How to control 12 V and 26 V coaxial relays through USB.

Good — that’s a nice setup for remotely switching antennas, receivers, or feed chains. Let’s outline how to get both coaxial relays (12 V and 26 V types) under PC control safely and flexibly.

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⚙️ Step 1: Electrical Characteristics

First, confirm the following for each relay:

• Type: SPDT, DPDT, latching or non-latching.
• Coil voltage: 12 V DC and 26 V DC.
• Coil current draw: typically 100–500 mA per coil.
• Control method: single coil or dual coil (set/reset).

You’ll need these specs to size the driver transistors or relay drivers.

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🧰 Step 2: Interface Hardware Options

Option A: USB relay controller module

Easiest route. Many commercial boards provide:

• 1–8 relay outputs controlled via USB serial commands (FTDI virtual COM port).
• Each relay output can switch 12 V @ 1 A easily.
• You can drive your coaxial relays through those, provided you add the correct supply voltages (12 V and 26 V).

For the 26 V relay, use the USB module relay to switch the 26 V supply (not provide it directly).

Examples:

• Numato 8-channel USB relay module
• SainSmart USB relay board
• KMTronic USB relay board

These are plug-and-play and scriptable in Python, C, or even via shell commands.

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Option B: Arduino or Raspberry Pi controller

If you prefer flexibility:

1. Use a microcontroller board (Arduino Nano, Raspberry Pi Pico, etc.) connected by USB.
2. Drive the relay coils via MOSFETs or Darlington arrays (e.g. ULN2803A).
3. Provide two DC power rails — 12 V and 26 V (from bench supplies or DC-DC converters).
4. Control via a simple serial protocol or Python script on the PC.

This allows integrated sequencing, indicators, and interlocks.

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Option C: RS-232/USB solid-state relay drivers

For high-isolation setups, you can buy USB/RS-232-controlled solid-state relay modules (e.g. Devantech USB-SR relay). Each output can handle up to 48 V DC, perfect for switching 12 V and 26 V loads directly.

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⚡ Step 3: Power Supply Arrangement

Voltage	Source suggestion
12 V	Common 12 V DC brick (2 A)
26 V	Adjustable DC bench PSU, or 24 V supply trimmed up to 26 V, or DC-DC boost from 12 V → 26 V

Both supplies should share common ground with the control electronics (if switching low side).

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🖥️ Step 4: Software Control

Once the USB interface appears as a serial port (e.g. /dev/ttyUSB0 or COM5), you can toggle relays via simple commands.

Example (Python):

import serial

ser = serial.Serial('COM5', 9600)

ser.write(b'relay on 1\n')   # energize 12V relay

ser.write(b'relay off 1\n')  # de-energize

ser.close()

For Arduino-based control, you can define ASCII commands or use a custom protocol.

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🧩 Step 5: Wiring concept

+12V  ──→ 12V relay coil ──→ NPN/MOSFET ──→ GND

+26V  ──→ 26V relay coil ──→ NPN/MOSFET ──→ GND

Flyback diodes across coils.

USB module output drives transistor base/gate.

Both coils can share ground with the logic interface.

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🛠️ Recommended Build (simple & robust)

• Controller: 4-channel USB relay board (~£15)
• Power: 12 V DC supply + 26 V DC supply
• Connections:
  • Relay 1 switches +12 V line to coil A.
  • Relay 2 switches +26 V line to coil B.
• Control: PC → USB COM port → relay board → coaxial relays.

No soldering to the computer, fully isolated, and scriptable.

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