Throat Microphone for bicycle mobile

I wanted to try using a throat microphone while operating the radio mobile on a bicycle. The big problem is wind noise on any conventional mic, but a throat mic is a contact microphone which picks up the vibrations directly through the neck.
Apparently they are often used by security guards and the like for discreet radio communications because they can be hidden under a collar.
I bought one of these.
https://www.amazon.co.uk/gp/product/B075D6PFPD

The two-pin jack plug connects to my TYT MD380 DMR radio, so it was quite easy to try out. The Ashford repeater GB7AS features an echo facility where you can talk for a few seconds, and then the repeater plays back a recording of your transmission. I found that the audio was quite muffled, but was best when I moved the microphone as high up the neck as possible.

But I also noticed another problem. On high power, there was a "puttering" noise on the transmission, which seems to be the radio frequency signal getting into the microphone and causing interference. The noise went away on low power, but was very noticeable on high power, where-ever I positioned the radio antenna.

Undaunted, I took the equipment for a 12 mile cycle ride up onto the North Downs. I was unable to raise anyone on the Charing repeater GB3CK (using analogue transmission), but I did manage to get a signal report from Peter GW6YMS on Anglesea in Wales on DMR, via GB7AS.
"Unintelligible"  was his verdict, and "there's terrible motor-boating on your transmission" - which didn't go away even when I switched to low power. I gave up and talked to him using the microphone in the radio.

The earpiece part of the device does work quite well, although it tends to fall out of my ear after a mile or two. It is quite clear to listen to, private and comfortable to use - and I can still hear traffic coming up behind quite well too.

Curiosity got the better of me and I opened up the microphone and the little "junction box" in the cable, to see what was inside.

 You can see that the "microphone" is just a very small "piezo bender". This seems to be connected directly to the microphone input of the radio. There is a soft pad of foam behind the bender, and a small piece of blank fibreglass PCB. The wiring looks pretty horrible, with joints insulated by little dobs of hot-melt glue.

The junction box is equally ugly inside with more blobby joints and hot-melt glue.

It was very inexpensive.

I might try to make a boom microphone like I use in the car, with an electret capsule in a wind shield of some kind. I could keep the earphone part of this and try and improve the RF shielding.

73 Hugh M0WYE

Sealed Lead-acid Battery Charge Circuit for Jump Starter

On the left is a "Jump Starter" which contains a sealed lead-acid battery and originally had two big cables with crocodile clips to help start car engines when the car battery is flat. I never used it for that, I took off the big leads and added a fused cable to power my FT847 radio for portable operation. Not only does the case make the battery safe and easy to carry, but it also has a built in volt meter and an emergency lamp. On the right hand side there is a "cigarette lighter" type accessory socket to power other items or smaller radios.

The Jump Starter came with a mains adaptor to charge the battery via a power socket underneath the meter.

This is the weak-point of the system, because the adaptor is unregulated and when the battery is fully charged, the voltage reaches over 20 Volts. This has almost certainly caused the premature failure of the original battery, because lead-acid batteries should not be charged to a voltage higher than 14.4 Volts.
If the Voltage is regulated to 13.8V then the battery can be "float-charged" continuously.

There is plenty of room inside the box, and I have wanted to install a Voltage regulator for a long time. But I discovered that I could buy (very cheap!) a little regulator board from ebay.
https://www.ebay.co.uk/itm/LM317-Adjustable-Converter-Voltage-Regulator-Module-Variable-Power-Supply-DC-DC/263896453817

The module came with a piddly little heatsink, but, as the regulator is close to the edge of the board, it is easy to add extra aluminium.

There are just four connections: input +/- (on the left) and output +/- (on the right). The voltage can be adjusted with the multi-turn preset at the bottom of the picture. It is wise to twiddle the adjustment screw to get 13.8 Volts before connecting the battery.

While adding extra heatsinking, I also added a smear of heatsink compound - as there was none on the device.

Now purists will complain that there should really be a current limit incorporated into the charge circuit, because a very discharged battery will draw a high current when charged. And this is true, however the existing circuit includes a one-ohm, high power resistor in series, and also the mains adaptor is rated at 500mA, and simply can't supply too much current for a lead-acid battery of this size. Whether the mains adaptor is really adequate is another matter.

Here is the circuit of the Jump Starter. The fused cable to the radio is connected straight across the battery terminals.

Hopefully the replacement battery will last longer now, although it is a smaller capacity than the original, which was claimed to be 12Ah. I say "claimed to be" as it weighed less than the 7Ah battery that I replaced it with and therefore probably had less lead in it!

Now the meter reads 13.8 Volts when the battery is charging.
This is good.

There is a data sheet for the LM317 regulator IC here:
http://www.ti.com/lit/ds/slvs044y/slvs044y.pdf

The internals are a spaghetti of wires, but it is all well insulated :-).

The LM317 with a piece of scrap aluminium as a heatsink, board is mounted top right.

If you want to be scientific, then the "waste heat" that the regulator has to get rid of is the difference between the input and output voltage multiplied by the current flowing through the regulator. Commercial heatsinks are specified with "degrees per watt" which tells you the amount that the temperature of the heatsink will rise when it is dissipating 1 Watt and can be multiplied by the wasted power to work out whether you can fry eggs on it.

Or you can leave it on for a bit and feel (carefully) whether it has got warm or not. In my case not.
73
Hugh M0WYE

PAM8403 Amplifier

I have some old audio-visual speakers, that were once used on a stand at exhibitions. They are quite battered, but the little 3" drive units seem ok. I thought I might make a pair of book-shelf speakers using them - sort of "PC speakers" that are a bit better quality than my old Sony ones.
I needed a little amp to drive them and spotted various amplifier modules on Ebay and Amazon. The speakers are rated at 3W so this PAM8403 module, at 3W per channel, seemed very suitable. PAM8403 Module from Ebay.

The module is tiny. It runs off 5V and has a stereo volume control with an on/off switch. There are input, output and power connections and that's it.
I hooked it up to a bench power supply, and a pair of the A-V speakers. I connected the headphone jack of my tablet to the input and was straight away listening to music at good volume level.
With everything turned up max, the music was very loud, but not distorting, so I am not sure that I had enough drive level to get the the full 3W. There was about 200mV going in when I looked on the scope.

Of course when I moved the scope to the output all I saw was a big blur! This is a "Class D" amplifier, that works using Pulse Width Modulation.

This is what I see on the speaker output with no signal going in. Here the timebase is set to 1uS per division, and the vertical scale is 2V/division. So this waveform is at 240kHz and is 8V peak to peak.

I guess this is going to radiate like crazy, and if I use this amplifier it may be best to use one module in each speaker and keep the speaker leads really short. Lots of filtering on the power-supply too.

The quiescent current was about 20mA and even when the volume was at maximum it was only drawing about 60mA. So it is incredibly efficient - 90% efficiency if the data is to be believed.

I found the datasheet for the chip here: https://www.diodes.com/assets/Datasheets/PAM8403.pdf

Anyway the amp module looks great for testing out the speakers. I can decide whether to carry on using it later.
73
Hugh M0WYE