This project is a DIY six-channel stereo speaker selector based on STC15W201 MCU. The purpose of this electronic switch is to share a pair of speakers between six stereo audio systems. This is an improved and low-cost version of the 6-channel speaker selector which we developed a few years ago.

This new speaker selector uses STC15W201 MCU. This MCU is a low-cost 8051 MCU designed by STC micro. At the time of this writing, the STC15W201 can obtain for less than US$ 0.8. To drive the seven segments and to control the relays, this design uses the popular 74HC595 8-bit shift register ICs.

To select each speaker channel, we use pair of 12V DPDT relays. These relays interface to 74HC595 through a ULN2001D 3-channel Darlington transistor array IC. This 3-channel Darlington driver IC is functionally equivalent to the famous 8-channel ULN2003 IC. This ULN2001D version is an 8-pin IC and is available in both DIP-8 and SOP-8 packages.

This new board is designed to work with a 12V DC power supply and in our prototype build, we use a 12V/5A SMPS unit to power the whole system.

To get the optimal results with this circuit, use an Omron G2R-2-12DC or an equivalent type of relay with a coil current lower than 100mA.

The firmware of the STC15W201S is developed using SDCC. To flash the MCU, we use stcgal. Thanks to the built-in bootloader, we can program this MCU using a generally available 5V USB to UART module/dongle.

The complete build process of the speaker selector is shown in the video below:

The functionality of the firmware is almost identical to the old PIC16F88 firmware. After powering up the system, the active speaker channel is displayed on the seven-segment display. To switch to a different speaker channel, press the button repeatedly with short intervals.

To mute or disconnect the current speaker channel, press and hold the same button for more than 3 seconds.

As described above, all functions of this circuit can be the control using a single push switch.

To reduce the cost and minimize the front panel complexity, we suggest using the dot indicator of the seven-segment display as a mute indicator. The front panel wiring layouts are available here.

This project is an open-source hardware project. All its design files, schematics, and firmware source codes are available at Github.com.

The PCB design, schematic, and other design files of this project are covered with a CERN-OHL-W 2.0 license. Firmware source code is released under the terms of the MIT license.