Hi,

I am trying to get this circuit to work better in terms of LED brightness and ensuring either LED is not lit when the other is. The LEDs are 5mm red (1.8 - 2.2V, 20mA) and blue (3.2- 3.8V, 20mA) LEDs as shown.

The 6.25V supply is constant, i.e. not switchable, but the 8V supply has a momentary switch, as shown. In theory the red LED should extinguish while there is power to the blue LED on the 8V rail, but in reality it glows a little. Furthermore, in order to limit the glow of the red LED I had to increase the value of R1 to 270R from 220R, but of course this limits the current supplied to the blue LED so it gets about 16mA rather than nearer 20mA. This also means the red LED is dimmer receiving only about 10mA. I am guess I may need a transistor to switch the power between the LEDs but asking for any suggestions, including adding transistor(s).



Thanks

 

8V - 3.5Vf = 4.5V
4.5V ÷ 270Ω = 16.7mA (16.666•••mA)

6.25V - 2Vf = 4.25V
4.25V ÷ 270Ω = 15.7mA (15.741mA)

Using both average forward voltages for each LED (Vf), the numbers work out to a difference of 1mA. Given the chances that the Vf of each LED, there's a possibility that the red LED is still seeing some current. Even that 1mA can be enough to cause the red LED to glow very faint. I've seen LED's light with as little as 5mA. Not saying below that is not possible, that's just been the sum total of my experiences, I've lit LED's with that low a current.

Is this circuit just an exercise? Or are you attempting to accomplish something particular? Tell us what your goal is and we can help.

 

hi 615,
Welcome to AAC.
Try adding another Red LED in series wit the RED.
E

 

hi 615,
LTSim plot.
E

Update:
You could use a couple or so of general purpose diodes instead of the extra RED LED.

 

Single 6.25V supply
Two resistors 270Ω
Blue LED Vf 3.5
Red LED Vf 2.0
Momentary switch

Push the momentary switch and the blue will extinguish and the red will illuminate.

My approach:

 

Hi, Thanks for replying to my post.

Yes, this is for a backlight on a transceiver analogue meter to indicate when the radio is in receive and transmit modes. In addition to ensuring LEDs are either fully on or off, I would like them at their maximum brightness (~20mA, without overdriving them) for maximum visibility of the meter scales.

 

Single resistor approach: Does the same thing.

 

If it's used in a transceiver shouldn't there be a voltage you can used that is OFF in the transmit mode?
Assuming the RED is for receive and the BLUE for transmit?

 

Thanks Tony. The 8V rail switch forms part of the PTT switch so I don't really want to have to add another switch as this would be a rather more complicated approach for the radio.

Thanks for your suggestion Eric. I was hoping to somehow gain some current through both LEDs to bring them nearer 20mA as well as dealing with the unwanted glow from the red LED.

 

Hi, Thanks for replying to my post.

Yes, this is for a backlight on a transceiver analogue meter to indicate when the radio is in receive and transmit modes. In addition to ensuring LEDs are either fully on or off, I would like them at their maximum brightness (~20mA, without overdriving them) for maximum visibility of the meter scales.

Reduce the size of the resistor. 20mA (RED LED) will require a resistance of (6.25 - 2) ÷ 0.02 = 212.5Ω. Use a 200Ω resistor and you'll have 21.25mA. The blue LED will be running a current of 13.75mA. But the blue LED is likely going to LOOK brighter than the red LED. If you drop the resistance even further, I wouldn't exceed 30mA on the red LED. Since it sounds like you'll be receiving longer periods of time than you would be transmitting, running the red LED occasionally at 30mA isn't going to shorten its life significantly. 30mA (red) requires 142Ω. At 142Ω the blue will see 19mA. Close to 20mA. The closest resistors to that value would be 150Ω. So red would be 28mA and blue would be 18mA.

 

If it's used in a transceiver shouldn't there be a voltage you can used that is OFF in the transmit mode?
Assuming the RED is for receive and the BLUE for transmit?

Yes, the 8V rail switches the blue LED off through having the PTT depressed allowing the red LED to light. What I would like is to increase the current through both LEDs while maintaining the binary state of light between the two LEDs.

 

Thanks Tony. The 8V rail switch forms part of the PTT switch so I don't really want to have to add another switch as this would be a rather more complicated approach for the radio.

Then I wouldn't worry about the red being ever so slightly illuminated. The red and blue mixed together will give a purple(ish) color. But you probably won't even notice it at all.

 

Yes, the 8V rail switches the blue LED off through having the PTT depressed allowing the red LED to light.

Your schematic shows the Blue LED on when the 8 volts is connected

 

The circuit in post #5 will not work as described.

 

Your schematic shows the Blue LED on when the 8 volts is connected

Sorry, yes, you are correct. I had to make up my own symbol for a momentary switch as I could not find one in LTSpice and forgot to show continuity rather than o/c.

 

The circuit in post #5 will not work as described.

Teach me. Why not? And what of post #7?

 

Reduce the size of the resistor. 20mA (RED LED) will require a resistance of (6.25 - 2) ÷ 0.02 = 212.5Ω. Use a 200Ω resistor and you'll have 21.25mA. The blue LED will be running a current of 13.75mA. But the blue LED is likely going to LOOK brighter than the red LED. If you drop the resistance even further, I wouldn't exceed 30mA on the red LED. Since it sounds like you'll be receiving longer periods of time than you would be transmitting, running the red LED occasionally at 30mA isn't going to shorten its life significantly. 30mA (red) requires 142Ω. At 142Ω the blue will see 19mA. Close to 20mA. The closest resistors to that value would be 150Ω. So red would be 28mA and blue would be 18mA.

Thank you, Tony. If you mean in my circuit, I already tried with a 220R resistor and although the blue LED meets ~20mA while on in Rx mode the red LED is even brighter as more current is allowed through it. Sorry, I may have caused confusion with my momentary switch showing o/c rather than continuity, as sghioto pointed out. It's true that the red LED is not hugely affecting the overall colour of the blue in this condition but it looked a bit cowboy to me if I left it that way and someone else came across this mod, perhaps with more experience.

 

asking for any suggestions, including adding transistor(s).

EDIT schematic using PNP transistor.

 

View attachment 239215

Nice solution.

 

it looked a bit cowboy to me

What's wrong with cowboys?