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Low voltage LED indicator

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jdp_osullivan

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Hi, This is my first post.
I left Uni 20 years ago (instrumentation and robotics) so the electronic part of my brain is a bit rusty.
Can anyone help with 2 really simple circuits (so simple that I can't do them!)

For an output that reaches 1 v I would like to power up an LED and leave it on until it drops below 1v again.

Secondly for an output that varies from 0 v to 0.7 v then up to 5.0v
I would like an LED to light when it gets to above 0.6v and then shut off above ,say, 1.0v and then a second LED to light when it gets to above 4.8 volts up to 5.5 volts
Both LED's need to turn off if the voltage drops below those numbers.

Any help would be greatly appreciated
 
Doable using comparators. What do you propose to use as a power supply for the LEDs?
 
To ask Alec's question another way, you are not planning to power the leds on the input voltages you quoted, are you?
 
You could do this with comparators with reference levels set respectively at 1V, 0.6V, 1V, 4.8V. I'm not clear how many LEDs you want; 2 or 3?
When you say "up to 5.5V", how does that happen if the power rail is 5V?
 
Hi Alec.
Basically I have a sensor circuit that will deliver an output up to 5.5 v.
This output will be the input for my LED display circuit which has a 5v supply but for ease of design you could assume a 5v max for both circuits.
There are only 2 leds on the second circuit. 1 will turn on and off around the 0.6v - 1.0 v range and the other turns on and off around the 4.8 to say 5.ov range.
Thanks for your input so far. I am looking at comparators online now to see if I can develop a circuit.
 
You will need to find comparators which have rail-to-rail input capability if you have only a 5V rail to supply them.
 
You can get around the rail to rail requirement by attenuating the signal slightly so it stays within the common mode range of the comparator, say a LM339.
 
Good point. The voltage reference levels would require the same attenuation factor.
 
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LoDet.jpg

Here is one way to do it. The colors on the plot correspond to the Led colors noted on the schematic. The X axis is the varying input voltage(s). Some of the circuits are ratiometric with respect to the 5V rail, so it has to be accurate.

You can buy TL431 for about the price of an NPN transistor. Since the Red Led turns on where the Green Led turns off, maybe the circuit could be simplified (unless V(i) comes from a different place).
 

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Mike, I have tried doing that with 431s and found the LEDs do not shut off because there is enough quiescent current in the 431 to keep them lit. A resistor across the LED will cure this.
Personally , I would rather use an IC than all those 431, but that's just my bias.
 
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I'd do this with a PIC 12F675. :)
Prob 15 -30 mins to code it up.:D
But then the OP prob doesn't have PICs or PIC programmers.:eek:
OP, get into microcontrollers as soon as u can!Gotta luv 'em.
 
Hi Mike ML, The simplicity of your first circuit is exactly what I'm looking for (The one that turns the Led on when V(i) is greater than 1v.
What changes would I need to make to this to turn another red led on when V(i) is greater than 4.7 v.
Also do you agree with EinarA on the requirement for a resistor across the LED to get them to shut off. If so what size could I use. Also...any particular spec for leds, there seem to be a million available
thanks again
 
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Hi Mike ML, The simplicity of your first circuit is exactly what I'm looking for (The one that turns the Led on when V(i) is greater than 1v.
What changes would I need to make to this to turn another red led on when V(i) is greater than 4.7 v.
Also do you agree with EinarA on the requirement for a resistor across the LED to get them to shut off. If so what size could I use. Also...any particular spec for leds, there seem to be a million available
thanks again

According to the TL431 data sheet, the off-state quiescent current is 1uA, which should not make the Led visible. If it does, I would put 10K to 47K across the Led. Maybe Einar bought some cheap Chinese Knock-offs. You can use any common indicator Led that is visible with 5 to 20mA with this circuit.

Look at the third circuit (Blue Led). Delete U5, R11 and R13. Now D3 will turn on at 4.8V and stay on. You asked for it to go off at 5.5V, so that was what U5 was for. If you really want turn on at 4.7V, make R10 (118K) slightly smaller.

Think of the TL431 as a switch that turns on (sinks current into Anode) when the voltage from Ref to Cathode exceeds 2.495V. By playing with the ratio of the resistors connected to the Ref pin (think voltage divider), you can move the switch point almost anywhere...

I have used these as a lo-voltage alarm to know when to disconnect a load from a battery to prevent overdischarge, for example... I am using one in my airplane to let me know that my alternator is not charging (Vbat<13.0V).
 
My 431s are not knockoffs but were purchased 30 years ago, when the specs were different? I have to use a 1K or 2.2K, but experiment as needed.
 
Mike's statements made me wonder if I had forgotten how the 431s behaved. My data book says Iq is 500 uA typical, and a test setup confirmed this, for both my old part and a newer one acquired two years ago. I had to look at the link he provided to see where we differ: that off state current is for when Vadj is zero volts, not just below 2.49. If you drive the adjust pin from a pot, you will find that the anode current jumps up above Vref, but as you go below Vref it stays around .5 mA and only drops slowly as the voltage decreases, not reaching zero until you go below 1V. Spice has been lying about this part. (There is a curve on the data sheet that shows this behavior .) Adding the parallel resistor can make it work, but it still doesn't appeal to me.
 
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