Circuit required

[ivenner]

Hi

I have been out of electronic design for 15 years now and therefore I
have not kept up to date with current component availability. However
the company I now work for requires a very simple circuit that has 2
LEDs on it - green and red. If the input voltage is < a predefined
level then the red LED lights, if above then the green one. the
circuit needs to be as low cost as possible and very low power.

Any ideas?

Cheers

Ian

 

[Rich Grise]

Hi

I have been out of electronic design for 15 years now and therefore I
have not kept up to date with current component availability. However
the company I now work for requires a very simple circuit that has 2
LEDs on it - green and red. If the input voltage is < a predefined
level then the red LED lights, if above then the green one. the
circuit needs to be as low cost as possible and very low power.

Any ideas?

Go to the other side of google and enter "comparator" into the search
box.

Good Luck!
Rich

 

[Luhan]

Hi

I have been out of electronic design for 15 years now and therefore I
have not kept up to date with current component availability. However
the company I now work for requires a very simple circuit that has 2
LEDs on it - green and red. If the input voltage is < a predefined
level then the red LED lights, if above then the green one. the
circuit needs to be as low cost as possible and very low power.

Any ideas?

Yep. One dual comparator and a few resistors. Somebody with that
ascii drawing program wanna post it here?

Luhan

 

[Fred Bartoli]

Yep. One dual comparator and a few resistors. Somebody with that
ascii drawing program wanna post it here?

Probably not. But with one comparator maybe...

 

[ivenner]

HI

Thanks for the really quick feedback. Not sure if a comparator is
neccessarily the right way. The input voltage is also teh supply
voltage. Think of this analogy, a guy on a bike has a dynamo attached.
As he padals faster the voltage output increases. What I need is that
th LED glows if the output is below a certain level (obviously greater
than the forward voltage of the LED of course!), but then illuminates
the green one above the threshold voltage.

I hope this makes more sense.

Cheers

Ian

 

[Jim Thompson]

HI

Thanks for the really quick feedback. Not sure if a comparator is
neccessarily the right way. The input voltage is also teh supply
voltage. Think of this analogy, a guy on a bike has a dynamo attached.
As he padals faster the voltage output increases. What I need is that
th LED glows if the output is below a certain level (obviously greater
than the forward voltage of the LED of course!), but then illuminates
the green one above the threshold voltage.

I hope this makes more sense.

Cheers

Ian

At WHAT voltage does the green LED light up?

...Jim Thompson

 

[John Fields]

Hi

I have been out of electronic design for 15 years now and therefore I
have not kept up to date with current component availability. However
the company I now work for requires a very simple circuit that has 2
LEDs on it - green and red. If the input voltage is < a predefined
level then the red LED lights, if above then the green one. the
circuit needs to be as low cost as possible and very low power.

Any ideas?

---
You haven't mentioned anything about the supply voltage, or input
voltages, or how low you need the power consumption to be, but
here's a general scheme that should work once the resistor values
are decided on. The LEDs should be high-efficency devices like
HLMP4700's so you can use something cheap like an LM393 for the
comparator. View in Courier:


..
..Vcc---+-------+--------+---------+
.. | | | |
.. | | [R] |
.. | | |A |
.. [R] | [LED] |
.. | | | |
.. | +--|---[R]--+ |
.. | | | | [R]
..Vin>-------+-|+\ | |A
.. | | >------+ [LED]
.. +----+-|-/ | |
.. | | | +---|-\ |
.. [R] | | | >--+
.. | +--|------------|+/
.. | |
..GND>--+-------+
..

 

[Tim Wescott]

HI

Thanks for the really quick feedback. Not sure if a comparator is
neccessarily the right way. The input voltage is also teh supply
voltage. Think of this analogy, a guy on a bike has a dynamo attached.
As he padals faster the voltage output increases. What I need is that
th LED glows if the output is below a certain level (obviously greater
than the forward voltage of the LED of course!), but then illuminates
the green one above the threshold voltage.

I hope this makes more sense.

