# Switch LED on negative input

Discussion in 'The Projects Forum' started by Man_in_UK, Feb 16, 2015.

1. ### Man_in_UK Thread Starter Senior Member

May 13, 2008
132
0
First let me say sorry for not being clever enough to have any confidence in this circuit.

I want to use a 3.3v logic input to trigger an LED while it is at logic 0. Is this the right way to do it?

http://i195.photobucket.com/albums/z278/man_in_uk/neg sw_zpsazw39dzf.jpg

R1 = 3.3k giving 1ma across opto output
R2 = 100 to 200 ohm
R3 = 5k to 10k

I will not be using a MX6 LED but it was the first symbol I found.

2. ### MikeML AAC Fanatic!

Oct 2, 2009
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The LED is backwards

Must you use the opto-isolator? or can the LED be powered from the 3.3V power supply that is driving the the upsteam logic that drives the input LED on the opto-isolator?

How much current can the upsteam logic source and sink at 3.3V?

3. ### #12 Expert

Nov 30, 2010
16,682
7,322
Simpler...
The questionable resistor depends on what color of LED you are using.

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4. ### MikeML AAC Fanatic!

Oct 2, 2009
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Many LEDs are sufficiently bright to be used as a visual indicator at 5mA.

Last edited: Feb 16, 2015
5. ### Man_in_UK Thread Starter Senior Member

May 13, 2008
132
0
Yes, I always draw LEDs the wrong way, but not intentionally.

The opto has to be there. It is triggered from a nasty AC source and its output is connected directly to a logic input of a processor. I had first put an LED directly on the logic line to ground via a resistor but the pull up resistor was not allowing enough current. I had thought about lowering the value of the pull up resistor but I do not want to put more strain on the opto.

The upstream source is an input pin on a Arduino. The opto does all the sink and R1 does all the pulling up.

6. ### Man_in_UK Thread Starter Senior Member

May 13, 2008
132
0
I think that 100 ohm will give around 5ma on the LED. It only has to be a visual indicator of the input state for fault diagnosing.

I am more worried about getting R3 correct but I realise that is going to depend on the PNP that I chose!

7. ### MikeML AAC Fanatic!

Oct 2, 2009
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Is not the Arduino running on 5V?

8. ### Man_in_UK Thread Starter Senior Member

May 13, 2008
132
0
There is 5 v on the PCB but the ARM7 chip does not use any, just 3.3

Jun 22, 2012
5,150
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Make R3 1k,

10. ### #12 Expert

Nov 30, 2010
16,682
7,322
Two reasons why R3 seems difficult:
1) You aren't paying attention to what anybody here tells you.
2) You put your LED on the emitter side of the transistor.

11. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
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Scratch the earlier circuit I posted and replace it with this:

The opto-coupler only needs to sink 700uA. Since its CTR is ~ 100%, that makes it pretty foolproof.

Last edited: Feb 16, 2015
12. ### Man_in_UK Thread Starter Senior Member

May 13, 2008
132
0
Not intentionally.
I did not fully understand your first diagram, I focused on the 82? and thought that it was not answering my problem. Finding the right current to drive the LED is not my problem but finding the right way to switch it .....is.
After thinking about it, your circuit is exactly the same as mine, only fixed and working correctly.

Cheers

13. ### Man_in_UK Thread Starter Senior Member

May 13, 2008
132
0
Thanks for such an in depth answer. Can I ask a few questions ?
I have 'Everycircuit' simulator (crap but free) and if I switch the LED in the same way as your circuit, it does not work. Even without a resistor feeding the transistor gate, I lose current across the transistor.
Is this a limitation of my rubbish circuit sim?
Is it OK to not have a current limit resistor on the gate?

Cheers

14. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
I'm not sure what you mean by that. Note that in this forward biased PNP transistor, the current into the emitter (same as the LED current) is ~= to the current outof the collector, which flows to ground. The current out of the base is the collector current (~5mA)/Hfe, so is only about 50uA. The current through the 4.7K pull-up is much higher than that: ~600uA.