Cables

BNC vs. Audio Cables

The cable attached to the signal output should be shielded and not too long, about 1 meter works well.
A BNC socket is easy to install in candy tin boxes and very robust, but smaller 3.5 mm audio sockets can be used as well.
BNC at the detector also allows the use of standard BNC cables for direct attaching to regular oscilloscope inputs.

Standard stereo/3-pin 3.5 mm jack to 3.5 mm jack audio cables can be used, but they should be short and shielded which is rare. Look for high-quality cables from a music/audio equipment store, e.g. microphone cables.

Since the crimping of BNC connectors onto shielded coaxial cables can be annoying, the easiest way for a BNC-based connection is to cut a pre-configured, 2-3 meters long, BNC-to-BNC (male/male type) cable in half. Then solder a 3-pin/stereo 3.5 mm jack onto the cut end of the coax cable. RG-174 coax cables have a rather thin outer diameter below 3 mm which makes them easier to attach and solder to the 3.5 mm jack connectors in comparison with more common and thicker RG-5x cables.

Part Suggestions

For example:

• RG-174 coaxial cable with BNC-BNC male connectors: Pomona Electronics, 2249-K-120 (3 meters to be cut in 2 halves)
• 3.5 mm 3-pin/stereo jack connector (TRS): Neutrik, NYS231BG
• Alternatively: 3.5 mm 4-pin/headset jack connector (TRRS): Cliff FC68124 (see instructions below)

Soldered cable using the parts suggested above:

Note how the center of the coax cable connects only to the tip of the stereo/3-ping audio jack (that is usually the left channel in case of stereo inputs). The middle ring is skipped but better to have instead of a mono/2-pin audio jack. Certain microphone input sockets, like the ones of CM108 USB Soundcards, apply a small bias voltage to the middle ring. This would be shorted to ground if using only a mono/2-pin audio jack and could affect the measurements (not tested).

Readily configured cables are rare but can be found online. Search for "BNC 3.5 mm". Make sure to check for the required polarity at the 3.5 mm audio jack connector or modify it if need. Metal-cased 3.5 mm audio jack connectors are usually easy to open.

Connection with a Headset Socket

Smartphones, tablets, and modern laptops usually have a socket for 4-pin TRRS connectors, the one that headsets use. They use 4 pins to connect stereo earphones as output, one (mono) microphone as input, and a shared ground connection. This requires a 4-pin 3.5 mm audio jack connector, where the second ring (counted after the tip) serves as ground and the sleeve/shaft at the connector base is the signal input. See the "TRRS Jack" drawing in the image below.
Some old 4-pin headset sockets may require a reversed polarity. If the pulses are upwards instead of downwards pointing, simply swap the ground and signal connection at the second rind and the sleeve. Some further technical notes are in the schematic.

The circuit board must be powered in case of certain smartphones and laptops (Macbooks/iPhones/iPads) before plugging the audio jack in. If the detector is not recognized as an external microphone, please check the value of the last output resistor on the detector's circuit board - it must usually have a value between 2 and 10 kilo Ohm to comply with automatic input detection requirements. I haven't found ready-made commercial BNC to 4-pin 3.5mm audio jack cables so far. If you do, please let me know or open an issue with the link.

Audio Jack Connector Overview (3.5 mm TRS/TRRS)

MIC = microphone input => connect with detector ouput

Further pinout drawings of typical TRS and TRRS connectors.