im tryin to make up an LED display to look roughly like this and im stumped on da techno babble in da pdf file about the chip (m5450b7(http://www.grifo.it/PRESS/DOC/ST/M5450_51.pdf)) using the following way of wiring up the LEDs it gives the following equation for working out R the LEDs im using have the following spec Forward Voltage 2.2V @ 20mA Forward Current(max) 30mA Reverse Voltage 5V Powe dissipation 105mW im expecting to have 34 connected (one for each output) and they could all be on at once though more likely 3/4 of them i just want to know if ive choosen the right way of wiring the LEDs and what rating R should have. cos i kno if i try it without askin ill end up with another repeat of Cheese's LED Destruction article cheers for ur help chris (im beggining to wonder why i didnt jus get an LCD hehe cheapskate i guess )
Welcome to the boards. That chip may not be perfectly ideal for what you're doing. But I'm not entirely sure what you're doing. The chip is a driver for seven-segment displays up to five digits. if you feed a number like "666" into the serial port, it will make the LED seven-segment readouts shine with "666". Assuming that's okay, and you're comfortable with the thing functionally, it can sink only so much current in configuration a) there. Before we launch into solving for R, can we first explore what you're trying to accomplish?
im tryin to make a display, based on a matrix of LEDs, to go in my case to which i can output text and pictures via da parrallel port and some hard graft programming. i just intend on using 34 individual leds on a piece of plastic or somesuch, in a pattern like the one above (and i done somin really stupid and bought dat damn chip so i hopes i can use it)
well, it's intended for seven-segment kind of layout. Maybe something like this was more what you were after? http://www.maxim-ic.com/appnotes.cfm/appnote_number/1033
The chip uses constant-current sinks on each led output, (the "8" symbol). The diagram you show is how to reduce heat produced in the ic package by producing some of it in an external resistor. Vout is set by your circuitry, then calculate the resistor so it can drop the spare volts when all the leds are on (worst-case current). The diode method shown looks easier to implement, as you'll drop around 0.75v per diode however many leds are on.
nah i was hopin to be able to use the chip ive got and to also keep the circuit fairly simple (its my first real shot at this). it says if the diodes are properly chosen that scares me i was planning on using the 5v internal supply, can i work out the values from there?
