DIY LED display offers extra features and PWM
This design idea (Figure 1) shows how to easily "add new meat to old bones" and how to add some new functions to the previously released simple LED display.
Figure 1 The upgraded simple LED display retains the same interface but adds more functions.
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The design retains the same interface, although some controls have additional functionality.
New features of the design are:
The display is recognized by the host μP;
Brightness control (2 levels or more);
The interface cable is longer because it is less sensitive to ambient electrical noise;
Protect IC from Vcc loss;
The display is reset on power-up (all segments are lit);
Can extinguish the light;
Some additional features for further modification or use in external applications;
Of course, there's also PWM for the true connoisseurs.
What if there are multiple devices sharing the same slot, the host? P can be done by reading bits S5…S0 and comparing the result with ? ? Code 110101 is compared to identify the display.
There are 2 levels of static brightness (although there may be more).
Levels are selected in the order in which the display numbers are programmed: from the number corresponding to LE0, to the number corresponding to LE1, or vice versa.
The selection is held by the RS flip-flop controlling BJT switch Q1 (BC807).
The sequence from LE1 to LE0 sets the output of the RS flip-flop to A=0 and BJT Q1 is activated. This corresponds to full brightness.
The sequence from LE0 to LE1 sets the output of the RS flip-flop to A=1, so the BJT switch Q1 is closed and the LED receives less current.
The same principles can be used to add more functionality to the interface.
0. Initial/display state: S3=0, S5=1, other pins are irrelevant;
The steps required to change the data on the display are:
1. Set the digital address: S2..S0 = address;
2. S3=positive strobe 0-1-0 to latch the address;
3. Set a segment address on S2..S0;
4. Set the value of D: if the segment must be ON, then S4=1; if it is OFF, then S4=0;
5. S5=negative strobe 1-0-1, latch the value of D;
6. Repeat starting from (3) for another segment and when all segments are completed, go to (1) when there are unattended numbers.
7. Turn off lighting: S3=1, S5=1
So, to get PWM, you just use S3 while keeping S5=1.
The power switch in Figure 2 is simpler and consumes less power.
The power switch of Figure 2 is simpler and consumes less power than the BJT switch shown in Figure 1.
It uses dual power distribution switch TPS2092 instead of BJT switch. This circuit will also limit current flow if an overload or short circuit is encountered.
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