# Make a Digital Logic Analyzer for Less Than \$1

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## Introduction: Make a Digital Logic Analyzer for Less Than \$1

A logic level sensor is a device that senses if the output of a component is 1 or 0 (positive or negative). You know those nice level sensors with the LCD screens that cost around \$25 ? This one is laughably cheaper and and does the same thing (It is a little less professional-looking than the store-bought ones, but still does what it is made to do).This one is designed for 5-volt logic circuits. I apologize for the lack of photos, but my parents do not yet have a digital camera.

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## Step 1: Obtain Components

you will need:

3 colors of 22-guage stranded wire (preferably red, green, and black)
2 100 ohm resistors
1 standard red LED
1 standard green LED
1 roll of electrical or duct tape, or a glue gun and gluestick
1 soldering iron and solder

## Step 2: Solder Parts Together

Cut the wires to your taste, about 3", and strip them 1/4" on each side. Green is 'IN', red is '+', and black is '-'. Solder the parts using the schematic and the wiring picture.

## Step 3: Put the Sensor in a Nice Package

Glob a glob of hot glue on it or wrap it up in tape.

## Step 4: Test It

Stick the red wire on the positive side of the power supply and the black one on the negative side. Both LEDs should be lit.When you touch the 'IN' wire to positive, the green light should come on. When you touch the 'IN' wire to negetive, the red light should come on.

## Step 5: Expand It

This is how one could make an analyzer with an infinite number of leads!(of course, the current output of the power supply would have to be infinite too). The picture explains it all. Make a ton of them and make all their negative and positive common.

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## 23 Discussions

WIth the schematic shown, aren't both LEDs on all the time (power by + and -, rather than (+ or -) and in)? Looks like you'd need a pretty low impedence signal to get one of the LEDs to go out. (However, I'm inclined to agree with the premise that a simple LED is often a useful logic-level indicator...)

I agree that they would be on all the time, until you touch something. Touch a low signal and the potential difference on each side of the green LED is zero, touch a high signal and the same happens to the red LED

As with charlie-plexing, the LEDs' working voltage is far above the voltage that can actually flow through both resistors and LEDs. 2 years later, I finally understand what I'm doing.

I'm going to make this with a double led! (That is, an led that has 3 pins, and two colors in one!)

use a 2-lead double led
`they are arranged like:|->-||-<-||    |`

Nevermind, It won't work... the two out side pins are the negative, and the middle pin is positive... So it would be impossible!

A good logic probe goes one step further - it indicates if the logic level is constant or changing (ie, data transmission). With a multimeter, you don't know if the output of a chip is sitting at 2.5 volts because something's fried or connected wrong, or if it's actually changing between 0 and 5 really fast and averaging out to 2.5. WIth a logic probe, you know immediately.

yes, a good probe. But this is a low cost probe and while a great idea I wonder how it would work in practice in your given scenario. If your were reading 2.5 when you expected 5 I would flip the mm to AC in. Just playing the devils advocate.

also, with this design I think that if it saw data you would have both LEDs on dim, no?

Depends on how the data is represented. If you're talking 3 volts is binary high and -3 volts is binary low, then you'd see activity on both LEDs. If 0 is low and 5 is high, then you would only see activity on one led. If the data transmission was slow, you'd either see blinking or a dimming effect on that one led. If it was fast, it would appear as brightly and constantly lit as a constant voltage signal.

I had to expirament because I couldn't visualize it working . You might as well build it to see for yourself. There's like four components, so no big project.

If it was logic data, then the LEDs would alternate. The only time both LEDs are on dim is when no electricity is passing through the probe's 'IN' wire.

they would alternate but if the data rate was at too high a frequency all you would see is either LED turn on/off really quickly and that would simply have a dimming effect. I guess there might be a little more going on inside the \$20 probe than you would expect. For example a probe probably presents infinite impedance to the trace you're sampling, this LED-resistor combination will possibly tie the line to the opposite power rail and corrupt the data. Its been a while since I've done any electronics, I'm happy to be corrected on my probing assumptions (o;

I did a little internet searching, and the common impedence is 1Mega ohm

You could add a NAND gate to the circuit, so if only one LED is on, then a yellow LED could turn on. Then if it looks like the red and green LED are on at the same time, and the yellow is on two, then you would know 'IN' is pulsing

Its 2 LEDs soldered together, i don't think its going to get large ammounts of praise. good beginner soldering project however.

perhaps you should check out the LED Throwie - now there's some high tech!