# LED voltage bar graph

Discussion in 'Electronic Basics' started by [email protected], Mar 24, 2008.

1. ### Guest

I'd like to have a "bar graph" of LEDs tell me how far along my 6V SLA
is on its charging.

I saw this, but I think it's overkill:
http://www.national.com/mpf/LM/LM3916.html
http://dan.pfeiffer.net/train/vmeter.htm

5 LEDs would be plenty.

Any suggestions for a discrete version of the LED voltage bar graph?
I'd like to tune it from 6.0V (first LED) to 6.8V (fifth LED) in
increments of 0.2V.

It's not particularly important whether all preceding LEDs are lit, or
if just the Nth LED is lit, if this simplifies the circuit at all.
(For example, if voltage is 6.6V, corresponding to the 4th LED, it's
ok if only the 4th LED is lit, and not LEDs 1 through 4, if this makes
design easier.)

Thanks,

Michael

2. ### Bob MonsenGuest

If you have a voltage reference, you can easily scale voltages so that a
couple of quad comparators, along with 100k trimmers can be used to do
something like this.

Each stage is simple, it's an open collector comparator with the - input at
the range, the + input at the battery voltage, and the output connected to a
voltage source through a resistor and LED.

Vbatt 12V
| |\| |
6.8V .-------)------|-\ ___ LED |
| | | >--|___|-->|-----o
.-. o------|+/ 1k |
10k | | | |/| |
| | | |
'-' | |
| | |\| |
o-------)------|-\ ___ LED |
| | | >--|___|-->|-----o
.-. o------|+/ 1k |
10k | | | |/| |
| | | |
'-' | |
| | |\| |
o-------)------|-\ ___ LED |
| | | >--|___|-->|-----)
.-. o------|+/ 1k |
10k | | | |/| |
| | | |
'-' | |
| | |\| |
o-------)------|-\ ___ LED |
| | | >--|___|-->|-----o
.-. o------|+/ 1k |
10k | | | |/| |
| | | |
'-' | |
| | |\| |
6.0V '-------)------|-\ ___ LED |
| | >--|___|-->|-----'
'------|+/ 1k
|/|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

You'll need to figure out how to get 6V, 6.8V, and 12V. You can use LM324s
instead of comparators, and use the extra three opamps to buffer voltage
dividers. If you have a 12V reference, like an LM7812, that would make this
easier.

Regards,
Bob Monsen

3. ### Phil AllisonGuest

** So YOU still refuse to use the proper charge method.

Constant voltage @ 13.8 volts.

Then the current drops over the charge cycle.

........ Phil

4. ### Guest

13.8 volts into a 6V SLA... ha ha ha. You're a hoot.

Actually I set my LM317T to 6.75V, and I just want to know how long it
will take for the battery to float up to that.

Michael

5. ### EeyoreGuest

Obviously 6.9V for a 6V battery.

Phil is right that a simple voltmeter is not a good indicator of the level of
charge, not least because the terminal voltage will rise whilst being charged
by varying degrees.

Graham

6. ### Guest

Yeah I know, I was just giving him a hard time. ;-)

Good point. I did notice that the battery would go up from its
original voltage to the charge voltage within a couple of minutes.

So if I've got a 10 A-h battery, and it charges at float at 500 mA, it
should charge for 20 hours at float to receive a full charge?

Michael

7. ### EeyoreGuest

More like 30 hours actually since the process isn't 100% efficient. Also, the
charge rate will taper off with time.

Graham

8. ### EeyoreGuest

I completely concur.

In this instance the OP could perhaps organise a suitable current limit around his
regulator.

Graham

9. ### Guest

Thanks a bunch.

6.75V not etched in stone... 6.75 to 6.90 seems more reasonable.

Michael