Lighting a Bicolor LED

[Mike]

I want to have a recharging device built with a transformer and a 5V regulator. It charges a 3.6V 280mA NiCd battery. This entire thing powers a headphone amp. I want to take a bicolor red/green LED to light up green when the amp is on (so between + of the amp chip and gnd) and it to light up red when it is charging (between the regulator out and gnd).<p>The regulator puts out 5V and the battery is rated at 3.6V but when charging the voltage into the chip goes to 5V and the battery goes over 3.6V when fully charged.<p>do i need resistors for the LED? if so, what value?<p>thanks, mike

[Chris Smith]

You always need Resistors when using leds, except for the regulated types like the flashing 12 volt types, and then only if your over 12 volts.<p>Leds cant current limit them selves, and even at the working voltage, they want to draw too much current.

[russlk]

150 ohms should work.

[ThomasHenry]

I had a circuit published back in 1982 that might be of some benefit. It used a tri-colored LED to indicate voltage levels: green for signals less than 5Vpp, orange for 5Vpp to 10Vpp, and red for signals greater than 10Vpp. The LED was driven by a simple pulse-width modulator to create the color effects.<p>Here are the bibliographic details: "Tri-Colored LED Indicates Voltage Peaks," _Electronotes_, Volume 14, Number 134, February 1982, pp. 17-19.

[dacflyer]

is there a link to see this circuit diagram,,,sounds interesting...

[Mike]

thanks!<p>i think that i need one resistor on each positive lead, but can i just use a 150 on the gnd? or since 1 resistor would be taking the place of two, a 300 ohm on the gnd?<p>thanks, mike

[dyarker]

Oh, a 3 lead Red/Green LED!<p>If you are going to use only one color at a time, then one 150 Ohm resistor on cathode would be okay. But the relative brightness of red to green might be different. If, for example, red is brighter than green; then try a 150 Ohm on red's anode, and 120 Ohm on green's anode.<p>If you want amber, red and green on at same time, then you definitely want a resistor on each anode; or amber will probably be dimmer than red or green. If this setup makes amber brighter than red and green; then try 120 Ohm red anode, 100 Ohm on green anode, and 33 Ohm on cathode.<p>I'm suggesting different value resistors for red and green because a red LED has a forward voltage of 1.7V to 2.0V and green 2.1V to 2.5V. The difference may not be needed because the eye is more sensitive to green light than red.

[Adam Y.]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr> It used a tri-colored LED to indicate voltage levels: green for signals less than 5Vpp, orange for 5Vpp to 10Vpp, and red for signals greater than 10Vpp. The LED was driven by a simple pulse-width modulator to create the color effects.<hr></blockquote>
That sounds like what my gameboy uses to show the battery status.

[Mike]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Dale Y:
Oh, a 3 lead Red/Green LED!<hr></blockquote><p>is that a good thing? or did you think it was something else?<p>most likely only one color will be on. Unless the amp is running and charging at the same time. I will just use one on each lead and if needed, the gnd. i may not even use it now since i just got the cases yesterday afternoon, and they are smaller than I thought. I only have 1 1/4 inch wide and 1 1/8 inch deep for the circuitry. i can get it all in, but dont know about an LED.<p>How much will the LED draw from the circuit? I know the reason the friends who want these made want them to turn it as loud as possible, another words, with extreme distortion they dont even notice. that drains the battery quickly. and i am worried that the LED would draw too much power. The battery is rated at 3.6VDC 300mA.<p>thanks, mike

[dyarker]

Not bad not good, just that there are two color LEDs with two leads. They are connected reverse polarity parrallel inside. Current one way shows red, current the other way for green, square AC for amber.<p>20mA is common figure for max continous current.

[ThomasHenry]

I don't believe in touting commercial stuff here, but since you asked, Dacflyer: my tri-color LED project appeared in the newsletter mentioned above. It hasn't been posted on the Web, but was reprinted in a book of mine which is still in print and available, called _Electronic Music Circuits: The Reprints_. A quick Google on the book title will show you how to find it.<p>I used a Schmitt trigger/integrator to drive a comparator based pulse width modulator. 0% duty cycle gives green, 100% gives red, and everything in between varies continuously from yellow to orange. Persistence of vision creates the illusion of continually changing color. Best of all, it's a one chip circuit.

[ThomasHenry]

Oh yeah, I used a three lead type LED, with the reversed back to back elements.