This ATmega8 project is a useful way to learn temperature sensing, PWM, multiplexed displays and PID-style control. It combines an LM358 thermocouple amplifier, an IRF540N heater switch, a 24 V supply and a three-digit display. It is not a universal controller for every “Hakko 907” handle: identify the exact handle’s sensor, heater and grounding before connecting it. The original design dates to May 5, 2016, and its stated temperature range of about 25–350 °C is a firmware mapping, not a verified accuracy or safety rating. See the original project and its files.
Check handle compatibility before building
This is the most important design decision. The original project calls for a Hakko-style clone with a thermocouple. A similar connector or “907-style” label does not establish that a handle has the right sensor or heater.
In contrast, Hakko’s official 900S/907/908-family manual specifies a 24 V AC, 50 W ceramic heater and a sensor resistance of approximately 43–58 Ω at room temperature. It also specifies tip-to-ground resistance below 2 Ω and tip-to-ground potential under 2 mV, with 0.6 mV given as a typical value. These characteristics do not establish compatibility with the project’s thermocouple amplifier and DC MOSFET heater driver. Check the Hakko manual.
Before wiring any handle, consult its documentation and use a meter to identify the pins and verify the following while it is disconnected from the controller:
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- Heater voltage, current and whether it is intended for AC, DC or both.
- Sensor type—thermocouple, thermistor, RTD or another device—and sensor polarity where applicable.
- Connector pinout, heater-to-sensor isolation, and tip-to-ground continuity or resistance.
- Whether the handle provides a protective earth or ESD connection, and how it must be connected.
A thermocouple amplifier will not correctly read a resistive sensor such as a thermistor. Do not connect a genuine 907-family handle or an undocumented clone on the strength of its connector shape alone.
How the ATmega8 station works
The circuit uses the sensor signal to estimate temperature, reads the desired temperature from a potentiometer, and adjusts heater power through PWM. The original project describes approximately 120× amplification of the thermocouple signal with an LM358. Its amplified output goes to ATmega8 ADC0; a 10 kΩ setpoint potentiometer feeds ADC1.
The firmware uses PID-style control: proportional action responds to present error, integral action corrects accumulated error, and derivative action responds to the rate of change. The project uses Brett Beauregard’s Arduino PID library and switches between more aggressive and more conservative parameter sets. This is hobby-grade control, not a published accuracy or response-time guarantee. Results depend on the sensor and amplifier, ADC reference, heater and tip construction, thermal load, PWM behavior, noise and calibration.
An ATmega8 PWM output drives an IRF540N MOSFET, which switches heater power. The original build uses a multiplexed three-digit common-anode seven-segment display. A common-cathode display or different segment pinout requires matching circuit and firmware changes; it is not a plug-in substitute.
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Original parts and electrical requirements
The following is the original article’s BOM and should be treated as a starting point, not a validated production specification for altered handles or boards.
| Reference | Part or value | Count |
|---|---|---|
| IC1 | ATmega8-P | 1 |
| U1 | LM358 | 1 |
| Q1 | IRF540N | 1 |
| R4 | 120 kΩ | 1 |
| R3, R6 | 1 kΩ | 2 |
| R1, R5 | 10 kΩ | 2 |
| C3, C4, C7 | 100 nF | 3 |
| Y1 | 16 MHz crystal | 1 |
| C1, C2 | 22 pF | 2 |
| R2 | 100 Ω | 1 |
| U2 | LM7805 | 1 |
| C5, C6 | 100 µF or lower | 2 |
| R7–R14 | 150 Ω | 8 |
The remaining build items include the verified compatible iron, a 24 V supply, a 10 kΩ potentiometer, five-pin connector, PCB, switch, headers, wiring, case, display and AVR programmer. The original author recommends a 24 V, 2 A supply. That is a nominal 48 W budget; it is not a guarantee that the supply can run every nominally 50 W handle. Check the actual heater’s current demand, startup behavior, supply’s continuous and transient rating, wiring and connector capacity.
