2 inputs control 3 LEDs - how to build?

Hi I want to build a driver for 3 LED diodes. They are to be controlled by

2 LED diodes on the front panel of the computer (hdd and power). The behavior of those three leds is to be follownig:

-red diode on the panel ON: diode 1 ON

-red and green diode ON: diode 2 ON

-both diodes off: diode 3 on

I designed a circuit, based on 7402 chip (4xNOR)

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but in reality I had some problems, especially with HDD led, which was always off (even on the front panel)

Now I want to build such cirtcuit, but based on transistors, and i'm a noob in this topc. If anyone would give me some suggestions (or schematics :)) i'd be gratefull [voltage of the diodes on the front panel is 2V, The whole circuit should be powered with +5V]

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Wanda Ch.
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Wanda, start with what we used to call a truth table: inputs on the hoizontal, outputs on the vertical. With that as a starting point, you'll see how gate circuitry can be used to create the outputs you want with the inputs you have.

Wanda Ch. wrote:

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The "logics" part is okay, because I simulated it in an appropriate software. However, when I soldered it, it came to be that the current passes thru the red diode and goes back to the Chip, disrupting its behavior. When I disconnected the red diode, 2/3 was working ;) - blue diode powred when PC is off, green when it's on. I think I need to separate the red diode circuit from the rest of it, so that voltage passing through it cuoldn't interfere with input signals.

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Wanda Ch.

Looks like you simply need to use one of the other nor gates to drive the hdd led.

The hdd light on my machine stays on for such a short time its almost imposible to see.

Colin =^.^=

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"Wanda Ch." schreef in bericht news: snipped-for-privacy@m79g2000cwm.googlegroups.com...

Seems you're over the edge both in logic and in electronic. In the first place you did not specify the problem properly a far as the logic is concerned. A good specification would look like:

PWR HDD | D3 D2 D1

------------------ 0 0 | 1 0 0 0 1 | 1 0 | 1 1 |

of course with the empty places filled in. Don't care and don't happen need to be specified. The schematic you drew will not work. Besides, what's the use of lighting a HDD-LED when there is no power? You did not tell under what circumstances the circuit is supposed to be used, but normally it will be a don't happen.

As for the electronics you need to know the voltage when a LED is on and when it is off. These voltages are not necessarily the same for both LEDs. I don't like to go further assuming all kind of things. So if you provide the missing details we can go on.

petrus bitbyter

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petrus bitbyter

First, get your semantics straight. LEDs don't control other LEDs. Come up with a truth table as to which LEDs should be on for the possible states of HDD(?) and PWR.Also, are they active high or active low. Second, you don't normally connect the LED polarities as you show. Connect them from VCC to the gate output through a resistor of a few hundred ohms. Then an LED being ON won't mess up the 1 level for that gate, plus you get a lot brighter LEDS.


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