For our Powered Para-Glider (PPG), and our flying for the Election Commission, we had a Lynx Headset and Helmet, the PTT system, the dual jack connectors, a set of VHF radios, and……the charger for the Lynx Headset.
The Lynx headset has an internal circuitry that amplifies the received audio signals from the radio set, while also buffering, and effectively lowering the microphone’s impedance for use with the VHF RT set. This requires power, and power is sourced from batteries, and these batteries are intended to be periodically charged by a charger.
And if you lose the charger, all is lost. Well, almost.
I took up the challenge of getting Nikolai’s Lynx Micro headset up and running, with these form words, “1. You’ll lose your warranty. 2. You stand a 5% chance of your headset not working at all". I had researched a bit on the Lynx Headset, and I felt I could bring his headsets back to life, considering he felt crippled without his charger.
So upon joining Nikolai today, and after a long wonderful drive up a part of west coast of India, with police escort and our trailer carrying the Powered Para Glider, I sat down to getting the Lynx Headset operated.
The left ear cup: the LM386 based circuit, and four wires running to the headband: the Green is Headphone signal (that runs to the other cup), the Blue is system ground, the Red connects to the +ve terminal of the 5 battery pack (and provides power to the circuit), The Yellow seems to have no function!
Nothing in the right ear cup!
We first pulled out the ear seal, speaker cover, and the speaker assembly from the right headset. There is no electronics in that ear cup save a 16 ohm headphone. We then ripped open the seal, cover, and speaker assembly from the left ear cup. We found the electronics inside, cantered around the popular LM386 audio IC. The PCB is soldered to the volume potentiometer, and cannot be taken out without de-soldering the leads of the potentiometer. Wires run into the cup from the circular, 7 pin female connector. Four wires run to the headband, and two to the microphone. With nothing else in the cup, the batteries had to be in the headband.
Nikolai carefully stripping off the headband cover.
We found the batteries!
Nikolai carefully stripped open the headband, and upon delicate surgery, we discovered five (5) odd shaped batteries. The Batteries are VARTA V450HR: NiMH batteries, each 1.2V and a capacity of 450mAh.
5 numbers of VARTA V450HR NiMH batteries inside!
After an electrical continuity check, we determined the five batteries to be in series. We thus concluded the system runs off 6 volt.
I now was faced with two options: Either disconnect the batteries and draw a line out of the headset, to a voltage regulator that feeds of the PPG’s on board 12V battery. Or, I could attempt recharging the headset, and risk more. I chose the second one.
The datasheet for the Vartha V450HR states multiple charging currents: 45mA (0.1C) requiring 14-16hr of charging, or an accelerated charging current of 135mA (3C) for a 4hr charge. A fast charge current of 450mA (10C) has been mentioned, but I believe that will damage the battery quickly.
We traced the leads from the series “battery pack" that lies in the headband of the headset, and identified the two pins in the 7 pin female connector responsible for charging. I next took a 15V 2A switching AC adapter (you could use a rectified step down transformer as well), and built a simple constant voltage linear DC regulator around the LM317, with a 10k multiturn potentiometer from Bourns that serves as the feedback control with which the output voltage can be set.
We built and tested the LM317 based charger!
The pins of the 7 pin female connector of the headset that are responsible for charging the batteries: Red is +ve, Black is ground.
The LM317 based “charger” consists only of the LM317, a heat sink, and 10k Bourns multi-turn potentiometer. Don’t exclaim an “Ouch”, Yes, the PCB cracked when I was cutting it out from the larger general purpose PCB, but I didn’t have sufficient components to build a new one. Getting road-rollers is a task; forget electronic components!
The output of the LM317 was set to 7V: the minimum it could achieve with no load. Upon plugging the output of the LM317 circuit to the headset connector, the voltage dropped to 1.5V. Slowly, the potentiometer’s screw head was turned, until the voltage (measured with a simple digital multimeter) increased to about 5V, with the circuit assumed to be “charging" the headset. We switched the multimeter to the current mode, and measured the charge current, which read “0mA", as expected. Slowly, we turned the potentiometer even further, until the current started increasing. At around 7VDC, the charge current was read at 100mA, which slowly fell as the batteries in the headset charged. We were pleased, the headset was responding!
After 30 seconds of charge, we disconnected the headset from the handmade charger, and plugged it into the radios. The Lynx headset was back to life! Satisfied, we’ve connected the handmade charger to the headset, and expect it to be sufficiently charged in 5 hours.
Mind you, all this was made on the go on an election campaign: and we are damn pleased with the thought of having RT communications restored!
Quick Reverse Engineering, minimal electronic tools, no repair manuals, on the go: all for seamless flight operations: Raw flying and raw engineering, at probably their best!