uart-relay-switch

Press a reset button or recycle the power of your board programmatically, with command-line interface remote switch!

A relay is an switch that operated by electricity. It could be controlled by a GPIO signal.

However, there is no GPIO available in most PCs. Instead, we have a USB to UART controller that can emulate one GPIO pin.

We can't control the state of the TX/RX manually. But RTS, Request To Send, is a software-controlled pin that could be used as a GPIO.

Prerequisites

1x USB to UART(TTL) converter
1x 5V relay
a few jumper wires

Wiring

                                  USB to UART
                                   converter
                          ┌─────────────────────────┐
                          │                     VCC ├──────────┐
   ┌────────────────┐     │        voltage          │          │        RELAY
   │                │     │        selector     GND ├───────┐  │  ┌───────────────┐          RESET
   │            ┌───┤     ├───┐                     │       │  └──┤ VCC        NO ├───x      SWITCH
   │            │ U │     │ U │      3V3        TXD ├───x   │     │               │       ┌──────────┐
   │      PC    │ S │=====│ S │    ┌─────┐          │       └─────┤ GND       COM ├───────┤          │
   │            │ B │     │ B │    │ VCC │      RXD ├───x         │               │       │          │
   │            └───┤     ├───┘    │     │          │       ┌─────┤ SIG        NC ├───────┤          │
   │                │     │        │ 5V  │      RTS ├───────┘     └───────────────┘       └──────────┘
   └────────────────┘     │        └─────┘          │
                          │                     CTS ├───x
                          └─────────────────────────┘

Note

To correctly power up the relay, the voltage selector at the UART converter must be set to 5V.
Since the state of the RTS pin is HIGH when idle, the relay will kept turned on. It will turned off only when the ON() function is called. This is why the switch is connected to the NC terminal instead of NO.
- Relay's NO and NC terminals are Normally Open and Normally Close.

Build

Implement your own switch logic in main.c and compile it with relay.c.

gcc relay.c main.c -o switch
gcc relay.c examples/push_return.c -o push-return # example

Then put the executable to somewhere in PATH.

Usage

./switch /dev/<your_uart_converter>
./push-return /dev/cu.usbmodem51850106861 # example

ON(ms) connects, and OFF(ms) disconnects the COM and the NC terminal of the relay electrically, for the duration of ms.

Examples

example/push_return.c: simple push-return(tact) switch press (ex. T2080RDB reset button)
example/push_toggle.c: push-toggle switch on&off (ex. IMA-FCC-T2080 power switch)