Introduction: Logic Probe With PIC: LogicProbePIC

A logic probe is not only the poor man's logic analyser but often more usable.

This logic probe is designed to

  • be cheep: below 10$
  • be tiny 0.4 x 3.5 inches, 1 x 10 cm
  • use the power of the circuit tested from 3V to 15V
  • be compatible to TTL and CMOS logic circuits from 1.8V to 15V
  • recognize (single) pulses about 20ns and 20Mhz (my 80Mhz)
  • have high impedance inputs
  • withstand input and supply voltages of +- 30V

The power is taken from the circuit tested. The LogicProbePIC needs less than 20mA.

The LocicProbePIC can be built on a veroboard and enclosed by a transparent heat-shrink tubing.

Step 1: Circuit of the LogicProbePic

The LogicProbePIC is based on a PIC12F629 and uses a LM2936 LDO voltage regulator.

The LogicProbePIC is powered by an external supply. This is in most cases the supply of the digital circuit tested.

It includes only a few resistors, capacitors and three LEDs. A push button can be used to control the LogicProbePIC.

The high valued resistors at the input prevent the probe to be damaged by high input voltages up to +- 30V. The smart voltage regulator LM2936 withstands power voltages up to +-40V.

A detailed description is at the Design ot the LogicProbePIC.

Step 2: Usage of the LogicProbePIC

The logic probe has 3 LED

  • The green LED indicates that LOW is applied,
  • red shows HIGH and
  • yellow that pulses were detected.
  • A flashing yellow LED indicates that there are pulses applied
  • The green and red LEDs can also be:
    • The green and red LED light up brightly when low- or high-rest with short pulses.
    • The green or red LED lights dim when low- or high pulses applied unequally duty.
    • The green and red LED lights dim when receiving low and high pulses with approximately equal duty cycle.
  • When neither the green nor the red LED light,
    • there is either no correct logic level,
    • there is logic in tristate or
    • the probe head is open.

Reset pulse detection

The yellow LED indicates that pulses were detected. It can be used to recognize a single pulse.

For many measurements, it is important to recognize individual pulses. The logic probe can be reset with a push button. The yellow pulse LED is deactivated. Connect the LogicProbePIC to the logic output to measure, and then press the button shortly. The first incoming pulse activates the yellow pulse LED. Strictly speaking, this LED indicates that at least one LOW-HIGH transition is detected.

A detailed description of the functionality is Description of the LogicProbePIC

Step 3: Test of the LogicProbePIC

The first tests of the logic probe was carried out on a breadboard.

From left to tight:

  • PIC with R, C and LEDs
  • voltage regulator LM2936
  • switch of the logic probe
  • push buttons for LOW and HIGH
  • potentiometer for testing the input voltage
  • glitch generator
  • clock generator 555
  • 80MHz quartz generator

Step 4: Board of the LogicProbePIC

All components are standard but the SMD capacitors C1, C2 and C3.

The LogicProbePIC build on a veroboard. All components are placed on top but C1, C2 and C3 are placed on the solder side. The blue connections are standard connections on the solder side of the veroboard. The red connections and the blue ones with "isolierter Draht von unten" are connection with isolated wires.

The connections to the power supply can be connected to the plug or be simply soldered.

An experienced person can build the LogicProbePIC in a few hours.

The case is simply a a transparent shrink tube:

  • You can show your construction,
  • the LEDs are visible and
  • the push button can be activated.

Step 5: Software of the LogicProbePIC

The assembler program of the PIC12F629 is attached. You need the MPLABX from Microchip . It is free. You need furthermore a prommer. I used the PIC-Kit 3. PICkit 3 .

Try on ebay "PICKIT3 Debugger Programmer Emulator PIC Controller" or "Adapter Programmer PIC".

You can get the PICKIT3 together with an adapter.

More information is on my site.