Light LED for a voltage input of 0.3 V - 40 V

Hi,

I have to create a circuit that can take in an input voltage that can vary between about 0.3 V to 40 V or can be set to off at 0 V. I wish to be able to light an LED when this any voltage is present.

The problem with doing it directly is if the LED series resistor is set to limit the current to drive the LED at 40 V, the current will not be sufficient to drive the LED at for instance 1 V.

I have considered using a voltage comparator circuit in order to take in the input and compare it with about 0.1 V, or a voltage just about the noise level. If the input voltage were above the reference level, the Op - Amp would saturate at the supply voltage. I wish this supply to be about 5 V.

However I am unsure of whether an Op - Amp can compare voltages greater than that of the supply voltage.

Any help would be greatly appreciated.

Regards,

Paul

Hi Paul,
If your circuit dont have an stand alone supply: Interesting dreams!
Otehrwise, its a good job for an Instr.Amp with high common-mode input range(has an input attenuator system) & a comparator, and you should place the LED on its output...
K.

Added after 8 minutes:

INA117, refer to figures 12..14 pls, price is at $ 5--6...:
https://focus.ti.com/lit/ds/symlink/ina117.pdf

K.

How about using a zener diode with a resistor?

This would reduce any large voltages to zener voltage (say 3.3V or 5.6V)

Any smaller voltages would be passed straight through.

If you need these smaller voltages ~0.5 V to light the led, you could put the output of the zener circuit into a comparator with a ~0.3V threshold and a 5V/0V power supply.

Any comments from anyone?

Hi,
Pauls idea with an OpAmp/comp is OK, but the must Amps can not live with input voltages over Usupply, or maximum slightly over that is possible...
In all case it need an input attenuator, ie.e for more-or less under the supply value & an RRIO amp.
Modern types are so good wit offset voltage, that it can work with so 10mV threshold or higher!
So will you have a cheaper circuit realied as with Instr.Amp...
K.