TEMP209

1. Project Overview

TEMP209 is a compact, open-source digital temperature controller designed for the ATmega microcontroller family (ATmega8 / 88P / 328). It reads temperature from a DS18B20 sensor, displays it on a multiplexed 2-digit 7-segment display, and manages thermal and user-defined outputs.

The project is written in BASCOM-AVR and released for educational and open-source purposes.

Figure 1: TEMP209 overview

Key Specifications:

Temperature Range: 0°C to 99°C
Sensor: DS18B20 (1-Wire protocol)
Display: Multiplexed 2-digit 7-segment
Remote Control: RC5 infrared protocol
Outputs: Out1 (reserved), Temp_out (thermal control)
Safety: Fault input (PORTC.3) — can be disabled or given 1–9 s delay
Storage: EEPROM for settings and data logging
MCU Support: ATmega8, ATmega88P, ATmega328 — separate firmware for each

2. Design Philosophy

Core question: There are countless temperature controllers on the market. What makes TEMP209 different?

TEMP209 uses a simple thermal controller to demonstrate alternative design choices.

❓ Why no physical buttons?

On ATmega8 and ATmega88P, the 8 KB flash is fully used — there is no room left for button-handling code, and there aren't enough free pins either.

On ATmega328, physical buttons can be added using ADC (analog keypad on a single pin).

In this design, however, the preference is toward remote + Bluetooth control — not because buttons are impossible, but because it avoids the difficulty of button placement, wiring, and mechanical design. A ready-made, elegant, optimized IR remote offers clean remote control with zero mechanical complexity:

The user can apply settings without approaching the controller — comfortably from anywhere.
Example: adjusting AC temperature from the couch.
Removing physical buttons → cleaner, cheaper, more robust design.

In the end, this is a matter of need and taste. Don't follow instructions blindly — decide based on your own requirements.

3. Key Features

🌡️ Temperature Measurement

DS18B20 sensor with 1-Wire protocol
Range: 0°C to 99°C
1°C resolution
Updated every 1 second

🖥️ Display

Multiplexed 2-digit 7-segment display
10-level brightness control
Special characters: P, b, d, F, H, L

🎛️ IR Remote Control

RC5 protocol
Full control without touching the device
Supports multi-command remotes
Any RC5 remote can be defined

🔌 Outputs

Temp_out (PORTC.4): thermal control relay output
Out1 (PORTC.5): user relay — reserved
Buzzer (PORTC.0): alert buzzer

🌡️ Control Modes

Heating (Heat): relay ON when temp < Setpoint
Cooling (Cool): relay ON when temp > Setpoint + Delta
Hysteresis (Delta): prevents relay chatter
Protection Delay: protects compressor

🛡️ Safety & Fault

Fault input (PORTC.3) — configurable reaction delay
F=0 disables fault monitoring · F=1..9 adds 1–9 second delay
Displays blinking "FF" on fault
Sends Fault on serial
Sensor disconnection detection

💾 Memory & Logging

Settings stored in EEPROM
Temperature logged every 1 minute
Capacity: 480 samples (8 hours)
Retrieve with report or IR command

📡 Serial Communication

UART at 9600 baud — the standard default for common Bluetooth modules
Simple text commands
Live debug output
Mobile app integration (SoifGo)

🔋 Stability

Auto-save setpoint
Restores state after power loss
Compatible with 3 MCU models

4. MCU Compatibility

MCU	Flash	SRAM	EEPROM	Status
ATmega8	8 KB	1 KB	512 B	Supported
ATmega88P	8 KB	1 KB	512 B	Current source target
ATmega328	32 KB	2 KB	1 KB	Supported

Separate firmware for each MCU: A dedicated HEX file is provided for each of the three microcontrollers. Download the one that matches your chip and program it directly — no modification needed.

