I’m working on a 7-segment, 0.56 inch tall, LED red display. The display must be readable in sunlight.

I’m looking for ideas to improve the segment brightness.

Should I consider another color of LED display?

What about putting some type of faceplate over the display to improve contrast? I’m thinking about the technique used on very early TI calculators. I don’t know what type of faceplate was used to make their displays visible under high brightness conditions.

What about a high current, low duty cycle drive to the display segments?

A PIC 16F676 multiplexes the three characters. Its limited to a source or sink current of 25 ma per pin. I’m using 220 ohm resisters to limit current from each display segment. Actual current draw is about 20ma on each segment.

Any other ideas?

If you are multiplexing then you need to increase the current through your display because each segment is only on a fraction of the time. Check the datasheet for the maximum pulse current of the display and use transistors to increase it to what is allowable.

Mike.

Displays are Kingbright SA56-11SRWA

They run 4700 to 24000 ucandles so the displays are high intensity. The peak forward current is 155ma for a .1ms pulse width.

Ha anyone information on an optical cover. Should it be a polarized screen?

If the allowable peak current is 155mA and your running them at 20mA then they won't be very bright. Covering them will make them less bright. Just increase the current but make sure the average current is within spec.

Mike.

Using a thin,tinted red plastic sheet should improve the display contrast.
I have used that solution in a few projects and it does make the display more readable.
Display filters are available from component suppliers but are very expensive.

Ideally you need it to be matched to the led colour but you could try getting an offcut sheet of tinted red acrylic sheet from places such as ebay.

Here is an article about the effect:
http://www.visionteksystems.co.uk/co...entfilters.htm

Originally Posted by bobledoux

The display must be readable in sunlight.

I put mine under a hood but it limits the viewing angle.

Three digits in the display.

Contact Texas Instruments and inquire about the opaque filter used in their calculators.

Use segment driver transistors (or IC) as well as column drivers. If 20 ma provides "full brightness" when at 100% duty cycle then you'll need approximately 60 ma "peak" current when running them at 33% duty cycle.

You're dropping too much voltage across the segment current limiting resistors (0.02 amps * 200 ohms = 4.0 volts)... 10 ohm resistors are probably more appropriate... or... eliminate them entirely... You'll still need segment drivers for full 60 ma "peak" current and full brightness...

Good luck... Mike

Thanks for the ideas.

The new display is supposed to replace an LCD display based on a 3 1/2 segment voltmeter. It needs to be read at a distance of about 8 feet. The LCD digit size and contrast is small making for reading difficulty.

The LED's are intended to improve contrast and readability. In normal room light there appeared to be little change in brightness as I reduced steady current flow from 15ma down to 5 ma. But at 15 ma the display washes out in bright sunlight.

The application calls for reading temperature from 0 to 255 degrees F. I have also considered using a 2x8 LCD display and creating custom characters so each 2 across and 2 down characters would be lighted for a single character. This would give me my three characters with vertical pair of LCD characters for each space.

The system is battery operated; currently wired on three AA cells. With the heavy pulsed current draw, when using segment switching transistors, I'd want to include a big cap in the supply line to smooth out current draw voltage spikes.

My solution sequence is as follows:

1. Wire the system up, as it stands, and see if the problem with visibility remains.

2. If there is a problem, add a red filter over the display and see if this solves the problem.

3. If problem remains, create new PCB with high current pulsing of characters and segments.

4. If all else fails, look for a big 2x8 LCD display and make characters that are two rows tall by two characters wide.

Thanks again for the feedback.