Use LEDs to indicate voltage

Question

I want to design a circuit that has 3 LEDs connected to it.

When the voltage of the power source connected to the circuit is...

3.6 volts I want only the first LED to light.

7.2 volts I want the second LED to light. The first LED can light also.

14.4 volts I want the third LED to light. The first and second LED can light also.

Is this kind of thing possible? and how do I make a start?

Thanks,

Karl

EDIT: sorry guys, the dashes were confusing. I'm trying to measure positive voltage. What I'm making is an enclosure system that allows you to make battery packs of various voltages, 3.6, 7.2 and 14.4v. I want to make a measurement tool that confirms there is power and indicates what voltage the pack currently has.

Answer 1

Here is a straightforward Zener-based circuit which accounts for the forward voltage drop over the LEDs themselves:

^{simulate this circuit – Schematic created using CircuitLab}

D1 is a 4.7V device, and D2 is an 11V device. These should give enough margin to work with different colours of LED, not just red (which tends to have the lowest forward voltage).

Note that since the full voltage is still applied to the first LED's limiting resistor, that will be the brightest one even when a higher LED is in fact lit. For that reason I do recommend that you use different coloured LEDs, taking advantage of the fact that red appears less bright than other colours.

An alternative circuit using transistors as LED drivers avoids this problem, and yields LEDs with identical brightness:

^{simulate this circuit}

In this case, D1 is a 5.1V type and D2 is 10V. The precise values are less critical than with the first circuit; the forward voltage that has to be added to the Zener voltage to obtain the threshold is that of the transistor (typically 0.7V), not the LED.

Notice also that all three LEDs now draw from the same limiting resistor. This makes the brightness of each LED more nearly the same regardless of which battery is connected, since although the current through the resistor is increased with higher voltage, that current is shared by more LEDs. However, because the LEDs are now effectively connected in parallel, they should all be the same colour (and hence forward voltage).

Answer 2

The device you need is called "dot bar graph driver", most commonly implemented as LM3914 integrated circuit. Here are instructions how to use it. You just need to adjust voltage dividers to suit your particular levels.

The LM3914 can safely operate up to 25V, so just connect the net labeled (5V) to positive terminal of your source, and ground net to negative terminal of the source you need to monitor. The circuit, however, will consume maybe 10-20 mA, so the source should be capable of supplying this current without affecting its voltage.

Answer 3

The cheapest and easiest solution I can think of at the moment is using zener diodes. Get the zener diodes for the voltages you want to indicate 3V, 7V, and 14V approximately.

Putting an led in series with the zener, and a current limiting resistor should work. The first light will turn on at 3V, at 7V the second light will turn on and the 3V one will remain on, and so on.

Answer 4

Use three comparators with inputs at -3.6V, -7.2V and -14.4V.

Answer 5

Well, this is the simplest as possible. Since diodes are current elements, it matters to them, that enough current is flowing through, which for a regular led is something between ~1mA and no more than 20mA.

(I'm often using LEDs, that you can see, if they're turned on at already 1mA)

3.6V - D1, resisor 960 R, the current will be about 3.75mA 7.2V - D2, resistor 1920 R, the current will be about 3.75mA 14.4V - D3, resisor 3840 R, the current will be about 3.75mA.

This schematics will work just fine and diodes and none of the diodes at voltage 14.4V will not be harmed.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 6

Here is a preliminary idea:

^{simulate this circuit – Schematic created using CircuitLab}

Here's a simpler implementation:

^{simulate this circuit}

I had to make one more try at optimizing this design:

Answer 7

Such simple but clever circuits of LED voltage indicators have always attracted me and made me come up with the simplest possible solutions. There was an additional difficulty here - the indicator did not have its own power supply, but had to be powered from the input source. This awakened the inventiveness in me and made me join the competition here.

Improving the transistor circuit

First, I improved (added current-limiting and current-equalizing resistors to) the @Chromatix's transistor circuit.

^{simulate this circuit – Schematic created using CircuitLab}

Inventing a new diode circuit

But the real challenge for me was to make a diode circuit in which the brightness of the LEDs does not depend on the varying supply (input) voltage. Guided by the notion that the path by which we arrived at the final circuit solution is more important than the circuit itself, I have shown below in four successive steps the course of my thoughts. Since my goal is to reveal the idea, I have considered a conceptual circuit with "ideal" diodes.

Basic idea

Such an LED bar indicator is a set of LEDs with step-increasing threshold voltages.
Parallel limiting. I came to the idea that in addition to threshold diode elements (Zener diodes or the LEDs themselves) connected in series, there is also a need for diode elements (Zener diodes) connected in parallel to fix the input voltage of each cell. Thus the currents will be equal.
Series limiting. Another solution would be to limit the current through the LED by a constant-current element but more on that later.

LED as a threshold element...

The single LED is itself a threshold element - it starts to pass current and lights up when the voltage across it exceeds its threshold voltage.

^{simulate this circuit}

It is a very sensitive indicator that sharply reduces its "resistance" at that moment. As a result, the current increases sharply when the voltage increases and exceeds the maximum permissible value.

... with current-limiting resistor...

To limit the current, we insert a constant resistor R in series. The requirements for its resistance are contradictory - on the one hand, it must be low (to ensure high sensitivity) and on the other hand, it must be high enough (to limit the current to the permissible value).

^{simulate this circuit}

But still the current varies and the LED changes its brightness when the input voltage varies.

... constant brightness...

We have to somehow make the current constant. We have two options to do it - either make the voltage constant or the resistance dynamic. Let's try the first one by starting to reason like this:

Once the LED starts to glow with its normal brightness, the voltage across the R-LED network should stop changing. Clearly, we have to connect a (Zener) diode in parallel to the network (I have used a forward-biased "ideal" diode for this purpose).

^{simulate this circuit}

However, there is no desired change in the current through the LED...

... but the current through the shunt diode D has become unacceptably high.

... another current-limiting resistor...

Ah, we had to include another current-limiting resistor R1 because of the D diode.

^{simulate this circuit}

Thus a kind of "voltage window" is obtained within which the current changes and then remains constant.

... and "lifted" threshold...

We can increase the threshold of the cell by "lifting" it on the right with a second (reference) voltage source Vref.

^{simulate this circuit}

Thus we artificially increase the threshold voltage of the LED.

... with reference diode

But there is no power supply here to get Vref from. So, we again use another (Zener) diode D2 through which we pass the current of the cell and use the voltage drop across it as Vref.

^{simulate this circuit}

LED bar assembled

Now it remains only to make a ladder of such cells and we get the desired LED bar indicator.

^{simulate this circuit}

I have shown only three cells, but it can be expanded with more cells, only the current consumed increases (a disadvantage of the parallel configuration).

Advantages

As you can see, two more clever tricks are added to this Russian Matryoshka-like structure:

• The shunt diodes (D1, D2, D3...) are not grounded but connected in parallel to the R-LED networks. The advantage of this connection is that the diodes have the same low threshold voltage.

• The reference diodes (D4, D5...) are connected in series. The advantage of this string connection is that the diodes can have lower threshold voltage (because they are summed).