5. Pin Mapping

Function	Pin	Description
Seg1	PORTB.3	Tens digit enable
Seg2	PORTB.4	Units digit enable
A	PORTB.1	7-segment A
B	PORTB.2	7-segment B
C	PORTD.7	7-segment C
D	PORTB.7	7-segment D
E	PORTD.5	7-segment E
F	PORTB.5	7-segment F
G	PORTB.6	7-segment G
Dp	PORTD.6	Decimal point
Out1	PORTC.5	User relay — reserved
Temp_out	PORTC.4	Thermal control relay
Buzzer	PORTC.0	Buzzer
Fault	PORTC.3	Fault input
RC5 IR	PINB.0	IR receiver
1-Wire	PORTD.4	DS18B20 data
TX	PORTD.1	Serial debug output (9600 baud)
RX	PORTD.0	Serial command input (9600 baud)

Note: Out1 is a reserved, unused output. Wire it up and use it if needed.

Figure 2: TEMP209 pinout (ATmega8 / 88 / 328 — same DIP-28 layout)

6. 7-Segment Display

Display Type

TEMP209 uses a 2-digit 7-segment display as listed in the BOM. But don't worry if you don't have this exact part — any type works:

Common Cathode
Common Anode
Even with pin rearrangement

Why So Flexible?

There are no current-limiting resistors in the connections. Current is controlled purely by switching timing (multiplexing). So you can configure the pins any way you need.

Common Display Driving Methods

Displays are typically driven by:

Multiplexers
Interface ICs (e.g., 74HC595, MAX7219)

Why TEMP209 Connects Directly

Instead of using external driver ICs, TEMP209 connects the display directly to the MCU. This provides:

📏 Brevity — simpler circuit
💰 Lower cost
🎛️ More control
💡 Brightness adjustment (PWM-based)
🔀 Pin flexibility — works with different display types
🔋 Component savings
🎓 Educational value — most important of all

Pin Connections

Segment	MCU Pin	Digit Select	MCU Pin
A	PORTB.1	Seg1 (tens)	PORTB.3
B	PORTB.2	Seg2 (units)	PORTB.4
C	PORTD.7	Multiplexed switching
D	PORTB.7
E	PORTD.5
F	PORTB.5
G	PORTB.6
Dp	PORTD.6

Character Map

Value	Display	Meaning
0–9	0–9	Numeric digits
10	P	Protection Delay menu
11	b	Beep menu
12	d	Delta menu
13	F	Fault Delay menu
14	H	Heat/Cool menu
15	L	Light/Brightness menu

Figure 3: 2-digit 7-segment display

⚠️ If your display's pins differ: If your display's pin configuration differs from this layout, you must update the pin configuration in the source to match.

7. IR Remote Control (RC5)

TEMP209 uses the RC5 protocol with a 3-slot array structure.

RC5 Array Structure (from firmware):

Irtxt(1) = Remote Address — used for filtering (default: 3142)
Irtxt(2) = Command 1 — primary command
Irtxt(3) = Command 2 — secondary command (optional)

Why Two Commands?

Some RC5 remotes (especially HVAC/AC controls) transmit multiple parameters in a single keypress (e.g., temperature and fan speed). The 3-slot array captures and processes all transmitted data.

⚠️ Important: Not all remotes use this multi-command format. Verify your remote via serial monitoring.

Default Command Codes (from firmware)

Address	Command 1	Action
3142	735	Toggle Power
3142	2759	Toggle Out1
3142	3137	Report Temperature Log
3142	264	Toggle Menu
3142	22	Setpoint +1°C
3142	227	Setpoint −1°C

Menu navigation (when Menu = 1):

Command 1	Action
2958	Menu Up
1738	Menu Down
1825	Increment parameter
285	Decrement parameter

Any RC5 Remote Works

Worried you can't find the example remote? Don't be. Most remotes are RC5, and any remote can be defined. Simply:

Note the address and command in the Bluetooth terminal
Replace the values in the code
Compile and program

You likely have a few spare remotes lying around at home. In the market, especially cheap Bluetooth audio module remotes (like the one used in this project) cost almost nothing.

