Logic probes are useful debugging tools that are used to analyze logical states (0 and 1) in a circuit, they are also cheap and easy to build. I took this concept and made a logic probe for e-textile makers, specifically beginner students, to easily test and debug their code and circuitry without needing to break out a multimeter.

The logic probe has two states - red and blue:

Red indicates a high logic state, meaning that the probe is in contact with a thread that is set to high (>2.5 volts).

Blue indicates a low logic state, meaning that the probe is in contact with a thread that is below .7 volts or is grounded.

The output from the logic probe can be broken down as such:

Output | Meaning

Red LED is on | Logic output is HIGH

Blue LED is on | Logic output is LOW

Both LEDs are on | The logic output is switching between HIGH and LOW

No LEDs are on | The probe is not connected to a thread, and/or the probe is not connected to the microcontrollers GND or VCC.

The logic probe works with supply voltages up to 12V and can be used to detect TTL and CMOS as well.

This material is based upon work supported by the National Science Foundation under Award #1742081. The project page can be found here.

This project was developed in the Craft Tech Lab and ATLAS Institute at The University of Colorado, Boulder.

Photo (1, 2, & 3) credit to Elliot Whitehead.

If you want to keep up with my work, or just toss around ideas, please do so on my Twitter: @4Eyes6Senses. Thanks!

Step 1: Materials

1x Custom PCB - while this project can be done with a regular protoboard, if you want to order the custom PCB you need to upload the "LP_GBR.zip" to any PCB manufacturer. I used JLCPCB and it costs about $2 for 5 boards.

1x Red LED

1x Blue LED

3x Different colored paracord (I use red, blue, and black)

1x Conductive thread

2x Alligator clamps

3x Heat shrink tubing (I use 1mm, 3.5mm, and 4.5mm)

1x Blunt needle (1mm diameter)

2x very thin silicone jumper wire

1x 100k resistor

1x 10k resistor

1x 470 resistor

1x 2N3904 transistor

1x 2N3906 transistor

1x Zenor diode (1N4148)

3x Male headers

3x Female connectors

1x Housing (Top, bottom, and needle tip 3D printed) - I used a Prusa MK3S to print mine.

Step 2: Print Housing and Needle Grip

I suggest printing the housing and needle grip first as they are essential for the following steps and take longer to print. You can also create the logic probe without the housing, but you will need to skip some of the steps later on.

Step 3: Populate PCB

Populate the PCB as follows (do NOT solder the LEDs yet):

T1: 2N3904 (The transistors need to be soldered flat, see figure 4 for how they should look)

T2: 2N3906 (The transistors need to be soldered flat, see figure 4 for how they should look)

R1: 100K

R2: 10K

R3: 470

D1: 1N4148

After soldering, your PCB should look like Figure 3.

Praktische Elektronic and Andy Mechanic were great resources while creating the circuitry.

Step 4: Create Probe

- Cut a piece of paracord so that it is about 12 inches, and remove the inner white nylon insert (Figure 1).

- Cut a wingspan length of conductive thread and thread it through the needle (Figure 2).

- Carefully thread the needle and conductive thread through the inside of the paracord (Figure 3).

- Place the 1mm heat shrink tube so that it is covering the lower part of the needle (including the eye) and inside the paracord. Then use the 4.5mm heat shrink tube to cover the paracord and about 3/4ths of the 1mm paracord (Figure 4).

- Use a lighter or heat gun to shrink the tubing (Figure 5).

- You should now have a probe the looks similar to figure 6.

Step 5: Create Clips

- Cut a piece of paracord so that it is about 8 inches, and remove the inner white nylon insert (Figure 1).

- Take the two silicon wires and strip them (Figure 2).

- Carefully thread the silicon wires through the inside of the paracord (Figure 3).

- Crimp the alligator clips the exposed sections of the wires (make sure the wire is making good contact with the alligator clips) (Figure 4).

- Place the 3mm heat shrink tube so that it is covering both the paracord and alligator clips. Then shrink the tubing (Figure 5).

- You should now have two clips that look similar to figure 6.

Step 6: Attach Probes and Clips to PCB

- Thread probes and clips through the outside of the 3D printed housing (Figure 2).

- Strip the two clips and crimp a female connector to each end (Figure 2&3).

- For both clips, place the 1mm heat shrink tube so that it is covering the female connector and inside the paracord. Then use the 4.5mm heat shrink tube to cover the paracord and about 3/4ths of the 1mm paracord (Figure 4).

- Shrink the tubing. You should have two clips that look like figure 6.

- Take the probe and attach a female connector to the exposed conductive thread (this can be a little bit difficult, I recommend tying a knot) (Figure 7).

- For the probe, Place the 1mm heat shrink tube so that it is covering the female connector and inside the paracord. Then use the 4.5mm heat shrink tube to cover the paracord and about 3/4ths of the 1mm paracord.

- The 2 clips and probe ends should look like figure 8, and be inside the housing.

- Solder the 3 male headers to the PCB (Figure 9 & 10).

- Attach the clips and probe to the male headers and then gently press them down until they are laying on the PCB substrate (Figure 11 & 12).