led
How could I make the led work only if a certain voltage exceeded?
I have four voltage regulators in my project to supply my ICs, and i want to put 2 leds on the box so i know if my regulators giving +15V and +5V.
I need to down it for 4 led, so it need to be simple .
I try to make a comparator with op amp but i cold not.
Placing LED's in a parallel-series combination like this is a technique to average out the brightness given constant voltage. It helps present a more even looking illumination cheaply (as opposed to active constant-current circuitry).
The Q-point of the two LED's in parallel will be determined by the voltage assigned to the pair by the current allowed through the 4 series steps.
The final Q-point will be V1 = V2 (diode voltages equal), I1 ≠ I2 (currents unequal).
However, in your case the parallel combination is itself in series.
That means that I1 + I2 = Ip is the same for each parallel pair of diodes in the same string.
In the case of LED's in the forward region, brightness is almost linearly related to current. So holding Ip constant gives you approximately the same amount of light from the pair of diodes even though it is difficult to know how the light output will be distributed between them.
Extending this to the string of pairs, you cannot find a closed form solution (I might be wrong, but I don't see one) so you would need to solve it iteratively (I like Newton-Raphson) like SPICE does.
In general, however, the more diodes in each parallel group and the more parallel groups in each string, the closer the light output of the parallel groups approach each other given a batch of LEDs, normally distributed, by light output.
PWM modulation is the way to go with this as it saves wasting a lot of power in heat that an adjustable current source would generate.
Building a PWM control can be done with many types of circuit designs but honestly the simplest to implement may be with using a microcontroller on a low cost board such as an Arduino.
If you want to build your own circuit there are a plethora of circuit resources available on line.
On the other hand if you want to keep your life as simple as possible at very low cost then consider visiting eBay and picking up a low cost adjustable module for a few bucks.
Best Answer
As the earlier comments and answer pointed out, if you want an accurate indication of voltage level, you need an accurate reference.
The more normal method is to use a voltage reference and multiple comparitors or op-amps to compare your input voltages to the reference. The output of the comparitors or op-amps would then drive the LEDs.
However, there is a simple trick that is inexpensive and very simple. It works best with positive voltages but can be made to work with negative voltages with some effort. Because you want to measure only two positive voltages, I'll offer it as a solution for you.
It uses only 3 resistors, a LED, and a TL431 adjustable shunt regulator per voltage rail to be monitored. Very simple and quite accurate.
simulate this circuit – Schematic created using CircuitLab
The TL431 is normally intended to be an adjustable shunt regulator - sort of an adjustable zener diode. It contains an accurate 2.5V reference and is normally configured so that the cathode connects to the voltage to be regulated.
But it will happily run open-loop. In that case, the TL431 has a high impedance from the cathode to the anode so long as the adjust pin voltage is less than the internal 2.5V reference. When the adjust pin voltage is more than the internal 2.5V reference, the cathode has a low impedance between itself and the anode.
It is that low impedance that turns the LED ON.
You will need to set the voltage divider ratio on the adjust pin so that the TL431 conducts when the input voltage is above your desired value.
Duplicate the circuit for each voltage rail to be monitored.