31 January 2021

QYT KT-8900 12V on the mic socket and more noise

Alright. I found a spot to get 12V inside the QYT KT-8900, which is pin 5 of the audio amplifier IC. It is located near the entrance of the DC cord. I bought it to pin 1 of the MIC socket, which is unused in this radio: that's where you can push the audio output for Packet/APRS (the documented mods mentioned in the previous post).

The result is that now the buzz is present both on transmit AND receive.

The oscilloscope explained everything. The wireless dongle generates digital spikes on its power supply line. Since it is designed to operate from the internal battery, nobody cares if there are 100mV 500Hz spikes.

Solution: add 47-100uF electrolytic capacitor on the power supply line of the dongle.

Thirty seconds after finding the solution I burnt the receiving dongle. (Hint: capacitors retain the charge.) Actually the LDO inside went short circuit. I removed and bypassed it, but while 3.3V to the dongle do power it up, it does not pair with the headset and shuts down after 10 seconds.

Considering that I am not operating /M on a daily basis as in pre-pandemic times I am now thinking of possible evolutions or restoring the same setup.

30 January 2021

Better 8V on QYT KT-8900 microphone socket

I want to simplify my mobile radio operations so that I do not have to switch on the wireless microphone receiver. In the car I use a [insert_your_adjective(s)_here] QYT KT-8900 VHF/UHF mobile, that I have described in the past. It outputs 8V on the microphone socket, so I wanted to use it to power the microphone receiver dongle bypassing the embedded battery.

I built a simple adapter with a 78L05 + the required capacitors + dropping diodes, but the voltage drop was too high when the dongle was powered through it. Then I checked online and a mod is suggested in order to get real 8V on pin 2 of the mike socket, which translates in adding a 10 ohm resistor between a point in the front panel board and the socket pin, like this:

How it is supposed to be done. But adds noise!

Well, I do get stable 8V out of the radio body, but now the transmitted audio has a strong buzz. The dongle drains about 35 mA at 4V.

I think the transceiver does not like when someone messes with the 8V line, as I had discovered when tracing the buzz on the received audio (see the first link above). Next move is to bring 12V out through the unused "pin 1" on the microphone socket. Fingers crossed (that nothing melts in the process!).

25 January 2021

Getting ready for some freeform electronics

Three sizes of brass wire.

Through hack-a-day stories and events I got to know Mohit Bhoite's freeform electronic projects (link goes to his Twitter page). He builds working circuits "in the air" as a form of visual art. He is not the only one on Earth publishing freeform electronics circuits, but he has a tendency to build "squared" shapes that better suit my current artistic abilities.

In order to get started I needed the wires that act both as conductors and supports. Mohit uses brass wires in 20 and 22 AWG sizes. That translates to 0.8 mm and 1.0 mm diameter in metric units. While I was at it I also bought 1.5 mm diameter brass, just in case I will need to support something heavier that a couple of ICs and a dozen of LEDs.

My first project will be a solar-powered super-capacitor LED flasher that promises to recharge even during short Winter days (already gone-by for Winter 2020-21).



22 January 2021

DMM Input Impedance

I am working on a very low power LED blinker and the need of measuring currents in the range of microamps (uA) has opened me new challenges in the world of lab instruments.

Once you can measure milliVolts, just let the uA current flow into a large-ish resistor (100k, 1M) and measure the voltage across it and apply Ohm's law. Then you remember that your instrument has a finite impedance and it might influence your readings. So, how to estimate a DMM/DVM input impedance? Or how to confirm what is written in the accompanying leaflet? 

In this experiment I have been using an ANENG AN8002 DMM, because it is small and uses 2xAAA cells instead of a PP3 9V battery. It should have 10 Mohm input impedance in DC Volts range.

In order to confirm or calculate the desired value, build a resistive voltage divider say, with a 10 Mohm resistor like this:

+V ------- 10 Mohm -------- DVM -------- GND

Let +V be a known voltage value (measure it in advance using the same DVM) and read what the DVM display says. The inverse formula says:

Rdvm = Vdvm*10 / (V - Vdvm)  [Mohm]

Using my values I get 11 Mohm of input impedance in the >2V range. I do have measured the 10 Mohm resistor with several instruments and they all agree on its value.

Since the AN8002 is an autoranging instrument, the input impedance may change with the auto-set range. And it does indeed, but it is always around 10 Mohm.

Do I need a microAmp meter? Maybe not yet.

Oh, by the way, the blinker takes 0.4 uA (that's 400 nA) when OFF. I have repeated the math several times and taken several measurements in different ways, but they do agree! It runs 24 hours on a 1F 5V capacitor ... if you don't mess with it with a DMM :)