Monday, October 29, 2018

Simple Logic (Captain)

One of the more useful tools when trying to troubleshoot faulty boards is a small, innocuous thing that is known as a 'logic probe'. So, what does it do? It probes logic, obviously.

What it actually does is give an indication of a logic level. Computers work on ones and zeros. A logic probe simply indicates if the point where you, ahem, stick it is a one or a zero. It's really that simple. And I don't have one. Of course I could buy one, but where's the fun in that? :) So I set out to build a simple logic probe from anything I had lying around.

First things first, lets think about a circuit. Fortunately, I didn't have to think too hard as a helpful youtuber called Aldrin Floyd created a very short video showing about as simple a circuit you could imagine to produce a working logic probe. See his video here.

The schematic he came up with is shown below:

It's operation is simple. When the probe is placed on a low signal, the transistor is completely off, resulting in the full voltage being shunted through the red LED to ground through the probe. Boom. Red LED lights up bright. When the probe is placed on a high signal, the base of the transistor becomes saturated by the probe and allows the flow of current straight down to ground through the green LED. Boom. Green LED lights up.

(When there are no signals, both LEDs light up quite dim, which I assume is because the voltage drop across the resistor and red LED is enough to allow the transistor to partly turn on which, in turn, allows the green LED to turn on slightly. I'm sure an electronics expert would be able to explain it better than I can.)

 A strobing signal also lights both LEDs but is easy to distinguish from the 'just on' state as both LEDs become significantly brighter (with red actually being the brighter of the two).

A list of parts required:

  • A stiff piece of wire or a brass road of about 2mm diameter (for the probe)
  • An NPN transistor
  • A red LED
  • A green LED
  • Two 1k ohm resistors
  • Some wire (two core about two feet)
  • A couple of crocodile clips

First, take your probe and file the end into a point - the pointy end will be the bit you use most
Solder the Base of your NPN transistor to the probe.
Then, solder one of the resistors to the collector of the transistor.
Next, solder the positive lead of the green LED to the other end of that first resistor
Now, on to the red LED. Solder the negative lead of the LED to the probe.
Then solder the second resistor to the positive lead and the end of the resistor to the emitter of the transistor.
Nearly done.
Take one length of wire and solder it to the point emitter and resistor meet.
Take the second length of wire and solder it to the negative lead of the green LED.
Solder a crocodile clip to the other ends of the wires.

Annnnd.... you're done.

Once done, it should look something like this:

Bottom View

Top View

I did this in about twenty minutes and, having coated it in hot-snot to prevent shorts, it's ready to rumble. It's not pretty, I'll give you that.

Hot Glue Gunned

So to test. I took an Amiga A500 motherboard and connected the croc clip from the transistors emitter (the positive) to a positive supply rail on the board, and attached the other croc clip (the negative) to an earth point on the board. At this point, both LEDs light up dimly.

To check it was working I probed the positive lead of another cap on the board. The green LED lit up brightly and the red LED went off. Then I probed another earth point on the board and the red LED lit up bright and the green LED went off. If data lines are probed which are normally 'strobing' then both LEDs light brighter than when in their quiescent state but not quite as bright as a steady high or low signal.

A 'high' signal

A 'strobing' signal

A 'low' signal

And that's it. It's primitive and I'm sure that there are probably better circuits out there but for what I need, it's perfect and almost completely free. To pay lots of money would be illogical. Captain.

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