snyderw@pawl1.pawl.rpi.edu (Wilson P. Snyder II) (10/22/88)
Since everyone is on the topic of LEDS - I have a little problem. I need a LED that will display RED, GREEN, and a decent YELLOW. I have experimented with about 10 R-G Tri-Pin LEDS (Three leads) The problem is that none of them form acceptable yellows (the yellow should be like a yellow LED.) If I over drive the green element (IE 100 ma instead of 20ma) I get a nice yellow, but of course the LED self-destructs. I also know about the two pin bicolor LEDS where you pass AC through the device. I would prefer not to have to use this type, but if I must I must. I expect that I will have the same problem getting good yellows though. Oh, yes, I do need the GREEN and RED also, so don't suggest Green/Amber LEDS. ______________________________________________________________________ Wilson P. Snyder II snyderw@pawl.rpi.EDU 15 Davis Parkway 518-276-2764 So. Burlington, VT 05403 802-658-3799 in summer (That's now) ______________________________________________________________________
byron@pyr.gatech.EDU (Byron A Jeff) (10/23/88)
In article <1491@imagine.PAWL.RPI.EDU> snyderw@pawl1.pawl.rpi.edu (Wilson P. Snyder II) writes: >Since everyone is on the topic of LEDS - I have a little problem. > >I need a LED that will display RED, GREEN, and a decent YELLOW. > >I have experimented with about 10 R-G Tri-Pin LEDS (Three leads) >The problem is that none of them form acceptable yellows (the yellow >should be like a yellow LED.) > >If I over drive the green element (IE 100 ma instead of 20ma) I get a >nice yellow, but of course the LED self-destructs. > >I also know about the two pin bicolor LEDS where you pass AC through the >device. I would prefer not to have to use this type, but if I must I >must. I expect that I will have the same problem getting good yellows though. > >Oh, yes, I do need the GREEN and RED also, so don't suggest Green/Amber LEDS. > > > >______________________________________________________________________ >Wilson P. Snyder II snyderw@pawl.rpi.EDU >15 Davis Parkway 518-276-2764 >So. Burlington, VT 05403 802-658-3799 in summer (That's now) >______________________________________________________________________ I ran an experiment with the following circuit: +5 ----------|---------- | | R1 R2 | | |_____RED LED_______| | GREEN LED* | | | |---------- | o | o / \ | / \ / \U1a | / \U1b ------- | ------- | |_________| | | | | ------- | | | 555 | I actually used a frequency generator but | Osc | would use a 555 oscillator in a real circuit. | | ------- R1, R2 - 220 ohm U1 - 7406 open collector inverter * - The LEDS are facing opposite directions. I used 2 discrete LED's because I didn't have a bi-color laying around. This flashes the two LEDs. I figure that by changing the frequency and duty cycle of the 555 and by changing the values of R1 and R2 you can get a good yellow from a bicolor LED. Also you can drive the LEDs above their rated values if you pulse them. Some simple control (like a nand gate) should allow you to get just red and green. If you overdrive them thoughyou'll need to gate with the oscillator. BAJ -- Another random extraction from the mental bit stream of... Byron A. Jeff Georgia Tech, Atlanta GA 30332 Internet: byron@pyr.gatech.edu uucp: ...!gatech!pyr!byron