Cheers

Ian

That's still a comparator thing -- split the power supply with a voltage
divider, compare it to a reference, light the lights. An LM339 is about
as cheap as they come and has enough oomph to drive an LED directly.
You can use one side for the 'real' compare and the other just to drive
the second LED.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

 

[John Fields]

HI

Thanks for the really quick feedback. Not sure if a comparator is
neccessarily the right way. The input voltage is also teh supply
voltage. Think of this analogy, a guy on a bike has a dynamo attached.
As he padals faster the voltage output increases. What I need is that
th LED glows if the output is below a certain level (obviously greater
than the forward voltage of the LED of course!), but then illuminates
the green one above the threshold voltage.

I hope this makes more sense.

---
It does, but it would make even more sense if you'd post what the
supply voltage limits are, at what voltage you want the LED
changeover to occur, and the power input requirements for the
circuit. Also, a description of the application wouldn't hurt.

I already posted a circuit based on your earlier description, but it
needs a reference if your supply is going to vary, so I'll fix it as
soon as you post back with what you _really_ need.

 

[ehsjr]

Hi

I have been out of electronic design for 15 years now and therefore I
have not kept up to date with current component availability. However
the company I now work for requires a very simple circuit that has 2
LEDs on it - green and red. If the input voltage is < a predefined
level then the red LED lights, if above then the green one. the
circuit needs to be as low cost as possible and very low power.

Any ideas?

Cheers

Ian

Use an LM324 as a comparator. See page 13 of
http://cache.national.com/ds/LM/LM124.pdf

On the output, put 1 Led/resistor to + and the
other led/resistor to gnd. Use a pot as a
voltage divider (outside pins to + and -)
and adjust so the the center pin produces Vref.

Ed

 

[ivenner]

John

Thanks for this. The supply will vary from 3v to 12v, with changeover
at 9v. It is for a solar application, to give a visual indication of
when a threshold voltage has been reached. Also ideally the output
voltage should be fixed at 9v once it is reached.

Cheers

Ian

 

[John Fields]

John

Thanks for this. The supply will vary from 3v to 12v, with changeover
at 9v. It is for a solar application, to give a visual indication of
when a threshold voltage has been reached. Also ideally the output
voltage should be fixed at 9v once it is reached.

---
You keep shifting the goalposts. First you want a level detector,
then you want a power supply threshold detector operating from the
same supply which threshold is being detected, and now you want all
of the above _plus_ an output voltage regulator for the supply, so
we need to know what maximum output current the regulator needs to
support, but you haven't supplied that data.

What next?

Why don't you take the surprises out of it and tell us everything
you want all at once?

 

[Jim Thompson]

---
You keep shifting the goalposts. First you want a level detector,
then you want a power supply threshold detector operating from the
same supply which threshold is being detected, and now you want all
of the above _plus_ an output voltage regulator for the supply, so
we need to know what maximum output current the regulator needs to
support, but you haven't supplied that data.

What next?

Why don't you take the surprises out of it and tell us everything
you want all at once?

Ian is only acting like a manager ;-)

...Jim Thompson

 

[Simon Scott]

Ian is only acting like a manager ;-)

...Jim Thompson

Yes, wait for him to tell you that it needs to be zero cost, he needs a
thousand of them, and he needs them tomorrow.

 

[Fred Bloggs]

---
You keep shifting the goalposts. First you want a level detector,
then you want a power supply threshold detector operating from the
same supply which threshold is being detected, and now you want all
of the above _plus_ an output voltage regulator for the supply, so
we need to know what maximum output current the regulator needs to
support, but you haven't supplied that data.

What next?

Why don't you take the surprises out of it and tell us everything
you want all at once?

The idiot is going to come back and say the 74121 did the job for him.

 

[ehsjr]

---
You keep shifting the goalposts. First you want a level detector,
then you want a power supply threshold detector operating from the
same supply which threshold is being detected, and now you want all
of the above _plus_ an output voltage regulator for the supply, so
we need to know what maximum output current the regulator needs to
support, but you haven't supplied that data.

What next?

Why don't you take the surprises out of it and tell us everything
you want all at once?

Yabut - if you design the thing for him, you get to change your
sig from Frustrated Professional Circuit Designer to
Professional Circuit Designer / Mind Reader Extrodinaire

Ed