- Start soldering the smaller and continue with the bigger components.
- Start with the resistors and the ceramic capacitors. You can find the resistors value for each component in the BOM above.
- