Kits are available for this project from Talking Electronics for $8.00 plus postage.

 A LOGIC PROBE is just one piece of equipment to help with designing/testing and diagnosing a project.
And for $8.00 you can't pass it up.
It has two features above other Logic Probes. A pulser to produce a waveform that can be injected into a circuit to produce "clocking" or into audio circuits to determine the relative amplification of a stage.  And a piezo to hear the presence of waveforms.
You won't be able to hear 1MHz signals but many signals in a project consist of scanning data and can be detected as audio.
This project forms part of our overall aim to get you into designing circuits using a microcontroller.
It teaches you how to use a Probe and Pulser and adds to your understanding and analysis of  circuits.
You don't need a Logic Probe until a project DOES NOT WORK.
That's when you reach for it.
And it is based on a very simple circuit.
It provides all the features you need and will detect HIGH/LOW voltages on 3v to 15v circuits because the power for the Probe comes from the leads clipped to the power rails of the circuit you are testing.
This provides automatic adjustment for HIGH and LOW values to equate to approx 50% of rail-voltage. 
The project also has a Pulse LED to show when a single pulse is detected. This is actually a "pulse extender" and is handy for the time when you are waiting for a single pulse to arrive to activate a circuit.
The piezo also monitors the input and you can hear the activity on the line and compare one line with another.
The PULSER mode delivers a low frequency waveform and this will override some of the waveforms in a circuit to slow-down the activation of a circuit to see what is happening.

This is the prototype Logic Probe. It was built on matrix board and
it worked very well, so a printed circuit board was made, using
surface-mount components. 
It is now available as a kit for $8.00. (the cheapest Logic Probe kit on the market) 

This project has already been used to solve the following problems:
No display on the Super Probe MkII project.
The clock pin was injected with the pulser and the Super Probe MkII showed segments on the display at a very low clock-rate to prove the microcontroller was working The problem was one of the 20Mhz crystal leads connected to 0v.
The 8x8 Module did not produce a display. The data and clock lines were probed and it was found the data line was not connected.
The Logic Probe also helped with the input of the Point Controller project to show when the photo transistor changed from LOW to HIGH. 
The Logic Probe helped to solve a problem with the Hourglass Timer project.  A track was missing !

You don't realise how many times you use a Logic Probe until you keep a list of the incidents. 
The illumination of the LEDs on the Logic Probe tell you a lot of things. They show if a line is predominantly HIGH or LOW and the piezo will let you hear the activity on a low-frequency line. This has helped with the display section of the Stroop and NIM projects.  

It won't solve all your problems, but it will quickly find open tracks (traces) and let you see the signal on each side of components, such as LEDs, that are not connected to the 0v rail (in a multiplexing or Charlieplexing arrangement).

Connect the red and black alligator clips to the project you are testing.
This will automatically allow the Logic Probe to recognise a HIGH and LOW and the change between these two values. 
It is the voltage-level at which the Logic Probe changes from HIGH to LOW or LOW to HIGH that is very important.
It must be very close to 50% of rail voltage.
Suppose you connect the Logic Probe to a 12v supply and work on a 5v microcontroller project.
When the Probe detects a 5v signal, this will only be 5/12 = 40% of the input voltage for the IC on the Probe and it will detect this as a LOW !
That's why the Probe is always connected to the project being tested.
Use the probe on lots of working projects to see how the LEDs illuminate and the piezo responds to the signals.
This is the only way to find out what to expect.
You must compare your observations with a circuit diagram as this will be your "library of resources" for future diagnosis.
When you are working on a faulty project, you will see the red and orange LEDs brightly lit and the green LED very dull. This indicates the line is mainly HIGH.
If the frequency of the pulses are between 50Hz and 10kHz, you will hear them in the piezo.

The project produces an output waveform of approx 330Hz when the button is pressed.
This can be injected into a circuit to produce lots of different effects.
The waveform is called a PULSER, CLOCK, INJECTOR, OSCILLATOR or SIGNAL INJECTOR, depending on the type of circuit being analysed.
It is especially handy when trouble-shooting audio circuits to detect the gain through a stage. Audio stages are very difficult to trouble-shoot, especially when three or more sections are DC coupled.    A signal injector will let you know if a signal is getting through as some sections are current amplifiers and the amplitude of the waveform does not increase.

All the parts fit on a slim-line PC board with surface-mount components used to reduce the size and make to the project appear very simple.

All the values of the surface-mount components are identified on the top (silk-screen) and the 10n is the same size as the 100n so don't get them mixed-up.


Logic Probe
Parts List

Cost: au
$8.00 plus postage
Kits are available

4   -  1k (102) SM resistors
1  -  15k  (153) SM resistor
2  -  220k (224) SM resistors
3  -  1M (105) SM resistors

1  -  10n SM capacitor
1  -  100n SM capacitor

1  -  3mm red LED
1  -  3mm yellow LED
1  -  3mm green LED

1  -  14 pin IC socket 
1  -  CD4069UB
1  -  13mm piezo
1  -  tactile switch
1  -  20mm x 1.2mm nail for probe
20cm red and black hook-up flex
1  -  red alligator clip
1  -  black alligator clip
20cm very fine solder 
1  -  Logic Probe/Pulser PC board

Tests LEDs.
$1.50 plus $4.00 post
Only responds to resistance less than 50 ohms.
Ideal for digital projects as it tests connections very quickly.
$12.50 plus $6.50 post

(buy a number of kits and pay
only one postage)
LOGIC PROBE with PULSER - slimline
Detects HIGH and LOW signals on both TTL and CMOS circuits.
The piezo allows you to hear low frequency signals and the signal injector (Pulser) will over-ride clock signals to make a circuit operate at a reduced frequency.
$8.00 plus $6.50 post
20 different functions.
See article for the complete list of functions.
$18.00 plus $6.50 post
Tests transistors and shows the gain of the transistor.
Also has Signal Injector probe.
$21.50 plus $6.50 post
Simple Transistor and LED Tester - 3
Tests PNP and NPN transistors and LEDs.
Also teaches the amazing property of an air-cored coil in producing a high fly-back voltage.
$4.00 plus $3.00 postage.
Detects 240v AC mains hidden in walls etc.
Will also pick up RF signals from a keyboard to show you where Electromagnetic Radiation is coming from and giving you a headache.
$10.00 plus $4.50 post
Traces cables when the power is OFF.
Uses an FM radio to pickup beeps.
$10.00 plus $4.50 postage. 

(buy a number of kits and pay
only one postage)

Teaches how an op-amp works by using pots to control the voltages on the two inputs.
$24.50 plus $6.50 post
(comes with instructions)
Learn to program PIC chips.
Comes with a pre-programmed PIC12F629 chip with 3 routines.
$12.00 plus $6.50 post
model railway POINT MOTOR CONTROLLER and TESTER   CDU-Inline
The cheapest CDU project you can get.
$8.50 plus $6.50 post