Pairing Procedure

Step 1: Capture Codes

Connect TX to serial terminal at 9600 baud and press a button. You'll see:

Address=3142
command1=735
command2=22

Step 2: Update Firmware

Replace values in the Ir_down subroutine:

If Irtxt(1) = "your_address" Then
    If Irtxt(2) = "your_command1" Then Call YourFunction()
    If Irtxt(3) = "your_command2" Then Call YourSecondaryFunction()

Sample RC5 remote

8. Serial Commands

9600 baud, 8N1 — the standard default for common Bluetooth modules.

Command	Function
`set_temp+`	Increase setpoint by 1°C
`set_temp-`	Decrease setpoint by 1°C
`power`	Toggle power state
`report`	Stream temperature log from EEPROM
`out1`	Toggle Out1 relay
`menu`	Toggle menu mode
`up`	Menu up
`down`	Menu down
`incr`	Increment menu parameter
`decr`	Decrement menu parameter

Serial Output Example:

Temp209
relay=1
temp=25
Address=3142
command1=735
command2=22
setpoint=26
Fault

Temperature Log Output (from report command):

The log starts with live=N — a reference index that points to the position of the newest sample — followed by raw temperature values (0–255), and ends with end.

live=240
25
26
253
24
27
25
...
end

The value 253 is a position marker (not the end of the log). There can be more than one 253 in the stream — the one closest to live is the valid marker. 255 means an empty/erased EEPROM cell and is shown like any other value.

9. Configuration Menu

Navigate using IR or serial commands. Parameters are stored in EEPROM.

🎮 Access & Navigation

Action	IR / SoifGo	Serial
Enter menu	Center button / ◉	`menu`
Up	▲	`up`
Down	▼	`down`
Increase value	▶	`incr`
Decrease value	◀	`decr`
Exit	◉ again	—
Auto-exit	After 8 seconds	—

⚙️ Parameters

P

Protection Delay

Protects compressor from rapid cycling

What: Mandatory delay between turning the relay off and back on.

Why: Prevents shock from fast power cycling. Compressors in ACs and fridges (especially low-back-pressure types) may be under pressure and fail to start if switched rapidly — leading to motor stall and damage without protection.

Range: 0 = disabled · 1-9 = minutes

Smart behavior: If power doesn't drop and the compressor stops due to temperature control, TEMP209 still counts the delay so the next start isn't delayed unnecessarily. Protection is always active.

L

Display Light

Brightness control

What: Adjusts brightness of the 2-digit display.

Why: Optimize power consumption, or increase brightness in well-lit environments for better visibility.

Range: 0 = lowest · 9 = highest

d

Delta Temperature

Hysteresis / temperature gap

What: Temperature gap between relay ON and OFF.

Why: Sometimes you need to store a bit of heat or cold, or avoid constant switching from rapid temperature changes. Delta lets the controller run a bit warmer or cooler than the setpoint.

Example: Setpoint = 25, Delta = 2

Heating: relay ON below 25, OFF above 27
Cooling: relay ON above 27, OFF below 25

Range: 0-9°C

F

Fault Delay

Reaction delay to fault input

What: Controls how the controller reacts to the fault input (PORTC.3).

Why: Motors with high inrush current may briefly trigger the fault input on startup. A small delay prevents false trips while still providing protection.

Range:

0 = fault monitoring disabled
1-9 = delay in seconds before reacting to the fault signal

Visual indication: When a fault occurs, the display shows "FF" blinking (eye-catching alert).

H

Heat / Cool Mode

Heater or cooler behavior

Range: 1 = Heating (behaves like a heater) · 0 = Cooling (fridge, AC, ventilation fan)

b

Beep / Buzzer

Alert volume

What: Buzzer volume for key feedback and sensor-failure alarm.

