This versatile LED display circuit uses an erasable programmable read-only memory (EPROM) to display various light patterns on LEDs. Since bicolour LEDs (comprising green and red LEDs) have been used, display is possible in three colours (green, red and amber).
Versatile LED Display Circuit
The circuit is powered by 5V DC. IC 555 (IC1) is wired as an astable multivibrator, whose oscillation frequency can be varied using preset VR1. The output of IC1 clocks 12-stage binary counter IC CD4040 (IC2), which, in turn, provides address data to EPROM IC 2716 (IC3). IC3 contains the code (see Table I) for the display.
The high logic at any data pin causes the corresponding LED to glow. When the data at address location 00H is addressed, the red LED of LED1 glows. The data byte 44H at address location 09H causes both the green and red LEDs of LED2 to glow (refer the table).
The binary outputs of IC2 comprising Q0, Q1, Q2, Q5, Q6, Q7, Q8 and Q9 have been connected to address pins A0 through A7 of EPROM IC3 (2716). Q3 and Q4 outputs of IC2 have not been used. This causes each display pattern to be repeated eight times before the next pattern is displayed. You can adjust the number of times a display pattern repeats by changing the output lines of IC2 connected to the EPROM’s address pins A0 through A7.
Construction & testing
The circuit uses a total of four bicolour LEDs. However, more LEDs in pairs of four can be added in the dotted lines (see the figure). Suppose you want to connect four more bicolour LEDs (LED5 through LED8, not shown in the figure). For this, you’ll have to connect them in parallel to LED1 through LED4, respectively. The speed of the display can be changed by varying preset VR1, which changes the clock frequency.
You can also create other display patterns by coding the EPROM accordingly. Note that the code should be burnt into the EPROM (by using a programmer kit) before it is inserted into the circuit.
The article was first published in January 2005 and has recently been updated.