Astable switching LED's colour

[TheAppleMac]

Hello all,

Just a quick question about switching the colours of a bi-colour LED when connected to the output of an astable. The LED itself is a two-pin standard red/blue bi-colour.
I tried the circuit here: https://www.pearltrees.com/#/N-s=1_...lay=1&N-f=1_5967759&N-fa=5753722&N-p=55007533

Looks like this was for single output colour changing (i.e. more yellow or less red etc rather than colour shifting)

With resistor values of the pot divider at 260 ohms. When the 555 output is high the first colour, red, comes on. But when the 555 is low, I don't think the 555 sinks the current from the pot divider, but rather the current just goes straight through the divider, this seems fairly obvious now. I tried larger 10k resistors, they didn't work either.

I was wondering if I could use a NPN transistor astable, perhaps bridge the two collectors together?
Is there any circuit I could use?

Thanks for your time,

TAM

EDIT: would connecting a NOT gate work like so? :

 

[alec_t]

Try this variant of the 555 circuit you posted:

R1 and R2 set the currents for the two LEDs, and R3 enables the mark/space ratio of the 555 to be adjusted around the 50% point for adjusting the relative 'on' times of the LEDs. R5 and C2 determine the 555 period. R4 should be just less than 1/3 the value of R5.

 

[TheAppleMac]

Hi alec, thanks for the reply.

I've tried the schematic, but I'm not sure. I don't know how this circuit could be converted to one with a bi-colour LED without making it the same as the first.
Sorry for bad news

 

[TheAppleMac]

Oh, another thing to note is that all bi colour LED circuits that I have seen use the same pot divider - so I am not sure, because in my circuit, it just seems as though the resistors in series just bypass the LED altogether.

EDIT:

After trying pot divider resistances on breadboard, I realised that the blue half of the bi-colour has a high resistance. This is basically my circuit (the LED with resistor symbolises the blue half & please bear in mind that I can't put another resistor across the red half as it a bi-colour)



This is what is causing problems, not sure how to fix it?

 

[alec_t]

Is your bi-color LED a 2-leg one or a 3-leg one?

 

[TheAppleMac]

two. I admit, three would make it much easier.

 

[alec_t]

Ok. Here's how you could do it with a 2-leg diode. It's an elaboration of the mod you added in post #1.

BTW, the blue LED has a greater forward voltage (~3V) than the red (~2V), which is why it seems to have a higher resistance and is why R1 and R2 have different values in my schematic to try and balance the currents for the two LEDs.

 

[Mr RB]

A normal 555 does not reach the supply voltages on it's output, it's likely you are not getting enough volts differential to run the LED.

You can change to a 12v DC supply, and adjust the two resistor values and it will work fine with your 2 pin LED.

 

[TheAppleMac]

Thanks guys, I eventually mixed the two ideas, so I used a transistor NOT from the astable to one pin and the direct astable output to the other pin. Now, one can sink the other's current to light each LED.
Mr RB, you were right, a simple resistor worked with the transistor.

Sorry for the late reply.

 

[audioguru]

The circuit does not "shift" the colors and since the 555 switches high and low then the colors do not fade.
If the 555 runs slowly then the LED is red, blue, red, blue over and over.
If the 555 runs fast then the red-blue LED appears purple.