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schematic http://www.perceptiveart.com/Arduino/Clock04_schem.jpg I am making a clock, using 6 rather large 7 segment blue displays. I am wondering 2 things.

1) Do I need a bypass capacitor on the 12V supply for each UDN2981 chip?

2) How can I control the dimming of the whole thing at once? I identified 3 options as potential solutions: A - a transistor at point A (on the schematic), controlled by the Arduino by PWM (which would in turn be controlled by a potentiometer via the Arduino) B - a potentiometer at point B (12V supply) C - some sort of voltage regulator at point B.

3) Any corrections or suggestions to my schematic are welcome.

Any help would be greatly appreciated. Thanks in advance.

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  • \$\begingroup\$ I would not put a transistor at the ground of the shifter, as it is possible ( though not likely) to reset the RAM. A potentionmeter doesn't dim LEDs, it will get very hot. I highly reccomend you cut that out. if the frequency of the source driver is high enough, you should 'flash' the LEDs. (put them on for Xms, and off for Yms.) and that will determine brightness. \$\endgroup\$ – user86234 Jul 14 '16 at 15:33
  • \$\begingroup\$ Flashing the LEDs seems to cause flickering rather be than a decrease in brightness. Maybe the shift register is the cause if this? \$\endgroup\$ – Fed Jul 15 '16 at 2:43
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1) Putting 0.1 uF on each UDN2981 is good practice but I would not go crazy on the decoupling. It does not have any "smarts" that will get confused (no sequential logic to mess up like a CPU or shift register or anything fancy). It's just a dumb power buffer. And your MCU is already on a separate regulator anyway.

2) For dimming, high side is not the best (requires level converter), and PWM on GND of UDN2981 may not work correctly. The simplest is to use an N-channel MOSFET to gate the GND connections of all LEDs and drive the gate with your PWM signal. N-channel MOSFETs are very efficient (2x lower resistance than P-channel of same die size). But you can also use NPN bipolar like 2N2222 with a base resistor if you want a bit cheaper.

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  • \$\begingroup\$ 1) what would classify as 'going nuts'? Using more than 1 cap? \$\endgroup\$ – Fed Jul 15 '16 at 2:50
  • \$\begingroup\$ 2) I'll try the MOSFET, thanks. I guess I'll have to watch the current and power ratings here, considering GND will be sinking 6*(3*7+1)=132 blue LEDs. Could you help me with figuring this one out? I estimate 1.4A and 17W, but I may be way off... \$\endgroup\$ – Fed Jul 15 '16 at 2:57
  • \$\begingroup\$ Wait! I finally figured out how to view your tiny schematic from the source so it is readable. A couple of problems: 1) driving single blue LEDs from 12V is bad idea. Most power is spent heating the resistor, not lighting led. Are LEDs really single LEDs or are they 12V? , 2) having PWM output drive the OE of each shift register will work fine to dim it, but you need a 10k (max) pulldown on each shift register output (see UDN2891 datasheet). Without this PWM will probably not work because OE causes outputs to go tristate. \$\endgroup\$ – Vince Patron Jul 15 '16 at 4:55
  • \$\begingroup\$ My schematic isn't entirely correct, sorry. 7 of the 8 segments are actually 3 LEDs in series, hence the 12V (3.3V each). DP is only 1 LED. \$\endgroup\$ – Fed Jul 15 '16 at 8:02
  • \$\begingroup\$ I tried 10k resistors on the shift register outputs, and no change. The off segments just stay at a low intensity level for some reason. \$\endgroup\$ – Fed Jul 15 '16 at 8:04
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1) You need a bypass capacitor whenever you expect the voltage to drop suddenly (usually by large transistors or by lots of small transistors being switched on and off) Since you are switching LED's on and off, Yes you do need bypass capacitors on each IC. This is because each piece of copper also has a few uH-nH (depeding on size/width) of inductance so each IC is really a dynamic load and turns your power system into a filter.

2) Dimming could be accomplished by either controlling the LED switch voltage (assuming the UND2981 can handle a wide range of voltage on VCC look that up in the datasheet OR dimming is usually accomplished via PWM, just google "PWM led"

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  • \$\begingroup\$ 1) Thanks for that, very helpful. 2) the UDN can handle 5-50V. But how could I control the switch voltage? As far as PWM to dim the LEDs, it doesn't seem to work. Maybe the shift register interferes with this method? \$\endgroup\$ – Fed Jul 15 '16 at 2:45

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