I have to make a 3-digit alfa-numerical indication. I have choosen to use 5*7 dots LED Matrix Displays. Since I need a big display - have chosen a 2.0" LED display from Kingbright
The schematic will work offcourse with any other LED display size.
To save controller pins, the solution was realized on shift registers 74HC594 and SPI peripheral.
Here is the schematic:
The controller needs only 3 pins to send data to display: MOSI, CLK_BUF, PL/RCLK_BUF. All three signals come from SPI peripheral of STM32F103. But they should be buffered to convert 3.3V levels to 5V. 74HC14 can do this job very well.
Signal OUT_CLR can be used to blank display at any time. IT can be connected to reset circuit or watchdog.
The circuit works as follows:
The first 74HC594 is used to drive 2981 Darlington transistor array. The array is needed to amplify row signal, as it can drive up to 15 parrallely connected LEDs in worst case. The array outputs are connected to Anode rows of TC20-11 LED Displays. During the indication cycle, the SPI should write there a running one. E.g. only one A2981 output should be at logic 1 at the same time.
The logic 1 activates appropriate row. To lit the dot in this row, the logic 0 should be written in appropriate column via second and third 74HC594 shift register.
In this case the duty cycle of 1/7th is achieved. The current limiting resistors define the current around 9ma per segment. This is not so much, but it should be so low, as 74HC594 has a limit of 70ma per IC.
When using TC20-11HWA type display the picture is not so contrast - but bright enough to read the image in artificial light. Using SRWA or SURKWA ultra bright displays allows also reading in sun light.
The number of digits can be easily extended as shown above. THe update rate should be higher than 100hz to avoid flickering.
This schematic is fully operational. The photos will follow. I can post also SPI function software.
The schematic will work offcourse with any other LED display size.
To save controller pins, the solution was realized on shift registers 74HC594 and SPI peripheral.
Here is the schematic:
The controller needs only 3 pins to send data to display: MOSI, CLK_BUF, PL/RCLK_BUF. All three signals come from SPI peripheral of STM32F103. But they should be buffered to convert 3.3V levels to 5V. 74HC14 can do this job very well.
Signal OUT_CLR can be used to blank display at any time. IT can be connected to reset circuit or watchdog.
The circuit works as follows:
The first 74HC594 is used to drive 2981 Darlington transistor array. The array is needed to amplify row signal, as it can drive up to 15 parrallely connected LEDs in worst case. The array outputs are connected to Anode rows of TC20-11 LED Displays. During the indication cycle, the SPI should write there a running one. E.g. only one A2981 output should be at logic 1 at the same time.
The logic 1 activates appropriate row. To lit the dot in this row, the logic 0 should be written in appropriate column via second and third 74HC594 shift register.
In this case the duty cycle of 1/7th is achieved. The current limiting resistors define the current around 9ma per segment. This is not so much, but it should be so low, as 74HC594 has a limit of 70ma per IC.
When using TC20-11HWA type display the picture is not so contrast - but bright enough to read the image in artificial light. Using SRWA or SURKWA ultra bright displays allows also reading in sun light.
The number of digits can be easily extended as shown above. THe update rate should be higher than 100hz to avoid flickering.
This schematic is fully operational. The photos will follow. I can post also SPI function software.
