Nexa Receiver and Transmitter

Generic 433 MHz transmitter

The AudioProtocolTransmitter module, can encode digital protocols as audio signals from the sound card/audio output from a PC. To be able to transmit the signals, some kind of hardware is needed. This is a description of how to build a generic RF-transmitter working on the 433MHz band.

Step 1, Signal Level

The first step is to transform the low level AC output signal from the sound card to a digital signal strong enough to drive a transmitter. This is done with a simple OP-Amp circuit as follows:

Transmitter schematics

The OP-Amp can be any general purpose type which can handle single power supply and high power output.

Step 2, Transmitter

The easiest way to build the transmitter is to modify an existing one. I took an ordinary Nexa remote control, disconnected its logic chip and hooked up the output of the OP-Amp to the transmitter stage of the remote control.

This is how the remote control looks (to the right):

Nexa Receiver and Transmitter

Disconnecting the logic chip requires a steady hand, here are the steps:

Locate the data output

Locate the signal output pin from the chip. This is where the digital protocol is sent to the RF-transmitter stage.

 Output pin

Disconnect the output pin

Cut the track on the board from the output pin and the pin next to it, remove the surface mounted capacitor. This is a bit tricky, because the tracks are small…

 Disconnect output pin

Connect our signal

The final step is to connect our digital output signal (from the OP-Amp) to the RF-transmitter. I used the hole in the PCB as guide and soldered it to the connecting pad of capacitor I removed:

 Connect our signal to the transmitter

Now any output from the sound card will be transmitted as a 433MHz signal, and we have a generic remote control transmitter!

Step 3, Package

There are many ways to hook this up to the computer. I used a cheap USB-headset converter as sound card, put the OP-Amp-circuit on a Veroboard and powered it from the USB-cable and finally put the whole pile into the box of an old WLAN-HUB. It looks ugly inside, but once I close the box I have a nice looking USB-connected generic 433MHz transmitter :-).

 Ready transmitter

The circuit board to the right of the Veroboard is actually a receiver I tried to put in the same box. I had to give that up, since it is to sensitive to the electromagnetic noise from the USB. I had to use the UPM-recevier connected via a few meter cable to get away from the noise.

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