How to drive higher current LEDs from ICL7107? - Page 1
Forum section: electronics / projects

author: Lomax
author_rank: Frequent Contributor
timestamp: October 06, 2016, 11:57:27 am
content: Hi all,I'm building a small LED display using 7-segment displays to show a voltage, using the ICL7107 display driver. Everything works the way I want, but I've found that the 8 mA current sinking ability of the 7107 chip to be too small to generate sufficient brightness - the 7-segment displays I use need 20 mA per segment to reach full brightness. So I'm thinking of adding a buffer chip to the 7107 outputs, something like theSN7407 Non-inverting Hex Buffer/Driver. But annoyingly this only has six channels, so I would need four chips to drive the three displays I'm using, leaving three SN74707 channels unused - a better option would be a chip with seven channels, but is there such a beast?Many thanks,Lomax

author: danadak
author_rank: Super Contributor
timestamp: October 06, 2016, 12:18:31 pm
content: I gather this is a static, non multiplexed design ?Some possibilities -http://www.ti.com/lsds/ti/logic/non-inverting-buffer-driver-products.pageRegards, Dana.

author: ebclr
author_rank: Super Contributor
timestamp: October 06, 2016, 12:58:46 pm
content: http://www.nxp.com/products/discretes-and-logic/bipolar-transistors/resistor-equipped-transistors-rets:MC_30588

author: Kleinstein
author_rank: Super Contributor
timestamp: October 06, 2016, 06:35:42 pm
content: There are lots of outputs so lots of drivers.Instead of more current I would consider using more efficient LED displays. It's normal that the intensity will go higher for 20 mA compared to 8 mA, but the visible difference is not that large. A suitable filter can have a bigger effect on visibility. It is also the question how much light is needed. In a very bright environment a reflective LCD might be the better solution.

author: Lomax
author_rank: Frequent Contributor
timestamp: October 06, 2016, 07:07:48 pm
content: Quote from: Kleinstein on October 06, 2016, 06:35:42 pmThese are red/green (and yellow) bi-colour displays, for which there's very little choice. Brightness is plenty high enough at the rated current.Quote from: Kleinstein on October 06, 2016, 06:35:42 pmThe intended environment is bright. Shaded daylight.Quote from: Kleinstein on October 06, 2016, 06:35:42 pmI don't see how filters could possiblyaddto the brightness?Quote from: Kleinstein on October 06, 2016, 06:35:42 pmLEDs have been chosen because they are readable at a distance.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 06, 2016, 08:10:04 pm
content: Using bicolor is unusual, as the 7107 only supports one LED string per output. Who is controlling the color ?Filters work rather well with red LED displays. They don't increase the intensity, but they make the dark segments to be darker. So the contrast gets better. However this will not work with bicolor LEDs of cause.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 06, 2016, 08:40:15 pm
content: ULN200x is a classic driver with seven outputs.

author: Lomax
author_rank: Frequent Contributor
timestamp: October 07, 2016, 09:44:04 am
content: Quote from: Kleinstein on October 06, 2016, 08:10:04 pmThe 7107 is a common anode driver - the displays are powered by two PWM channels from an MCU (via two transistors). The 7107 only chooses which segments to light up; colour and intensity is controlled by the MCU. This is used for messaging (green for nominal values, yellow to warn, red to alert).Quote from: Kleinstein on October 06, 2016, 08:10:04 pmThere are contrast enhancing filters that will work with any colour, but they all reduce the brightness somewhat.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 07, 2016, 10:20:46 am
content: If there already is a µC involved, one might chose a different ADC (could be as small as MCP3421) and than use shift register based output drivers (like tpic6c595) with binary to 7 seg. conversion in the µC.

author: Lomax
author_rank: Frequent Contributor
timestamp: October 07, 2016, 10:34:14 am
content: Quote from: Kleinstein on October 07, 2016, 10:20:46 amOne might. If one had more than one output pin available.

author: dannyf
author_rank: Super Contributor
timestamp: October 07, 2016, 10:43:26 am
content: QuoteSome PNP switches? Or to add a buffer if you wish.

author: Lomax
author_rank: Frequent Contributor
timestamp: October 07, 2016, 12:10:34 pm
content: Thanks everyone! Having thought about this for a while I think I'm going to to go with the SN7407 buffers after all; I realised I might as well design the board for four displays as there may be cases where I'll need the full 3 1/2 digits, and that means exactly 24 segments! Not looking forward to laying this out though...

author: Zero999
author_rank: Super Contributor
timestamp: October 07, 2016, 12:30:46 pm
content: Perhaps a bit overkill but what about the MIC2981?http://ww1.microchip.com/downloads/en/DeviceDoc/mic2981.pdf

author: Infrared_Fred
author_rank: Regular Contributor
timestamp: February 08, 2021, 07:15:46 pm
content: I know this an old post, but for the sake of searching... Another option for the lower current, at least for red, can be AlGaAs LED displays, they are brighter at low current than typical red LEDs and they are distinguishable by >2V forward voltage and longer wavelegnth (650-670nm).