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First of all, I'm new to designing circuits so bear with me.

I'm looking to create LED test devices that will light up when they see an output from a 24 Vdc supply. I would want to be able to connect 1, or up to 10 devices. And these devices would be connected in series.

I want to be able to connect just 1 device and have it light up when it gets a voltage, or connect 4 devices and have all 4 light up, or connect up to 10 devices and have all 10 light up. with the same 24 Vdc supply.

What kinds of resistors would be needed in each test device? Could I use zener diodes to control the voltage?

The color of the LED does not matter, it is just needed for a visual cue.

Mark
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2 Answers2

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Basically, depending on your criteria, this design can be fairly simple or fairly complex. Starting simple, you want to know if you have power on a single jack. For that, you can place a LED with a series resistor between the + and -. The resistor value can be calculated from the LED you select. Let's say you have an LED that requires 2.8V@10mA. You can compute the resistor value by the following equation R=(Vsupply-VLed)/CurrentLED. In my example, this would give you a 2k resistor. They can be a bit bright at that value so you can place a higher resistor value (2.7k for example).

If you want 10 LEDs detecting 10 inputs, you can do the previous circuit 10 times. No need for anymore complexity.

Now, if you want to ensure that the voltage is between 22 and 26V, this is a bit more complex. You could do it with transistors and diodes, but, the simplest way to do it is with two comparators. In that approach, your device will require a power source of it's own. So the topology of those 2 comparators is to tie both their in+ together. Connect it to your input signal with a voltage divider (1:10 seems a good ratio). Place you max threshold on the first comparator (with the ratio). You can use a pot to configure the triggering voltage. Then do the same for your low threshold. Finally, place your LED between the output of both comparators with a series resistor. Replicate this circuit for each input (they can share the same Vref). Of course, this circuit is very generic, but without more context, I can't comment any more.

Good luck!

Julien
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Doing some back-of-an-envelope maths. Lets say we want a maximum of 20mA with one LED. Assume that the voltage across the resistor is about 23V, we get V/I = 1150. Call it 1.2k ohms.

You want 10 LEDs in series to still light. That assumes a voltage drop per LED of less than 2.4V. Red LEDs are about 1.6V, so they will work. Green ones at about 2.2V might work, but it's pushing it. Forget blue or white ones.

So each test LED is a red LED in series with 1.2k ohms.

BUT, with a total of 12k ohms, and a voltage drop for 10 LEDs of about 16V, we get a current for all 10 in series of about (24 - 16) / 12000 = 0.67mA. They will glow dimly, enough to see, but no more than that.

Simon B
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