On market there are lots of options for sound card-based data modems, like Signalink, Digirig and lots of DIY and cheap modems. I haven’t seen any Bluetooth based audio data modem apart from Mobilinkd which is unfortunately Bluetooth serial port TNC.
In order to keep the cost to minimum I got together components I already had just had to order the microcontroller as I didn’t had any small form factor microcontroller. One reason for the project internal looks so crude.
I’ll be working on a proper PCB design but as I don’t have any experience designing surface mounted level PCB will be looking for some expert help. Might switch SMD to Through hole components.
Blew is the basic circuit which I had in mind but many changes were made wile making the modem.
Power:
Initially, I had planed to use 3.7V BMS to charge the battery and was hoping it would have internal 5V DC boost circuit. but I had to switch to a old power bank BMS circuit with modified USB C board. I had to solder a push button in parallel with BMS push button. Out put had a STSP slider switch to cut off the power to the rest of the circuit.
Power out from the switch was solder to a PCB patch to serve as Power bus to connect rest of the components power.
BT002 Bluetooth Module:
BT002 Bluetooth module is a cheap module with audio in/out and few other controls. I only used +5V, Ground, Mic, Audio Left, Audio Right.
Microcontroller:
I had to order a small form factor microcontroller, I went with Chinese Microcontroller “Pro Micro” It can be programmed in Arduino IDE as Arduino Leonardo. I needed only 2 pins 1 as input and 1 as out put, in future I’ll add a 10K Potentiometer to add programable PTT trigger delay. right now it is done while programming the microcontroller.
*No External LED was used as displayed in the circuit above.
Relay module:
I went with relay based PTT for few reasons, not all radios like Transistor/ Mosfet based PTT. Using Relay based PTT make it much simple. I used CW-025 5VDC Relay module.
Modem to Radio connection:
To keep all simple I used RJ-45 Port to connect Radio to Modem. It easy to make RJ-45 based cables then using any special connector.
Audio I/O and PTT:
For Audio I/O from bluetooth and radio I used following configuration:
Blutooth Audio Left goes to radio mic via 10uF capacitor.
-Bluetooth Audio Left goes to Radio mic input via 10uF capacitor.
-Bluetooth Audio right goes to Pro Micro using Voltage divider via 10uf capaitor.
-Bluetooh Mic in is connected to Radio’s audio out via 10uF capacitor.
Pro Micro Code:
simple code is written in Arduino IDE. Audio level is monitors on analogue input pin. Once the audio level meets the threshold PTT is triggered. I added 1 second delay after the audio goes below threshold before the PTT switched off.
Code:
// Blue Dmod // // https://uhpowerup.com/ // // Author: 2E0UMR // // use at your own risk // const int audioPin = A1; // Audio input pin const int RPin = A3; // Relay pin const int threshold = 200; // Adjust based on your audio signal bool RState = false; // Tracks Relay state unsigned long offTimer = 0; // Tracks time to turn off the Relay const int offDelay = 1000; // 3-second delay in milliseconds void setup() { pinMode(RPin, OUTPUT); Serial.begin(9600); } void loop() { int audioValue = analogRead(audioPin); // Read audio signal Serial.println(audioValue); // Debugging: Print the value if (audioValue > threshold) { // Audio signal exceeds threshold digitalWrite(RPin, HIGH); // Turn Relay on immediately RState = true; // Update Relay state offTimer = 0; // Reset off timer } else if (RState && offTimer == 0) { // Start the 3-second timer offTimer = millis(); // Record the current time } if (RState && offTimer > 0 && millis() - offTimer >= offDelay) { digitalWrite(RPin, LOW); // Turn Relay off after 3 seconds RState = false; // Update Relay state offTimer = 0; // Reset off timer } }
I have tested the “Blue Dmod” with Wouxun KG-UV9D, Wouxun KG-UV9K and TCA/PRC 152. On TCA/PRC 152 I had to add a ferrite core on high power.
Below are some images of “BlueDmod”. Please note it was a crud build so it doesn’t look professional.
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