About
Revision 1 was a 5 V heater ring: simple, effective, and happy on a Raspberry Pi or USB port. Revision 2 keeps the idea and adds the brains.
It accepts a wide 5–24 V input, so it runs from a USB supply on the patio or a 12 or 24 V observatory supply. Constant-current drive means the heater delivers the same power whatever the supply voltage, so there's no re-tuning when you move the rig. Your controller sets how hard it works with a PWM input, and an onboard thermostat flags when the dome is getting too cold or too hot.
The result is a clear dome until dawn, and a usable all-night record of meteors, clouds and whatever else the sky gets up to.
Connections
| Pin | Signal | Notes |
|---|---|---|
| 1 | PWM | Input, default high (heating) |
| 2 | HOT | Open-drain, active low |
| 3 | VCC | 5–24 V DC supply |
| 4 | COLD | Open-drain, active low |
| 5 | GND | Ground |
Constant-current drive
The heater is an array of 49 resistors of 300 Ω each, all in parallel. The onboard driver holds the voltage across that array at 4.13 V, whatever you feed the board between 5 and 24 V. Heater power is P = V² / R, and with V fixed, P is fixed too.
The math:
- Array resistance: 300 Ω ÷ 49 ≈ 6.12 Ω
- Heater power: 4.13² ÷ 6.12 ≈ 2.8 W
- Per resistor: 4.13 V ÷ 300 Ω ≈ 13.8 mA, or about 57 mW each, so the heat is spread evenly around the ring
Revision 1 heated by whatever voltage you gave it, so 5.5 V instead of 5 V meant about 20 % more heat. Revision 2 doesn't care.
PWM input
The PWM input defaults to on: with nothing connected, the heater runs at its full 2.8 W. Wire it to your controller to dial the heat down, or leave it unconnected for a simple always-on dome heater.
Jumper-selectable undervoltage lockout
A jumper sets the undervoltage lockout (UVLO) to suit your supply, so the heater stops before it flattens a battery. The heater turns on above the ON voltage and off below the OFF voltage.
| Mode / jumper | ON, typical | OFF, typical |
|---|---|---|
| Open | 4.48 V | 3.65 V |
| 1–2 / SLA | 12.52 V | 10.96 V |
| 2–3 / LiFePO4 | 12.91 V | 11.32 V |
Thermostat outputs
Two thermostat outputs report the board temperature to your controller. Each asserts LOW at one temperature and releases HIGH a few degrees later, so it doesn't chatter around the threshold. Both are open-drain, so they need a pull-up to your system's logic level (3.3 V or 5 V, for example). An external resistor works, or use your microcontroller's internal pull-ups where it has them. A Raspberry Pi's GPIO pins have software-enabled pull-ups of roughly 50 kΩ to 3.3 V (in Python, gpiozero's Button(pin, pull_up=True) turns one on). On a Pi, keep any pull-up to 3.3 V, never 5 V, because its GPIO isn't 5 V tolerant.
| Output | Asserts LOW | Releases HIGH |
|---|---|---|
| HOT (pin 2, thermostat OUTA) | Warming above 60 °C | Cooling below 55 °C |
| COLD (pin 4, thermostat OUTB) | Cooling below 25 °C | Warming above 30 °C |
Hardware over-temperature cut-out: join HOT (pin 2) to PWM (pin 1). Above 60 °C, HOT pulls PWM low and the heater switches off; once the board cools below 55 °C, HOT releases and your PWM signal takes over again. It works with no software at all, which also makes it a good backstop when the software is yours and still in progress. Otherwise, the board itself doesn't act on the thermostat; that's your controller's job.
Built to survive the field
Overvoltage and short-circuit protection are built in, so a pinched cable at 2 a.m. doesn't have to be the end of the night.
KEY FEATURES
- 5–24 V input
- 2.8 W heating, constant-current drive
- PWM input with onboard power driver
- Thermostat outputs at 25 °C and 60 °C
- Jumper-selectable undervoltage lockout
- Overvoltage and short-circuit protection
- Still round, still with a hole in the middle
QUICK SPECIFICATIONS