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Generic Calculator Shield for Arduino Photo-journal
03-11-2015, 10:58 PM
Post: #19
RE: Generic Calculator Shield for Arduino Photo-journal
(03-11-2015 09:32 PM)Maximilian Hohmann Wrote:  Forget about the ugly LCD and stick with LEDs :-)

(03-11-2015 07:20 PM)MarkHaysHarris777 Wrote:  Should you want to, a larger display can easily be made with more digits... just add more seven segment display chips and an equal number of 273 chips.

Have you looked at the max7219 display multiplexer? With a single chip that only needs three lines from the arduino you can drive an 8-digit seven segment display. (For example: Or an 8 by 8 dot LED matrix (less than 3$ on eBay directly from Hong Kong, shipping and multiplexer chip included...).

'ey Max! ... good to hear from you. The 7219 is a pretty cool chip; worth playing with sometime. I have often thought about making a modern replica of the Wang 700c. I would put more function into it, of course, but it would have the classic click button panel on the bottom with the rising two register 12 digit red display above. The Wang 700c had nixie tubes-- but we don't need to go that far!. The trouble with the LED displays is power. And with a dual 12 digit display it would also require a switching 5v power supply.

If I were to make my own LED seven seg driver for a large display I would not use the 273. I would find a shift register like the 74HC595, or the 74HC164. I used the d-type flip flops (273) because I had them.

Well, you can run the leds (as you know) with about half of their max forward current, comfortably. You probably noticed I used 330 ohm limiting resistors; gives a forward current of 7.8 mA per pin on the seven seg devices, and 7.5 mA per pin on the led bar display. The cool thing about the 273 is that it can 'source' and 'sink' 8 mA per pin! But still, even at that, with all 24 bars 'ON' my little display is drawing about 184 mA. Even limiting the current a dual twelve digit seven seg display would draw a little more than 1.3 amp--- the 7805 can tolerate about 1.5 amp for short periods but really isn't supposed to be used for loads more than about an amp. So, maybe two of them?

Anyway, I am looking for a way to drive the 12 digit display (LCD) in my Royal cl130 (dead, from 1993). Otherwise, I may put some LEDs in that thing! I'll keep the 7219 in mind... thanks.


Kind regards,
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RE: Generic Calculator Shield for Arduino Photo-journal - MarkHaysHarris777 - 03-11-2015 10:58 PM

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