# Adjusting LED currents same level for different voltage levels

I have different voltage levels 1 V, 1.2 V, 1.8 V .. 5 V, 12 V and I want to connect a led each of them to see the voltages are clearly set. In this case, I want same brightness for each LED. I can do this by using simple resistor for each voltage level but low voltage sides (1 V, 1.2 V and 1.8 V) aren't efficient. So I thought maybe there is some voltage shifter that accepts different voltage levels as input, fixed voltage level for the output that I can use. Anyone know anything like that? or some other suggestions to handle this?

Thank you

• Do you want to detect the presence of some voltage, or a specific minimum voltage?
– CL.
Feb 6 '17 at 11:31
• If a voltage is lower than desired, you get a lower current, which makes the LED less bright, but probably not so much that it would be visible. Is is that what you want?
– CL.
Feb 6 '17 at 11:37
• No, Assume all the voltage levels are correct and all the leds should be the same brightness. How can I assure the same brightness for each leds? Feb 6 '17 at 11:46
• @a-f-a: small and fast broken? what are you talking about? Feb 6 '17 at 11:59
• In case all share the same ground: 5 Comparators to compare the voltage against 5 references. Possibly up to 5 voltage dividers to discriminate the level. All Comparators supplied from the 5 Volt rail. What are the accuracy requirements? To be sure: "different voltage levels ..." means you have 5 power rails or does it mean you have a signal line which may get these distinct voltage levels? Please clarify your question. Feb 6 '17 at 12:34

Apparently you want to indicate that various voltages are present. I'll assume the indication threshold doesn't need to be all that accurate, and that you mostly just want a quick way to see whether a power supply has come up at all.

I had a similar problem where I wanted to show quickly that the 24 V, 12 V, 5 V, 3.3 V, and 3.0 V supplies were up. Here is a snippet of what I did:

The bottom rail is ground, but the ground symbol is cut off in this snippet of the schematic page.

The 24 V is used to derive all the remaining supplies from, so the LEDs are actually lit from the 24 V supply. I used TL431s as voltage threshold detectors to turn on the LEDs. There is a different resistor divider feeding the threshold input of each TL431, depending on the voltage of the particular supply being tested.

The resistors across the LEDs (R13 and R14 in the snippet shown) are to avoid the LEDs being dimly lit when they should be off. A TL431 requires some current to operate, which would otherwise come thru the LED. The resistor across the LED passes that current but at a voltage so low that the LED won't light visibly.

This method works for supplies down to 2.5 V, since that's the threshold built into the TL431. It also requires a high enough voltage to run the LEDs from. In this case I used green LEDs with a forward drop of about 2.1 V.

For your supplies below 2.5 V, you can use a NPN transistor instead of a TL431. You can either just use a single base resistor to show that the supply has come to about 600 mV, or a resistor divider to raise the effective threshold. In this case, you can get rid of the resistor across the LED since a bare transistor doesn't draw operating current when off. The threshold for lighting the LED won't be as crisp and accurate as with a TL431, but may be good enough for your purposes. Basically, you'd be using the B-E junction drop of the transistor as a voltage reference.

• TLV431 is a 1.25 V version of the TL431; for the 1.0 and 1.2, one might consider a voltage divider using a pullup instead of pulldown to GND. Feb 7 '17 at 0:39

See simple bar graph display driver for what you want. You can also go more fancy and use a microprocessor with an analog to digital converter.

• I don't think this is what the TS is looking for as it doesn't offer the possibility to choose the voltage levels. Feb 6 '17 at 12:44

If you just want the LEDs to show whether the voltage is present or not, you could use a mosfet to switch the LEDs. Then you connect the LEDs all to the same voltage rail.

simulate this circuit – Schematic created using CircuitLab

You could use low threshold devices for the the small voltages (<2V) and for the rails higher than 5V be sure to choose a transistor that is rated for the right gate voltage.

The smaller voltages would make it more difficult to light an LED with the same intensity using the method you talked about. I'd probably use the different output voltages turn on N CH MOSFETS with very low Threshold voltages. Vdd should be 12 volts. Adjust your resistor values between the Cathode of LED and the Drain of MOSFET.