Range: 0 = off · 1-9 = increasing volume

10. Thermal Control Logic

The Heat_cool() subroutine manages automatic temperature regulation:

Heating Mode (H=1): Temp_out = 1 when Currenttemp < Settemp
Cooling Mode (H=0): Temp_out = 1 when Currenttemp > Settemp + Deltatemp
Protection Delay (P): Enforces a delay between cycles to protect compressors
Sensor Error Override: If Sensorerr = 1, the relay is forced OFF for safety
Fault Override: If fault input is active (after the configured Fault Delay), Temp_out = 0 and display shows blinking "FF"

11. SoifGo Mobile Interface

SoifGo is a mobile app that connects to TEMP209 via Bluetooth, giving you full control from your phone. The UI is customizable — you can modify or extend it as you wish.

Figure 5: SoifGo main screen

🔌 Bluetooth Connection

Pair your phone with the controller's Bluetooth module and connect. The module communicates at the standard default 9600 baud.

📺 Live Display (Top)

Live temperature — updated every second
Setpoint — current target temperature

🖥️ Terminal Panel

The most educational feature! When connected, pressing any remote button shows its address and command in the terminal:

Address=3142
command1=735
command2=22

Use this to discover codes for any remote, then replace them in your source, compile, and program.

Figure 6: Terminal showing remote codes

📊 Report Button & Temperature Graph

Press the Report button
The command is sent to the controller
The controller streams the latest log
Wait ~4 seconds
The 8-hour temperature graph appears

Graph features:

🔍 Zoom in the upper half
📍 Pan by dragging
📏 Cursor bars show time and temperature (lower half)

Figure 7: 8-hour temperature graph

🎮 Bottom Buttons (Mimic Remote)

Button	Function
Out1	Toggle reserved output
Power	Toggle device power
+1	Increase setpoint
-1	Decrease setpoint
Report	Request temperature log

◉ Center D-Pad (5 Buttons)

▲ Up
▼ Down
◀ Left (decrease)
▶ Right (increase)
◉ Center (enter / exit menu)

Figure 8: Menu D-Pad

SoifGo = virtual remote + code-discovery tool + graph viewer.

12. Fault & Safety

When the fault input on PORTC.3 receives a HIGH signal (2.7V–5V), and after the configured Fault Delay has passed:

Immediately sets Temp_out = 0
Sets Faultstatus = 1
Displays blinking "FF" on the 7-segment display
Sends Fault on serial

Fault clears when the signal returns LOW or the device is reset.

Fault Delay (F Parameter)

0 = fault monitoring disabled (no fault reaction)
1-9 = delay in seconds before reacting to the fault signal

This is useful for motors with high inrush current that may briefly trigger the fault input on startup, preventing false trips.

Hall Sensor Interface

The Fault input is typically driven by the Hall-sensor current comparator (see Section 14). Any over-current triggers a fault, cutting the relay for protection.

The choice of detection method is yours. You can either:

Use a UGN503 Hall sensor with a washer and 8-turn wire coil (the method used here), or
Use a ready-made ACS-series current sensor module (ACS712, ACS724, etc.)

Recommendation: The ACS series is the better choice — compact, high accuracy, and stable output.

⚠️ Critical Safety Warning:

Do NOT rely solely on the F (Fault) protection
Always add a proper miniature circuit breaker (MCB) to the load side
Add an inline glass fuse for the MCU power supply
These are your primary protection; the Fault input is a secondary safety net

13. Hardware

Two-Board Design

TEMP209 is designed as two separate boards:

Board 1: Display / control / sensor (controller)
Board 2: Motor / relay / power

Mounting Options

Integrated: Place both boards side by side in one enclosure
Split: One near the motor, one in the room or control panel

🛡️ Safety Clearance

On the relay board, sufficient physical spacing is provided between the mains AC section and the low-voltage MCU power section. Be careful when handling the board, and always observe electrical safety practices.

⚠️ Fusing Recommendation:

Load side: Miniature circuit breaker (MCB) matched to your load
MCU supply: Inline glass fuse (e.g., 500 mA)
Never operate without fuses

Figure 9: Board 1 — controller

Figure 10: Board 2 — power/relay

Figure 11: PCB layout

Figure 12: Schematic diagram

14. Current Sensing with Hall Sensor

The Challenge

Reading AC/DC current to detect over-current and trigger emergency shutdown.

