#ne555

These days good quality oscilloscope is not an expensive instrument and, we can see it in most workbenches. With the advancement of oscilloscopes, simple testing tools such as logic probes are not as popular these days. However, if the oscilloscope or logic analyzer was out of reach, the logic probe is a handy instrument to check digital circuits.

Also, in some cases, a logic probe is an easy option to check the functionality of low-speed logic circuits because it provides a real-time visual indication of the logic state without adjustments or calibrations.

The logic probe design described in this post uses common and inexpensive ICs, including the popular NE555 timer and LM393 low voltage comparator IC. At the time of this writing, both ICs were prevalent in the market and cost less than LKR50 (US$ 0.25).

This circuit design using SMD components to minimize the board size. The dimensions of the soldered PCB are around 70mm × 14mm × 6.6mm, and the 3D printed housing is 90mm × 26mm × 14mm only.

This probe got three LED indicators, and it displays logic levels (high and low states) and pulse signals up to 1.8MHz (on 60% duty cycle).

To operate this logic probe, connect the power cable of the logic probe to the DC power supply. The voltage supply should be from 3V to 15V. Voltages higher than this range can damage the ICs of the logic probe.

Depending on the circuit, set the logic family switch to the CMOS or TTL position and start testing by touching the probe tip to the components of the circuit board. Refer to the table below to interpret the LED output.

The pulse indicator (Red LED) is sensitive to higher frequency pulse signals ranging from 13kHz to 200kHz with an average 50% duty cycle. The effective range of the pulse indicator illustrates in the below chart. The maximum frequency support by this logic probe is 1.8MHz on a 60% duty cycle.

The suitable enclosure design for this logic probe is available at TinkerCAD. 3D print the top and bottom covers of the enclosure using ABS and attach the PCB to it as shown in the video. This enclosure design assumes that the PCB populates with 3mm LEDs and 12 SWG (2.6mm) probe-tip. If the components differ from the above limits, then the enclosure layout may need to adjust accordingly.

In our prototype, we construct this probe-tip using 12 SWG enameled copper wire. The probe-tip preparation and installation steps are available in the video. Also, in our prototype, we 3D print the enclosure using 1.75mm PLA filament and bind both top and bottom covers using PVC solvent cement.

The test setup used to test this logic probe consists of a NE555 timer, two CD4040 12-stage counters, and CD4017 Johnson counter ICs. In this test setup, NE555 delivers 16kHz output with approximately a 50% duty cycle. CD4040 counters are used as frequency dividers to produce 500Hz and 15Hz waveforms. This test setup is needed if you do not have the frequency synthesizer/generator to test the logic probe functionality. The schematic of the test setup is available with the archive file mention below.

The KiCAD PCB design files, schematics, Gerber files, and test setup schematic of this project are available to download here. The enclosure design (both top and bottom covers) is available at TinkerCAD. The STL files of the enclosure are also available with the above archive file.