Hello,

I have to make an indication device which has 120 LED's.

I have connected that LED's to one port of micro controller via 74HCT574 and ULN2004.

i.e Micro controller port -> 74HCT574-> ULN2004 -> LED's

For 120 LED's I have used multiple 74HCT574 IC'c and ULN2004 IC's.

I am sending common data on all 74HCT574 and enabling particular 74HCT574 by giving clock to it.

Is this circuit okay?

Please suggest.

thanks,
paddy

 

can you post the circuit?

 

You should multiplex it, so you need less ICs.

 

Please have a look on attached circuit.

I have to check 120 values with some defined limit values and depending on result i want to turn on/off the LED's.

So,
1. I am comparing first 8 values
2. Set/clear corresponding pin according to result
3. Giving clock to 74HCT574 latch
4.Again comparing next 8 values and so on.

So the output of the latch will drive the LED's through ULN2004

 

if you are not doing multiplexing and driving bunch of LEDs at once (rows or columns), it looks like a waste of ULNs and PCB space, because LEDs can be driven by 74HCT directly. common way is to use sink/source matrix with bunch of LEDs between them. for example using 8 x8 outputs you can drive 64 LEDs using only two chips (plus MCU). if you add one more, you can drive 128LEDs. multiplexed LEDs have lower duty cycle so they need a bit more current that usual. if you have two 8-output chips with all LEDs on at a same time, you need to sink lot's of current. for example let's say we use 30mA per LED. 16*30mA=480mA. granted, this is the worst case but it is at the limits of ULN.

 

Is it possible to turn on all 120 LED's at a time with this configuration?

Is there any effect on intensity of LED's?

I am having confusion with this. That's why i am doing it with another approach.

Please suggest so that i can modify my circuit according to your suggestion.

thanks for your kind support.

 

this would require multiplexing, you would load 16bits representing row, and turn on column_0 (ULN); wait a bit, then load another 16-bit to rows and turn on column_1, etc.

the scanning need to be continuous as LEDs are turned on intermittently (only for a short time). only one column is active any time.

if you don't like that, you can use shift registers like 74HCT595, each has 8 outputs so you would daisy chain several chips to get number of outputs you need. the good thing is that outputs can stay on (latched) indefinitely so you don't need to scan. downside is that this requires more chips and deep queues require more clock cycles to update outputs.

if you don't mind soldering fine pitch, there is single chip solution for this - AS1130 (there are others too).

 

It would be great information for me. I will check out with this.

Thank you very much.