How to light an LED when a voltage hits a certain point

Question

Lets say I need an LED to indicate when a DC voltage reaches 5 volts or more. I don't want the LED to be dim when it is at 4.5 or anything. I just want it to be full bright at 5, 0 bright at anything below 5. Is there a way I can do this with discrete components (no IC's, just resistors, capacitors, transistors etc.)? I do not require the input has the same power source as the meter.

Answer 1

To narrow the voltage range over which the LED turns on, you can use a transistor in order to create voltage gain, to drive a second transistor which drives the current into the LED.

^{simulate this circuit – Schematic created using CircuitLab}

Results from simulation, where the input ramps from 0V to 8V over the period of a second:

Input Voltage:

LED Current:

The LED goes from zero to fully on over a range of something like 80 mV.

Zener diode D1 biases the input so that the turn on takes place at around 5V.

Note that only about 150 microamps of current are required across R1 to develop 0.7V across Q2's EB junction. That small amount of current does not require the BE junction of Q1 to be fully turned on! This is why the Zener value isn't based on 5V - 0.7V. From the simulation, the transition occurs between 5.12V and 5.2V, and not starting at 5.3V as the approximate junction model would suggest. Higher values of R1 do not significantly improve the upper 5.2V voltage, but degrade the soft turn on voltage. The next available lower Zener model in the simulator is a 4.3V unit, which leads to a turn on that falls short of 5V. Raising that toward 5V requires lower values of R1, which cause Q1 to pull as much current as Q2.

Answer 2

Didn't test it, but it should work. The SCR latches when its gate current exceeds a certain value. Notice that the LED won't turn off unless V(much) is interrupted.

^{simulate this circuit – Schematic created using CircuitLab}

Sizing is very depending on the thyristor, leaving that to the reader.

Answer 3

I would use a comparator, which of course is an op amp. You can get a quad package like the LM339 to do 4 at a time. Resitors or zener diodes can set different on voltages.