Candidate Methods

Method	Pros	Cons
Rectification + capacitor	Simple	Loses useful signal, complex circuit
ADC sampling	Accurate	Heavy MCU load, large code
Comparator op-amp ⭐	Cheap, reliable, compact	Needs initial calibration

🏆 Selected Method: Comparator Op-Amp

Hardware Setup

Hall sensor sandwiched between two metal washers
8 turns of wire wound around it
Hall output with 2.56V offset

How It Works

AC current: Output oscillates sinusoidally at mains frequency
DC current: Level shifts above/below the 2.56V offset (depending on direction)

Figure 13: Hall sensor with wound wire

Calculations

Each 1 A of current → 60 mV at the op-amp output
4 A → 4 × 60 mV = 240 mV + 2.56 V = 2.80 V
5 A → 5 × 60 mV = 300 mV + 2.56 V = 2.86 V

Calibration Procedure:

Apply the expected maximum normal current (e.g., 4 A).
At 4 A, the Hall output reaches 2.80 V; the op-amp begins oscillation and the MCU Fault input is triggered.
The fault LED indicator should be OFF under normal current (e.g., 4 A) — turn the potentiometer until the LED turns off.
When an over-current occurs (e.g., above 4 A), the fault LED turns ON, the 7-segment display shows blinking "FF", and the relay cuts off.
The Fault message is also announced on the serial output.

Depending on the op-amp and circuit, fine-tuning may be needed — but one-time calibration is enough.

Figure 14: Comparator op-amp circuit

⚠️ Educational Note

This method (UGN503 + metal washer + wire coil) is educational and may introduce some design error.

Recommended: Use an ACS-series current sensor instead — cleaner and more reliable.

Both methods produce the same type of output (analog voltage proportional to current) — only the scaling values differ.

Main drawback of the UGN503 method: A UGN503 with a core and wire coil is more sensitive to nearby magnetic fields compared to an ACS sensor.

15. 8-Hour Temperature Logger

TEMP209 automatically logs one temperature sample every minute to EEPROM. The buffer works like a CCTV recorder — a 480-sample circular buffer that always holds the last 8 hours. The newest sample is always available, even after power loss.

How It Works

Every 1 minute: a temperature sample is written to EEPROM
Capacity: 480 samples = last 8 hours
Circular buffer: when full, the oldest sample is overwritten
Position marker: the value 253 is written right after the newest sample and travels with it
Persistent: samples survive power loss

Circular buffer + position marker (CCTV-style):

Each minute, the current temperature is written to the next slot, and 253 is written right after it.
The previous 253 is overwritten by the new temperature sample — so the marker always sits right after the newest temperature.
253 is a position marker (not the end of the log). It tells the reader where the newest sample is located in the circular buffer.
255 = empty / erased EEPROM cell (all bits = 1). It is displayed in the graph like any other value, so blank areas remain visible.

Viewing the Report

Method	How
Serial command	Send `report` via Bluetooth or cable
IR remote	Press the button coded `3137`

The full report is sent to the SoifGo app, where you can see the temperature graph on your phone. 📊

Report Format

The stream starts with live=N — the reference index of the newest sample — followed by raw temperature values (0–255), and ends with end.

live=240
25
26
253
24
27
25
...
end

253 is a position marker, not the end of the log. If more than one 253 appears in the stream, the reader uses live to pick the correct one (closest to live).

