IC Transistor with LED - Page 1
Forum section: electronics / general

author: modernjack3
author_rank: Contributor
timestamp: January 15, 2016, 07:32:50 pm
content: Hello guys !First of all , sorry for this maybe stupid post but I really need some help ...I am currently working on a LED based clock, but i want to build it oldschool so no adruino, raspberry pi oder other "advanced" devices, just a quartz crystall signal tht will be transformed into a 1Hz signal,some (probably about 18 of them) Shift registers (8-Bit) of the type "74HC595" and the LEDs...But here comes my problem - the shift register has by far not enough power to supply an LED so what I need are transistors - there comes my next problem - which ones should I use for tht job ? I will be using about 140 LEDs to show the time so Id prefer some kind of IC tht is packed with ~10+ Transistors for this use.If it matters how tht clock will work, ill post it in another post right here .So already a BIG "thanks" to all the ppl tht are willing to help me out !(I am not a native speaker so please excuse my mistakes)J.

author: Dielectric
author_rank: Regular Contributor
timestamp: January 15, 2016, 07:41:25 pm
content: The easy answer is a ULN2803. 8 channel darlington array, maybe overkill on the current but super easy to use with a bunch of shift registers. The harder answer would be some kind of fancy TI LED driver, they have a shift-register interface but the packages tend to be little leadless things.

author: modernjack3
author_rank: Contributor
timestamp: January 15, 2016, 08:02:34 pm
content: Hey mate thank you for helping me out .Could you tell me how i regulate the output voltage and if this thing might need any cooling?

author: Dielectric
author_rank: Regular Contributor
timestamp: January 15, 2016, 08:13:45 pm
content: If you need less than about 20mA through the LEDs, you could just use a resistor in series with each LED. The size depends on the Vf of the LED, the current, and the voltage you're using to drive the LED array. Easy math. If you Google "ULN2803 LED) you'll find 1001 schematics to choose from. Most of them are even correct!You shouldn't need any active cooling. Unless you're making a clock that's visible from space in daylight?

author: modernjack3
author_rank: Contributor
timestamp: January 15, 2016, 08:40:09 pm
content: Since you seem to know quite alot about tht stuff ..Can i use this to measure the time? i am not looking for something extremely accurate ( maybe 1 sec loss a day or so ...)http://www.electroschematics.com/4843/1-hz-pulse-generator/can I use tht?

author: Dielectric
author_rank: Regular Contributor
timestamp: January 15, 2016, 09:55:05 pm
content: No, the frequency will be all over the place. It is based on the 10uF cap and the resistors, so as they drift so will your 1Hz timebase, even if you have a trimpot to get it set to 1Hz, it will change dramatically with temperature. You probably want a crystal-based oscillator. This one looks cute, uses easy-to-find stuff, and has a nice writeup:http://www.hackersbench.com/Projects/1Hz/

author: modernjack3
author_rank: Contributor
timestamp: January 15, 2016, 09:57:50 pm
content: Ok thanks a lot ! May I send you like a overall "collection" of my plans and you can check what works out and what doesnt ? Id love to hear what you think about my project or if you have any improvements!

author: Dielectric
author_rank: Regular Contributor
timestamp: January 15, 2016, 11:10:57 pm
content: Actually, you should move on over to the Projects section since this is decidedly not a microcontroller or an FPGA. Show all of us your plans!

author: 0b01010011
author_rank: Regular Contributor
timestamp: January 19, 2016, 02:20:06 am
content: TPIC6B595 - almost the same interface as HC595 but much more current capability... 150mA per pin, to a max of 500mA per device.There is also a TPIC6C595 that has a slightly lower rating - 100mA per pin to max of 250mA per device.Note these are open drain devices as opposed to the 74HC595 which is bipolar.

author: macboy
author_rank: Super Contributor
timestamp: January 19, 2016, 04:51:19 pm
content: Modern LEDs can be very bright with just 1 mA of current. You don't need to run them at their maximum rated current!

author: matseng
author_rank: Frequent Contributor
timestamp: January 20, 2016, 04:13:04 am
content: But.... Using shift registers like the '595 or their higher power brethrens like the TPIC will be incredibly hard in a non-processor clock.For someone that have to ask about how to supply enough current to LEDs it would be a next to impossible task.What you need is simple counters like the 74LS162 and then BCD-to-7-segment-decoders like 74LS47 - counters and decoders is what have been used for many decades to build clocks.