Introduction: Build Your Own Logic Probe on the Cheap Using a 555 Timer!

Hello! By clicking on this instructable you're one step closer to building a cheap, easy to construct, and more importantly useful little gadget....a Logic Probe! A logic probe is helpful when working with various chips. By using a logic probe, you can tell if a pin on a chip has high or low voltage by observing the two LEDs on the circuit.

This circuit uses a 555 timer in power ranges of 5V-18V. It also can be used to test TTL circuits! I won't be going through too much of the theory on how logic probes work but, I'm sure you can search this site for an explanation of the inner workings of logic probes.

Step 1: Gather the Things You Need!

For this project you will need:


  • 0.1µF (This is a non-polarized capacitor which might be marked "104" or "104Z")


  • 3.9K Ω
  • 390 Ω (x2)


  • 1N914 (x4)
  • Yellow LED
  • Red LED


  • 555 timer

Other items:

  • Wires (recommended that you have red, black, and a third color. I'll be using yellow.)
  • Gator clips (x3)
  • PCB
  • 8-pin chip socket
  • a wire with clips on both ends for the probe/testing (optional)
  • confetti

You will also need solder and a soldering iron as well as a wire cutter/stripper.

Step 2: Building the Circuit!

A photo of the circuit diagram can be found above.I encourage you to be creative when arranging the components for this circuit. I'm only providing a proof of concept/functional prototype. If you make this circuit, please share it by posting a pic in the comment section! :)

I started with soldering the pin socket onto the PCB. I then added each of the components attached to their respected pin on the 555 timer.

Adding Diodes:

When adding the diodes to the circuit, make sure that they are aligned properly. On each 1N914 diode, you will see one end with a circular band. That is the end that is blocking positive voltage. On the circuit diagram, the circular band is represented by a line adjacent to the black triangle (see photo). If you have the ends reversed on the circuit, you might end up causing a diode to smoke and even pop if given too much voltage!

  • Diode D1 will be used to connect the circuit to a power supply (positive) for a circuit being tested. The side of the diode with the circular band should be going to pin 8. A wire with a gator clip attached will be added later.
  • Diode D4 will be used to connect the circuit to ground. The side of the diode that does not have the circular band will go to capacitor C1. A wire with a gator clip will be added to the circular band side later.
  • Diodes D2 and D3 are connected in a loop that will connect to pin 6. Opposite the 555 timer side of the loop is where you'll attached the probe. Let one of the diode's leads stick out for testing the circuit (step 4).
  • D5 is the yellow LED and D6 is red. The LED's have a flat side (shorter lead). This is the negative side (lead). For this circuit you will the positive side of D5 going to pin 8 and the negative side of D6 going to pin 1 (photo of LED orientation above).

Adding the capacitor:

Capacitor C1 is attached to pin 5. This capacitor is not polarized.

Adding resistors:

ResistorsR2 &R3 are both 390 ohm (orange,white,brown). They are is series between the LEDs. I used one of the resitors leads to connect to pin 7. Save one of the clipped leads for bridging pin 2 to pin 6. R1 is 3.9K ohm (orange,white,red) and will have one lead between pin 5 and C1. The other lead of R1 will be connected to a wire with a gator clip later which will be used to set the probe in TTL mode.

Bridging pins 2 and 6:

Use the lead clipping you saved from the resisitor and use it to attached pin 2 to pin 6. This will put the 555 timer in astable mode and, by adding it to the soldering side of the PCB, it will keep you from having to add any extra wires.

Adding wires:

On my circuit, I used a red wire to connect pin 4 (between D1 and D5) and a black wire to ground the 555 timer (beteen C1 and D6).

:::Once you have everything mounted on the PCB, the next step is to add 3 wires with gator clips.:::

Step 3: Building Wires With Gator Clips!

Cut three wires about 14 cm in length (that's about 5.5 inches for those who don't want to embrace the metric system like the rest of world ;) )

One wire should be black (for ground) and red (for positive) and a third color, in my case yellow, which will used to set the probe in TTL mode. Each of the following steps will be repeated for each wire. Photos provided for reference!

  • Use your wire stripper and expose about 2.5 cm (1 inch) of wire.
  • Take the exposed part of the wire and feed it into the hole on the clip (...sounds a bit dirty...but anyway)
  • Wrap the exposed wire around the end of the clip and seal with some solder.
  • Add the cover back onto the clip.

When you're done, connect the wires to the PCB.

  • Red connected to D1
  • Black to D4
  • Yellow to R1

For the probe:

The probe I used is just a wire with a gator clip on each end that I had lying around. Since that serves my purpose just fine....which is in no way me being lazy.......promise. You can make a simple probe by attaching a clipped lead to one end of a wire and then attached the opposite end to the diode loop (D2,D3). I'll let you be creative here!

Step 4: Testing the Probe!

To test the probe:

Connect the red clip to positive and the black clip to negative of a power supply between 5-18V. No need to attach the TTL wire. While these are connected, touch the probe to the positive terminal. The yellow LED should be on and the red off which indicates "High". Then touch the probe to the negative terminal. The red LED should be on and the yellow off which indicates a "Low". If this happens then you're all set! Congrats!

Use the confetti and throw it in the air! (Yelling eureka is optional)

Step 5: Using Your New Probe!

Here's a brief explanation for how to use this logic probe:

Connect the red wire positive end of the circuit being tested and the black wire to the negative end.

For CMOS chips:

If the voltage at the tip of the probe is below 30% of the circuits power supply, it will light up the red LED (low). If the voltage is greater than 70%, then the yellow LED (high) will light up.

For TTL mode:

Connected the same way as you would for CMOS chips but also attached the 3rd wire (the one coming off of R1) to the negative terminal as well. The supply voltage of TTL circuits will operate at 5V. Any reading above 2V will make the yellow LED turn on. The red LED will turn on if the voltage is 0.8 volts or lower.

That about does it for this project. Thanks for checking out my instructable! If something needs to be clarified please leave a comment!