16. Debugging

Monitor serial output at 9600 baud, 8N1 — the standard default for common Bluetooth modules:

Temp209                    - Startup message
relay=1                    - Current relay state
temp=25                    - Current temperature
Address=3142               - Remote address
command1=735               - Primary command
command2=22                - Secondary command
setpoint=26                - Setpoint changed
power=1                    - Power state changed
Fault                      - Fault condition detected
live=240                   - Report start (reference index of newest sample)
25                         - Temperature sample
26                         - Temperature sample
253                        - Position marker
24                         - Temperature sample
end                        - End of report stream

Common Issues

Symptom	Possible Cause
Display shows "--"	Sensor disconnected or faulty
Display shows blinking "FF"	Fault input triggered (after Fault Delay)
No serial output	Wrong baud rate, TX not connected
IR not responding	Wrong remote codes — verify via terminal
Relay chatters	Delta too small — increase it
False fault trips on motor start	Increase Fault Delay (F parameter) to 1-9 seconds

17. Building & Programming

🔌 On-Board Programmer Socket

The board includes a programmer socket for easy development and in-circuit programming. You can use any standard AVR programmer, such as USBasp.

Note: Due to space and layout constraints, the on-board socket is a single-row 6-pin header, while the USBasp programmer uses a 2×3 (two-row, 6-pin) connector. To bridge the difference, use male-to-female jumper wires, or build your own socket adapter.

💾 Ready-to-Flash HEX Files

A dedicated HEX file is provided for each of the three supported microcontrollers (ATmega8, ATmega88P, ATmega328). Download the one that matches your chip and program it directly — no modification needed.

🔧 Not Familiar with BASCOM?

Don't worry. If you don't know BASCOM-AVR, simply paste the source code into an AI assistant and ask it to translate the code into a language you're familiar with (C, Arduino, MicroPython, etc.).

Recommended Workflow

Pick the HEX file that matches your MCU
Connect USBasp to the programmer socket (use jumper wires if needed)
Flash the MCU
Verify via serial terminal at 9600 baud

Advanced users who want to modify the source can open the BASCOM project, adjust remote codes or parameters, recompile, and program the resulting HEX.

Figure 15: On-board programmer socket

18. Downloads & Links

PCB and Schematic Files Atmega 328P

📦 Download All File (MEGA)

📑 Table of Contents

1. Project Overview

2. Design Philosophy

❓ Why no physical buttons?

3. Key Features

🌡️ Temperature Measurement

🖥️ Display

🎛️ IR Remote Control

🔌 Outputs

🌡️ Control Modes

🛡️ Safety & Fault

💾 Memory & Logging

📡 Serial Communication

🔋 Stability

4. MCU Compatibility

5. Pin Mapping

6. 7-Segment Display

Display Type

Why So Flexible?

Common Display Driving Methods

Why TEMP209 Connects Directly

Pin Connections

Character Map

7. IR Remote Control (RC5)

Why Two Commands?

Default Command Codes (from firmware)

Any RC5 Remote Works

Pairing Procedure

8. Serial Commands

9. Configuration Menu

🎮 Access & Navigation

⚙️ Parameters

10. Thermal Control Logic

11. SoifGo Mobile Interface

🔌 Bluetooth Connection

📺 Live Display (Top)

🖥️ Terminal Panel

📊 Report Button & Temperature Graph

🎮 Bottom Buttons (Mimic Remote)

◉ Center D-Pad (5 Buttons)

12. Fault & Safety

Fault Delay (F Parameter)

Hall Sensor Interface

13. Hardware

Two-Board Design

Mounting Options

🛡️ Safety Clearance

14. Current Sensing with Hall Sensor

The Challenge

Candidate Methods

🏆 Selected Method: Comparator Op-Amp

Hardware Setup

How It Works

Calculations

15. 8-Hour Temperature Logger

How It Works

Viewing the Report

Report Format

16. Debugging

Common Issues

17. Building & Programming

🔌 On-Board Programmer Socket

💾 Ready-to-Flash HEX Files

🔧 Not Familiar with BASCOM?

Recommended Workflow

18. Downloads & Links

PCB and Schematic Files Atmega 328P

🔗 External Links

19. Test Videos

🎬 Video 1: Programming & Flashing

🎬 Video 2: Setting Up a Different RC5 Remote (Example)

🎬 Video 3: Temperature Logging & SoifGo Connection

🎬 Video 4: Fault Potentiometer Calibration (Example)

20. Changelog

v1.0 — Initial Release

21. About & License

License

Credits