| Author |
Message |
Guest
|
 Posted:
Fri Nov 11, 2005 5:35 pm Post subject:
Simple LED circuit.. HELP! |
|
|
I am having problems with this one, and it shouldnt be that hard...
I have a 9v battery, and I want an LED to be on above 6.5 volts and off
below 6.5. I don't want it to dim, I want it to be bright at 6.6 and
off at 6.4...
How do I do this?!
|
|
| Back to top |
|
 |
CWatters
Guest
|
| Posted:
Fri Nov 11, 2005 5:35 pm Post subject:
Re: Simple LED circuit.. HELP! |
|
|
<gordonjs@gmail.com> wrote in message
news:1131718251.805949.315030@g47g2000cwa.googlegroups.com...
| Quote: | I am having problems with this one, and it shouldnt be that hard...
I have a 9v battery, and I want an LED to be on above 6.5 volts and off
below 6.5. I don't want it to dim, I want it to be bright at 6.6 and
off at 6.4...
How do I do this?!
|
For that level of accuracy you need something like a voltage comparator. |
|
| Back to top |
|
|
Don Taylor
Guest
|
| Posted:
Fri Nov 11, 2005 5:35 pm Post subject:
Re: Simple LED circuit.. HELP! |
|
|
gordonjs@gmail.com writes:
| Quote: | I am having problems with this one, and it shouldnt be that hard...
I have a 9v battery, and I want an LED to be on above 6.5 volts and off
below 6.5. I don't want it to dim, I want it to be bright at 6.6 and
off at 6.4...
How do I do this?!
|
One way might be to include an op-amp and a few resistors to your
circuit.
If you google for
circuit op amp hystresis
you can find some examples. The idea is that using a few resistors
you can make the op amp output switch from low to high at a fairly
specific voltage. You would probably use two resistors to create a
voltage divider to make your 6.5 volts. Then you would use two more
resistors to create the feedback needed to get the op amp to turn on
and off at that voltage. Then you would use the output of your op
amp to drive your LED and resistor.
|
|
| Back to top |
|
|
Charles Schuler
Guest
|
| Posted:
Sat Nov 12, 2005 1:35 am Post subject:
Re: Simple LED circuit.. HELP! |
|
|
<gordonjs@gmail.com> wrote in message
news:1131718251.805949.315030@g47g2000cwa.googlegroups.com...
| Quote: | I am having problems with this one, and it shouldnt be that hard...
I have a 9v battery, and I want an LED to be on above 6.5 volts and off
below 6.5. I don't want it to dim, I want it to be bright at 6.6 and
off at 6.4...
|
Google for "window comparator." |
|
| Back to top |
|
|
DaveM
Guest
|
| Posted:
Sat Nov 12, 2005 1:35 am Post subject:
Re: Simple LED circuit.. HELP! |
|
|
<gordonjs@gmail.com> wrote in message
news:1131718251.805949.315030@g47g2000cwa.googlegroups.com...
| Quote: | I am having problems with this one, and it shouldnt be that hard...
I have a 9v battery, and I want an LED to be on above 6.5 volts and off
below 6.5. I don't want it to dim, I want it to be bright at 6.6 and
off at 6.4...
How do I do this?!
|
View this with a fixed pitch font such as Courier
+9V
|
+--------------------------------+
| |
| ------
| \ / LED
| \/
| -----
| |
| |
| \
| / 620
| \
| /
| |
| 10K |\ |
+--/\/\/\/\-----+-----| \ |
| | - | \ ______|
\ | | /
1.5 / +-------------| / LM339 or similar comparator
K \ | | + |/
/ | |
| | \
+-------+ / 27K
| \
-----/ /
/ ^ |
/ \ 4.7V |
--- 400 mw |
| |
| |
+---------------+
|
-----
---
-
Resistors are 1/4W 5%
If you want to adjust to exactly 6.5 V for switching, replace the 10K
resistor with a 7.5K fixed resistor in series with a 5K trim pot. Caveat...
this circuit does not provide hysteresis, so if the source is a bit noisy or
fluctuates around the switching point, the LED may flicker on/off. But
this certainly meets your original spec.
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in
the address)
Never take a laxative and a sleeping pill at the same time!! |
|
| Back to top |
|
|
Anthony Fremont
Guest
|
| Posted:
Sat Nov 12, 2005 1:35 am Post subject:
Re: Simple LED circuit.. HELP! |
|
|
<gordonjs@gmail.com> wrote in message
news:1131718251.805949.315030@g47g2000cwa.googlegroups.com...
| Quote: | I am having problems with this one, and it shouldnt be that hard...
I have a 9v battery, and I want an LED to be on above 6.5 volts and
off
below 6.5. I don't want it to dim, I want it to be bright at 6.6 and
off at 6.4...
How do I do this?!
|
Are you talking about the 9V battery that powers the circuit itself
discharging to 6.5V and the LED indicating the charge level of that
battery, or are you talking about a separate voltage input? Either way,
I'd probably use a comparator (see LM393), but it's a little more
complicated to measure your own supply voltage reliably. |
|
| Back to top |
|
|
Paul Perry
Joined: 12 Nov 2005
Posts: 4
Location: Melbourne Australia
|
| Posted:
Sat Nov 12, 2005 1:02 pm Post subject:
re:Simple LED circuit.. HELP! |
|
|
If you want a sharp on/off for the diode, I think a comparator is needed (or just a cheap op amp wired as one).
I'd use a resistor and a 3.0v zener to give 3.0v, and put a 35K and 30K resistor in series from + to ground, to give 3.0v when the battery is at 6.5v.
Then run the 3v reference from the zener & the voltage from the divider to the appropriate inputs of he op amp. And then the output of the op amp drives the led, (via a suitable resistor) if you got the inputs the right way about. And, as a bonus.. put another diode to the + rail, to tell when the battery is flat. (maybe a red/green diode, if the pinout is right). |
|
| Back to top |
|
|
Guest
|
| Posted:
Mon Nov 14, 2005 5:35 pm Post subject:
Re: Simple LED circuit.. HELP! |
|
|
Alright, thanks for all the info guys! I will look into all your
solutions this morning. For the record, I am measuring my own supply
voltage. |
|
| Back to top |
|
|
|
|
|
|
FAQ
Memberlist
Register
Profile
Log in to check your private messages
Log in
