 16bit signed value into display compatible string.

After the conversion made, the actual display buffer needs to be updated with the new value, here is a tricky part, by converting 1234 into display compatible data we will have [DigImage1, DigImage2, DigImage3, DigImage4], by having DigImage1 at the first position and my display has  D4,D3,D2,D1 physical configuration the data displayed will look like this: 4321, it gets reversed. This can be overcome by reversing the selection lines PD4..PD7, or by copying the reversed string into the display buffer(see void StrUpdateDisplay(uint8_t* DisplayBuffer, uint8_t* UpdateData, uint8_t InvDir)  ).

After the conversion, the result is a string(not ascii!), so the string manipulation techniques can be easily used, the dot can be added to any digit, simply by setting the 7-th bit in that byte(see macro WITHPOINT(a) in SSDisplay.h ). There is also possible to display a limited amount of characters like E,F,c,C.., (see SSDisplay.h) it won’t be pretty but it’s readable.

Last but not least, since the refresh is done in interrupt, the display buffer update operation should be made atomic, in other words first disable the refresh interrupt, and then update the buffer, otherwise the image will flicker, and don’t forget to re-enable the interrupt after!

If you use only the refresh interrupt, than it’s easier to just disable all interrupts globally, although if you have other interrupt sources too, then disable only the refresh interrupt, this way you won’t hold back the other incoming events.

For data source I used the ADC to measure the potentiometer output, you can also tweak the macro’s used to handle the ADC, the dot in the 2th digit is activated if the ADC value  is above 512.

The demo project was made using avr-gcc and avrstudio, some variable type notations may differ from the ANSI C standard since I use the avr-gcc typedefs, the entire project among with schematics, pdf’s is available for download.

In the next article I will explain the usage of the matrix keyboard.

Seven segment display explained: [download]

May 29th, 2009

Wearable Computing

Wearable Computing

Here we are one step closer to a future that’s starting to look more and more like Sci Fi movies. After I’ve seen Johny Mnemonic for the first time, i was wondering when will we be able to type or surf the net just by moving our fingers in the air on a invisible keyboard. Seems that day is closer than i thought thanks to the research done by Dr. Masaaki Fukumoto Executive Research Engineer at Docomo.

Dr. Fukumoto is researching ways to interface the human body and machine  and he managed to brake one barrier with a headset capable of reading the movement of the eye and use it to control other devices or applications. The headset measures the electrical potential of the cornea and tracks the eye’s movement. This is called electrooculogram in the medical field.

Using common electronics components in building this headset, Dr. Fukumoto can control the functions of a media player just by moving his eyes or making a camera focus on whatever his eyes are looking at. Although more research must be done before this technology becomes reliable, Dr. Fukumoto says we could see this technology on the market in a few years.

Wearable Computing: [Link][Via]

I just discovered this video on youtube, which shows a couple of guys hacking into a building’s light control circuitry to get access to it and turn it into a giant pacman game. I don’t know if it’s real or not, because they are just connecting a bunch of wires without any logic but it sure looks like their having allot of fun doing it and theoretically this is possible. I’m not sure about the legal actions that these guys could be facing if the hacking is indeed true. Leave a comment and tell me what do you think about the hacking, real or fake ?

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May 27th, 2009

Weather Indicator

Weather Indicator

Not all of us understand weather mechanisms, how storms develop or what a low pressure front means so having a weather station that measures these parameters and display them as numbers might not be that helpful. However a team from Cornell University designed and built a station that displays a graphic representation of the current weather condition. If its stormy outside you will see a cloud with some raindrops and a lightning, all this on a 88 RGB LED matrix.

There are two main modules making this station. The outdoor module who makes all the measurements and then sends the data wirelessly to the indoor module which will display the icon corresponding to the data received. The data transmission is done on  433Mhz  by two Linx Technologies modules models RXD-433-KH2 and TXD-433-KH2. These modules also take care of data encoding and decoding using an address code so that only your specified modules will receive the transmission.

The outside station has five sensors to measure weather parameters. A solar cell, which also provides power, measures the light intensity and so it can tell if it’s day or night, cloudy or sunny. A LM34 is used as the temperature sensor and outputs a 10mV with every degree F. Rain sensor is from a Hot Wheels Radar Gun, using a Doppler Radar operating at 10.525GHz is sensitive enough to measure velocity of rain drops. A Diy anemometer was the choice as wind speed measurement tool and a capacitor who changes capacitance based on humidity is used for the last measurement.

ATmega644 is the microcontroller gathering data from all the sensors. This is then transmitted as a 10 bits code, each sensor data having  2 bits in that message. Inside the indoor module we find another ATmega644 who controls the LED matrix. Using PWM to drive the LEDs a very high number of colors is achieved instead of the usual 8 colors.

This project offers a large amount of information and insight into weather monitoring systems. It will require some skills to built it, specially the anemometer, it not as easy as it looks but it’s worth it.

Weather Indicator: [Link][Via]

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