Back then when I was looking for new ways of controlling a model railroad, I remembered using  DTMF for remote control, combined with a low-cost Arduino microcontroller, I was all set to use it for controlling a model railroad.

You might have heard those sounds from your mobile phone when pressing number keys on the dialer. Each key is a unique combination of frequencies which is recognized by one of these:

This is a DTMF decoder module for using with DIY projects. It receives audio signals and checks if the incoming audio contains any recognizable sound. If there is, it turns the four of its output pins to digital high in a combination depending on the decoded number. 

Using a microcontroller to interpret the combination, we can assign a task for each button press. For example, the button '2' can be used to increase the track voltage, '1' and '3' can be used to switch a turnout, etc.

What is the use of yellow wire connected to the board?

One problem I encountered in this project that through the 4 digital outputs of the decoder, there is no way to let the microcontroller know when the button is released. Luckily, most of the decoders like this one have an on-board LED which lights up whenever the decoder receives a DTMF signal. Since there was no pin connected to it, a wire had to be soldered to connect it to the microcontroller's input pin to let it 'know' when was the button pressed and released.

How to make it wireless?

After figuring out the controls, in order for the decoder to receive the DTMF signals, it had to be wired up with the headphone jack of the mobile phone running a DTMF tone generator app. To solve this problem, I got a low-cost Bluetooth audio receiver from a local electronics store.

This had an aux audio output that could directly be connected to the DTMF decoder's input.

You can get the full documentation, including the parts list, source code, and instructions here.

All the best!

For future

Being a quite old project, this still uses the Adafruit's discontinued Motor Shield V1. In future versions, I will be going to add more features and improve the design.