The supply also feeds a 5 V LM7805 regulator for the MCU and analog circuit. Check regulator dissipation and provide suitable heat sinking or thermal design. The IRF540N is not an ideal logic-level MOSFET at low gate voltage: verify its gate drive, voltage drop and temperature under load instead of assuming it will run cool. Use adequately rated conductors for the supply-to-board and MOSFET-to-heater paths, as the original project advises.
Assemble and test in stages
- Document the handle first. Confirm pinout, sensor and heater specifications, grounding and compatibility with the DC switching circuit.
- Inspect the design files. Obtain the schematic and PCB files from the original project page, inspect the schematic, and confirm the board matches the parts and handle you intend to use. The available information does not establish that the downloads are currently complete or tested with a current IDE.
- Build the low-voltage board. Check component values and polarity, solder joints and supply paths. Place the crystal capacitors and decoupling capacitors close to the MCU and relevant ICs.
- Check for shorts before power. Inspect the board and measure between supply rails. Do not connect the iron or mains wiring during these checks.
- Test the 5 V rail by itself. Confirm its voltage and check for regulator overheating before installing or powering the MCU circuitry further.
- Program the MCU and test the display. Verify that the display lights correctly and that the ADC inputs produce plausible readings before connecting a heater.
- Wire the verified handle. Recheck connector orientation and ground or ESD wiring against the handle documentation. Do not rely on wire colors alone.
- Test heater switching cautiously. Begin at low duty cycle, measure heater current, and monitor the MOSFET and wiring for excessive temperature. Stop if the supply current is abnormal, the MOSFET heats rapidly or the displayed temperature is implausible.
- Calibrate, then enclose the unit. Verify temperature with an appropriate tip thermometer. Add a correctly rated fuse and switch, secure strain relief, ventilation, insulation and separation between mains and low-voltage wiring.
Program the ATmega8
Use a dedicated AVR ISP programmer
Connect the programmer to the MCU’s +5 V, ground, MISO, MOSI, SCK and RESET signals, observing the pinout and orientation. Compile the firmware for the actual MCU and clock configuration, then program it through the ISP tool. The project specifies a 16 MHz crystal; a mismatched clock or fuse configuration can prevent reliable operation.
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Use an Arduino as ISP
- Connect an Arduino Uno or Nano to the computer and open the Arduino IDE’s File → Examples → 11.ArduinoISP → ArduinoISP example, where that example is available.
- Upload the ArduinoISP sketch to the Arduino.
- Wire the Arduino’s SPI and reset signals to the corresponding ATmega8 pins, along with the required power and ground. Check the board pinout before connecting.
- Select Tools → Programmer → Arduino as ISP, then use Sketch → Upload Using Programmer with the correct target selected.
The 2016 article flags an ATmega8 issue with Arduino IDE versions later than 1.6.0 and points to additional instructions. Treat that as a historical warning, not a guarantee about current IDE menus, board definitions or library compatibility. Current compilation and upload success are not established here. Confirm the selected MCU is ATmega8—not ATmega8A, ATmega168 or ATmega328P—and do not change fuse bits casually.
If the programmer cannot see the chip, first verify VCC and ground, RESET, and MOSI/MISO/SCK orientation; use short ISP wiring. Check the crystal and capacitors and confirm the configured clock source. If the MCU was set to use an external clock and no longer responds, a temporary clock source may be needed for fuse recovery. Do not guess at fuse values.
Calibrate temperature readings
The original firmware maps ADC input to a nominal 25–350 °C range using map(Input, 0, 510, 25, 350). This is a scaling assumption, not proof that a particular handle reaches or accurately measures that range. The original calibration approach is to measure the hot tip with a thermocouple-equipped multimeter or tip thermometer, compare that measurement with the station display, change the ADC-to-temperature mapping in firmware, reflash and repeat.
