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.
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 to the tip of the stereo/3-ping TRS audio jack (that is usually the left channel in case of stereo inputs). The middle ring is skipped but better to be present instead of using 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).
The shield of the coax cable connects the ground of the detector circuit with the sleeve of a stereo/3-pin TRS audio connector.
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.
Smartphones, tablets, and modern laptops (e.g. Macbooks from 2013 and later) 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.
A 3-pin 'TRS jack' is required for regular mono or stereo sound inputs (like on external USB soundcards).
A 4-pin 'TRRS jack' is required for most integrated headset sockets of smartphones, tablets and modern laptops.
MIC = microphone input => connect with detector ouput (marked as 'signal' on the board)
GROUND = connect with the outer shield of the cable (marked as '-' on the board)
Important Note: some PCs and laptops have flexible 4-pin sockets and can work in both modes: headset and stereo/mono microphone. Please check your audio system settings carefully to decide which one you need. Some systems (like all Apple hardware) switch only then automatically into the headset mode if the detector is powered before plugging it in.
Further pinout drawings of typical TRS and TRRS connectors.