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NFC Microphone Kit ("21c lite")

$125 USD | To purchase, contact: ear2sky at oldbird dot org

 

All parts shown above are included except the batteries and the white cable. The user supplies the batteries (6 x AA) and the length of cable needed for the span between the mic and their listening/recording station.

INTRODUCTION

This is a DIY microphone system for tuning into avian nocturnal flight calls (nfcs). The improvement from the former Old Bird DIY design is in the simplicity of construction (no soldering), the fact that the electronics are supplied in the kit, and that it has much better functionality for detecting nfcs (better signal to noise ratio). Like the old design, the user is responsible for obtaining the microphone housing & audio cable, and building the microphone. Once the parts are in hand, construction takes about half an hour.

PARTS

Kit includes the following parts (shown above):

Microphone element mounted in acrylic pyramid

Preamp circuit board

Battery connector cable with housing for 6 AA batteries, which power mic for 3-4 months

Signal out connector cable (gray cable above)

Terminal-block 3.5 mm (1/8 in) miniplug connector

Screws & twist-on electrical connectors

Suggested parts for user to acquire (<$20 USD):

~2-gallon paint bucket or similar (recommend white or light color to minimize heat absorbtion; e.g., Lowe's Item #954439)

~32 oz. tall container or similar (e.g., typical large yogurt container)

Wire for transmitting signal. 2-conductor is needed for mono. 4-conductor round telephone wire is durable & economical for outdoor use (e.g., Lowe's Item #303015 or #303007) - for one microphone, just use the black and red conductors for ground and signal respectively.

 

CONSTRUCTION

Step 1: Before inserting circuitry into containers, review how the circuit board and wires are to be connected in the picture below. The circuit board has three 2-pin male connectors, each inscribed with its function and + sign by one of the pins. The PWR port receives the 2-pin female from the battery pack, the MIC port receives the 2-pin female connector from the mic element mounted in the pyramid, and the OUT port receives the 2-pin female connector from the signal out cable, which connects to the audio cable running to the listening/recording station. The reds wire in all three 2-pin female connectors goes to the + terminal of the male 2-pin connector on the circuit board. Be careful with the 2-pin connectors as they are delicate and repeated bending may result in the wires breaking.

 

Step 2: Mount the circuit board in the small plastic container ~ 3+ inches up from the bottom. Use an exacto knife or drill to make two small holes in synch with two diagonal holes on the circuit board. Orient the circuit board so that the 2-pin PWR connector is on the low side, and use the screws provided to secure the circuit board in place. Once the circuit board is mounted, gently connect the battery pack's 2-pin connector (red wire side to +) to the PWR port and rest the pack at the bottom of the container - use tape or such to secure the pack from moving. Next, make a small hole with an exacto knife or drill midway down the side of the container with a 90-180 degrees from the circuit board - this is where the audio cable from the listening/recording station enters the mic. Prepare the end of the audio cable by removing ~1/2 inch of the plastic cover on the two wires to be connected to the ground and signal. Then thread the end of the cable through the hole at least 12 inches then thread the cable through the hole about 12 and twist the wires together with the corresponding exposed wires on the kit's short connector cable. Use the twist-on electrical connectors to secure the connection, then pull the excess audio cable back out the side until the twist-on connectors meet with the hole. Now, connect the 2-pin connector for the conjoined audio cable to the OUT port on the circuit board, making sure the red wire goes to the positive. Finally in this stage, connect the 2-pin connector from the mic element mounted in the acrylic pyramid to the MIC port (red marked side to +), letting the pyramid then rest atop the container.

 

Step 3: Drape a piece of plastic food wrap over the pyramid so that it hangs half way down the container all the way around. Tape one side of wrap to side of container, and then on the opposite side pull the wrap taught and tape that side down - continue all the way around, being careful not to stretch the wrap, just pull it taught. The idea is to form a taught waterproof membrane over the top of the pyramid and to prevent water from entering the chamber of the container so that the electronics are protected.

Step 4: Drape a piece of thin nylon cloth (e.g., translucent curtain) over the mouth of the bucket and secure in place with rubberband (e.g., make a chain of 3-4 office-sized 1/4 inchers) and pull taught.

Step 5: Finally, at the listening/recording station end of the audio cable, strip the plastic cover off latter 1/2 inch of the wires in the interior of the cable - the corresponding wires that lead to the black (ground) and red (signal) wires on the 2-pin connector. In the included screw-in miniplug connector, unscrew the rightmost screw and the leftmost or center screw, then slide the ground wire in to the rightmost port and the signal wire into the which ever of the other two was unscrewed and then tighten both screws  Your mic is now ready to test!