- Use a suitable tip thermometer and let the iron stabilize before comparing readings.
- Adjust the mapping only after confirming the sensor and amplifier are operating normally and the ADC is not saturated.
- Check more than one setpoint if you need useful performance across a range. A one-point correction does not establish accuracy elsewhere.
- Repeat calibration after changing the handle, heating element or tip. Tip geometry, sensor placement and soldering load affect the relationship between sensor and tip temperature.
A contact probe may not read the same temperature as the tip during active heating or soldering. A displayed number is an estimate derived from the sensor and calibration; it is not automatically the tip’s true temperature. Hakko’s manual also recommends recalibration after changing the iron, heating element or tip and identifies a tip thermometer as the measurement tool.
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Enclosure and electrical safety
The handle may operate at low voltage, but the power supply connects to mains. Use an enclosed, appropriately certified supply with adequate current capability, and keep mains wiring physically separated from the low-voltage control board. A casually 3D-printed case is not, by itself, a safe mains enclosure.
- Use a properly rated fuse and switch, insulated terminals and secure strain relief.
- Maintain protective earth where required; do not treat the handle’s earth or ESD connection as an optional signal wire.
- Keep mains conductors and exposed terminals away from the PCB’s low-voltage area, and do not route exposed mains traces across a hobby control board.
- Provide ventilation for the supply, LM7805 and MOSFET as their measured temperatures require.
- With power disconnected, test the handle’s grounding arrangement and wiring. If you cannot verify isolation, earth continuity or safe enclosure construction, do not put the station into service.
Troubleshoot by symptom
The display is blank, reversed or ghosting
Check the display’s common-anode or common-cathode type, segment pinout, resistor placement and firmware assumptions. Confirm the 5 V rail and MCU clock before debugging display code.
The display works, but temperature is fixed or implausible
Check that the handle actually has the sensor type expected by the circuit, that its pins are correct, and that the amplifier output remains within the MCU’s ADC range. Excessive gain, op-amp offset or a disconnected or mismatched sensor can saturate the ADC and make readings appear stuck.
The heater does not warm, or remains on
With the handle disconnected, check wiring, supply voltage and MOSFET gate drive; then confirm PWM operation and heater current under controlled conditions. If the heater stays on regardless of setpoint, disconnect power rather than relying on firmware to stop it. Investigate a shorted MOSFET, wiring fault or control-output fault before trying again.
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The MOSFET or wiring gets hot
Stop the test and disconnect power. Measure heater current and check the MOSFET’s gate voltage and drain-source voltage under load. Excessive dissipation can result from insufficient gate drive, an unsuitable heater or undersized wiring.
Temperature readings jump or control oscillates
Look for loose connector contacts, sensor wiring noise from PWM switching, unstable 5 V or ADC reference, poor grounding, op-amp offset and airflow or changing soldering load. PID tuning cannot compensate for a wrong sensor type or unreliable electrical connections.
The programmer cannot detect the MCU
Recheck supply and ground, RESET, SPI wiring, selected MCU, clock and programmer setting. Avoid repeatedly writing fuse bits without understanding the clock configuration; an external-clock fuse setting can make ordinary ISP access fail until a clock is supplied.
Should you build this station?
Build it if your main goal is to learn analog sensing, ADC use, PWM, display multiplexing and PID control, and you have a verified compatible handle plus the tools to debug and enclose the circuit safely. The ATmega8 design offers customization and repairability, but handle identification, board fabrication, firmware compatibility and calibration are part of the work.
Choose a supported commercial station if you need dependable daily use, documented grounding and ESD performance, predictable calibration, manufacturer support or less electrical risk. Consider a newer DIY controller if you specifically want modern integrated-heater cartridges, standby or sleep features, motion sensing, OLED output or USB-C power. A Hackaday build illustrates another route: using a ready-made digital controller module with a 907-style handpiece rather than designing the controller from scratch; it does not establish universal handle compatibility. See that alternative project.
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