Making a Lynx Micro Headset Charger on the Go

For our powered paraglider (PPG) and for our flying for the Election Commission, we carried a Lynx headset and helmet, a push to talk system, dual jack connectors, a set of VHF radios, and a charger for the Lynx headset. Then the charger was lost, and without it the headset was useless. Almost.

The problem

The Lynx headset has its own circuit. It amplifies the audio that comes from the radio, and it buffers the microphone and lowers its impedance so it can work with a VHF radio. That needs power, which comes from batteries inside the headset, and those batteries need a charger from time to time.

I took on the job of reviving Nikolai's Lynx Micro headset. He felt crippled without his charger. I gave him two warnings first: that he would lose the warranty, and that there was a 5 percent chance the headset would not work at all. I had read up on the Lynx before, and I believed I could bring it back. I started work after a long and pleasant drive along part of India's west coast, with a police escort and our trailer carrying the paraglider.

Opening it up

We pulled the ear seal, the speaker cover and the speaker from the right ear cup. It held nothing electronic except a 16 ohm headphone. The left cup was different. Inside was a circuit built around the common LM386 audio chip. Its board is soldered to the volume control, so it cannot come out without unsoldering that part. A round 7 pin female connector feeds wires into the cup. Four wires run to the headband and two go to the microphone:

• Green carries the headphone signal to the other cup.
• Blue is system ground.
• Red goes to the positive end of the battery pack and powers the circuit.
• Yellow seemed to do nothing.

Since the cup held no batteries, they had to be in the headband. Nikolai carefully stripped the headband cover, and we found five odd shaped batteries. They were VARTA V450HR nickel metal hydride cells, each 1.2 V and 450 mAh. A continuity check showed they were wired in series, so the headset ran on 6 volts.

Choosing a fix

There were two options. One was to disconnect the batteries and run a line from the headset to a voltage regulator fed by the paraglider's 12 V battery. The other was to try to recharge the headset and accept more risk. I chose to recharge.

The datasheet gave several charge rates. A current of 45 mA (0.1C) needs 14 to 16 hours. A faster 135 mA (3C) needs about 4 hours. A 450 mA (10C) rate is also mentioned, but I believed it would ruin the cells quickly.

The charger

We traced the wires from the battery pack to the two pins of the 7 pin connector that handle charging. Red is positive and black is ground. For power I used a 15 V, 2 A switching adapter (a rectified step down transformer would also work). Around it I built a simple constant voltage linear regulator using an LM317, a heat sink and a 10k Bourns multi turn potentiometer, which sets the output voltage by feedback. The small board cracked when I cut it from a larger one, but I had no parts to make another. Parts are hard to find on the road.

With no load the output could go no lower than 7 V. When I plugged it into the headset, the voltage dropped to 1.5 V. I turned the potentiometer slowly until a multimeter showed about 5 V. Then I switched the meter to current mode. It read 0 mA, as expected. I kept turning until the current began to rise. At about 7 V DC the charge current was 100 mA, and it fell slowly as the batteries filled.

The result

After just 30 seconds of charging we plugged the headset into the radios, and it worked. We then reconnected the handmade charger and expected a full charge in about 5 hours. All of this was done on the go during an election campaign, and we were very pleased to have radio communication back.

It was quick reverse engineering with few tools and no repair manual, done to keep flying operations smooth. Raw flying and raw engineering at their best.

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