Voltage indicator

[Uncle Krunkus]

I'm trying to do a simple voltage indicator. Maybe just three LEDs. I need low parts count, but I can afford an op-amp already on it. (I actually need 3, and the design has 3 spare op-amps waiting)
Do I have to go for comparators?
I've got some LM3914s, but I want to save them for a couple of 5-pulsers. Besides, I don't have room for that many LEDs.
I'd like the mid one to cover +/-1V and the outer two to cover >+1V and <-1V.
Any ideas?

[Tim Servo]

Hey Unk,

You might be able to rig up a chain of op-amps configured as comparators. Each comparator triggers at a slightly higher level, giving you a 'bar graph' effect as the input voltage rises. Here are a couple of examples I found:

http://www.ecelab.com/circuit-simple-led-voltmeter.htm

http://www.elecfree.com/electronic/bargraph-voltage-indicator-by-lm339/

This one only uses a single op-amp
http://www.elecfree.com/circuit/Meter-test-circuit/bargraph-light-meter-by-ic-741/

This one uses transistors, and has one stage enable the next
http://electronicdesign.com/Articles/Index.cfm?AD=1&ArticleID=6246

And a whole bunch of good info / theory on the op-amp type
http://www.opamp-electronics.com/tutorials/the_operational_amplifier_3_08_03.htm

Hope this helps!

Tim (if you put it upside-down, is it a hparg rab?) Servo

[Inventor]

A divider/ladder type structure as shown in the above links is a good start. Also, why not utilize the fact that LEDs have different knee voltages? A red LED is only 1.7V or so, so that could be your +/- 1V indicator (with gain of 1.7), and the other color LEDs would turn on at a higher voltage. That would be more of a parallel approach. In fact, if you don't need buffering then the opamp itself may not be necessary - depends on whether you have a low-impedance source.

something like this:

[BOB-SNARE]

Or something like this...maybe use the opamp to level shift/amplify.

[Uncle Krunkus]

Thanks heaps everyone for all the great ideas and links. I'll keep you all updated on my progress.

[urbanscallywag]

Three ideas:

A microcontroller sampling the voltage, (re)quantizing the analog value and turning on appropriate LEDs (as few/many as you'd like).

A microcontroller sampling the voltage and PWM output to an LED.

An analog voltage controlled PWM circuit.

[Uncle Krunkus]

Hmmm,
I think the microcontroller ideas would be a bit too complicated for what I want, but I'm intrigued by the PWM idea. How do convert that to a 3 LED indicator?

[urbanscallywag]

I was thinking of controlling a single LED with analog PWM like in the microcontroller PWM in the suggestion above it.

A bicolor LED with PWM could be really cool. The color would dictate polarity, the intensity would be the magnitude.

[Inventor]

I tell you what would be trick, would be to use a bilateral current source. This circuit appears in National Semiconductor's Linear Applications Handbook and requires only one opamp and five resistors. You apply an input voltage and the output current is proportional to the input current, magically in *both* positive and negative directions.

Then you could just put side-to-side diodes on the output and the intensity of the diodes would directly track the input voltage, with one diode for positive and the other for negative. That's because, as you surely know, the intensity of an LED is proportional to its current.

I have the page opened up and can give you the info but it's detailed. If you want to use this circuit I'd suggest obtaining the PDF file of AN-29 from www.national.com if you are interested. I can help you find this file if you cannot find it (I think so anyway). Cheers!

[frijitz]

[Inventor]

What does the Howland circuit look like?

[frijitz]