[SOLVED] How to detect if an LED is illuminated?

[abuhafss]

Hi

I am working on a project which is actually an enhancement of this circuit:

https://www.redcircuits.com/Page83.htm

This circuit shows the result with different combination of LEDs and we have to manipulate the actual results from the LEDs.

I am working to somehow sense which LED is lit and then feed into my circuit and show the results as per attached. I have almost completed my design however, I am stuck and could not figure out how to fetch signal for each LED of the three pairs.

Shall be grateful if anybody can help me. Of course, these LEDs would not be required and if required they may be replaced with some suitable devices. I need to have HIGH signal when the LED is ON and LOW when it is OFF.

Thanks in advance.

 

[Audioguru]

You could fasten a Light Dependent Resistor to each LED.
Or you could replace the LEDs with opto-isolators that have transistor outputs.

 

[abuhafss]

Thanks for your response.

I have been thinking of LDR set-up with each LED but I wanted to replace the LEDs.
Yes, phototransistors would be the best option!

Thank you very much.

 

[abuhafss]

I tried to replace the LEDs with opto couplers but, could not get the results.

I connected each collector to the power supply (9V) and emitter to ground thru a pull down resistor 560R to 1k.

My objective is to have a positive output (5-9V) against an illuminated LED and (0V) against an off LED or vice versa.

Can you please guide me how should I make the connections? Kindly keep in mind the illuminated LEDs are actually blinking at high frequency around 350Hz.

 

[godfreyl]

You need to use a bigger resistance - maybe 100K

 

[abuhafss]

There is no improvement.

All the outputs are giving 9V whereas, in present connection of the transistor, I expect the output for RED-J2 to be near zero volt.

 

[godfreyl]

I expect the output for RED-J2 to be near zero volt.

How do you figure that out?
Q1 is a PNP transistor. When the output of U2 is high, neither of it's junctions are forward biased, so no current should flow through the base (or are you hoping for avalanche breakdown of the BE junction?)

OTOH, when the output of U2 is low, the J2 LED is reverse-biased so no current will flow either.

Have you got this circuit working with ordinary LEDs so you can see which are on and which are off?

 

[FvM]

There is no improvement.

You are talking about simulation results. Did you verify that the logic ICs are supplied at all and that the oscillator is operating?

 

[abuhafss]

I simply converted the schematic to LTSpice. As per description given at https://www.redcircuits.com/Page83.htm when a PNP transistor is connected, the GREEN (right side) LED should illuminate for the Base pin, and both LEDs for Collector and Emitter should illuminate.

The simulation did not worked with NAND model for CD4011 (5V) however, it worked with my own NAND model. And the same is working with opto couplers, I can get 0V for an illuminated LED and 9V for an off LED.