I'm trying to control a large 12V 7-segment LED by way of a HEF4543B driver. I've connected my Arduino Uno R3 to a 12V power source (wall-wart). See the attached schematic to see my test-setup.
I'm somewhat of a noob in the electronics area, so the (wiring described in) the schematic probably makes no sense at all, but that's (part of) why I'm posting here right now.
As you can see, the test-setup is not connected to any of the data pins, it's just hard-wired to light up (almost) all of the LED's.
Now here's the problem: the segments do light up, but only about half as bright as they do when directly connected to the Arduino's Vin + GND.
I also have some ULN2803's, which I wired up with and without the HEF4543B and the result is the same: way less brightness from the LED segments.
What am I missing?
All help is greatly appreciated!
allert:
Now here's the problem: the segments do light up, but only about half as bright as they do when directly connected to the Arduino's Vin + GND.
So how have you connected them when you get half brightness? Do you mean you get full brightness when you bypass the hef4543? What series resistors are you using?
I'm trying to control a large 12V 7-segment LED by way of a HEF4543B driver. I've connected my Arduino Uno R3 to a 12V power source (wall-wart). See the attached schematic to see my test-setup.
Thanks for taking the time to think about my problem!
@PaulRB:
The half-brightness-scenario occurs when the wiring is like the attached schematic, so WITH the 4543. (Half-brightness also occurs when 'replacing' the 4543 with a ULN2803 darlington array...)
The benchmark ('full-brightness') is directly connecting a segment to Vin and GND from the Arduino.
It probably sounds silly, but I'm not using resistors anywhere yet, thinking this would only make the LED's less bright thus not solving my problem...
Firstly, the critical factor that is necessary for any useful discussion, the link to the actual specification of this 12V 7-segment display.
Secondly, a LED driver. Reference to the datasheet for the HEF4543B demonstrates that is is not a LED driver. It is in fact, a LCD driver; it is not designed to drive LEDs (other than the HP "bubble" displays rated at 1 mA or so!).
As to the ULN2803, it should drive nominal "12V" displays quite well, though it does have a minimum saturation voltage drop of about a volt. Without the LED data, it is impossible to know more.
Totally forgot to include a link to the specs for the display. Sorry about that! Here they are:
I'd seen the 4543 being used for just this purpose on multiple occasions, so I just figured that's what it's for. Furthermore, it fit my requirements (capable of 12V, fit for common anode usage, etc) and I just bought it. But would you say it's useless for driving my 7-segment LED?
Regarding the ULN2803: would the 1V voltage drop explain such a radical decrease in brightness?
The forward voltage of those displays is 12V, with a max of 12.4V. There is no guarantee they will light with a 12V supply, you are just lucky. The extra voltage drop of the driver chips you have tried is causing the dimness. Again, you are lucky they light at all with the driver chips. Bottom line, you need higher voltage supply, perhaps 15V, and series resistors to prevent damaging the displays.
No, the Uno can take up to 20V. But you need to be careful above the recommended max of 12V. It depends what else the Uno is doing. As long as the total current draw of the Uno is kept really low, you should be ok. But if you draw signifiant current, eg. driving several leds directly from Uno outputs, then there is a danger of the onboard voltage regulator overheating and failing. So if you need to draw significant current at 5V, the answer is to use an external regulator that can provide more current than you need from 15V without overheating. An ordinary 7805 regulator will probably do fine. It is rated up to 1A, so you won't be anywhere close to that and overheating won't be a problem. You would feed the 7805's output directly to the Uno's 5V pin, bypassing its onboard regulator.
Thanks!
But in this case, I need to get 12+ Volts to the LED's after the voltage drop caused by one of the IC's (be it the ULN2803 or the 4543 driver). Say I had a 15V power supply, what kind of setup - powering the Arduino as well as the LED's - would be the most sensible?
Eventually, I'll have three of those big 7-segment LED digits mentioned in my earlier posts. What kind of difference would that make, regarding the power source requirements?
My suggestion of an external voltage regulator is just for powering the Arduino and anything connected to it running on 5V. If there is little or nothing other than the Arduino and few of those driver chips, you shouldn't need it and can use the Uno's onboard regulator.
For the displays themselves, you would wire them to the 15V (regulated) supply via series resistors, one per segment. The value of those resistors depends on the current each segment draws. Suppose that is 20mA. The driver chip is probably dropping 0.5~1.5V. Lets assume that's only 0.5V for safety. The segment needs 12V. So we need to "drop" the remaining 15 - 12 - 0.5 = 2.5V. Using Ohm's law, your series resistors need to be 2.5V/20mA = 125 Ohms. So 150R or 120R should do. Try the 150R first for safety and measure the current with a DMM.