Hi All
I am in the process of designing a comparator circuit I want the output of this comparator to then go into a simple BJT latch which keeps an LED on (i.e. if an error occurs, the LED remains on). I have attached screen shots of the simulations I carried out the issue I am having is that I have been able to get the latch work on it's on and the same for the comparator however when connecting the two up I am having an issue the latch functionality ceases to exist I am quite puzzled as to how I can fix it and any assistance would be greatly appreciated.

Kind Regards

Art

 

Hi All
I am in the process of designing a comparator circuit I want the output of this comparator to then go into a simple BJT latch which keeps an LED on (i.e. if an error occurs, the LED remains on). I have attached screen shots of the simulations I carried out the issue I am having is that I have been able to get the latch work on it's on and the same for the comparator however when connecting the two up I am having an issue the latch functionality ceases to exist I am quite puzzled as to how I can fix it and any assistance would be greatly appreciated.

Kind Regards

Art

Use SCR. CD106M and a 2N2222A. 2N2222A will be used to turn off the SCR.

 

hi 547,
If you check Q1 and Q2 Bases you will see they are biassed so that they are never switched Off.
When used in isolation, the latch, I guess your Vsource for the test pulse has zero ohms output resistance.???
E

 

Use SCR. CD106M and a 2N2222A. 2N2222A will be used to turn off the SCR.

The OP is asking why his circuit is not working, he is NOT asking for a different circuit.

 

used in isolation, the latch, I guess your Vsource for the test pulse has zero ohms output resistance.???

Thank you so much I really appreciate the explanation, can you suggest whether this circuit could be improved or what I could use for the same effect?

Kind Regards

Art

hi 547,
If you check Q1 and Q2 Bases you will see they are biassed so that they are never switched Off.
When used in isolation, the latch, I guess your Vsource for the test pulse has zero ohms output resistance.???
E

 

Sorry I mean can I ammed

Thank you so much I really appreciate the explanation, can you suggest whether this circuit could be improved or what I could use for the same effect?

Kind Regards

Art

the circuit so it behaves in the way I intent or can you recommend any other typologies that I could use?

 

hi 547,
This is one option.
E

 

hi 547,
This is one option.
E

Hi thank you, is there a way for the LED to have zero current flowing through it when there is no signal from the comparator I think atm the LED I am thinking of using the following:
http://uk.farnell.com/kingbright/kp...ategoryId=&searchView=table&iscrfnonsku=false
So I think with the simulation when the pulse is zero at the start it comes out to 13mA flowing so the LED will be slightly ON

 

hi 547,
This is one option.
E

If I change R4 to say 10Ohms would that be suitable or would that much current damage the BJT's?

 

If I change R4 to say 10Ohms would that be suitable or would that much current damage the BJT's?

I can to the following BJT's if that works out better: BC817

 

I can to the following BJT's if that works out better: BC817

*BC817-40

 

hi,
To isolate your LED driver I would add a final transistor, you can drop that intermediate transistor.
E

 

hi,
To isolate your LED driver I would add a final transistor, you can drop that intermediate transistor.
E

Brilliant thank you so much, really appreciate the assistance learning a lot from creating designs and your input is really helping out with the learning process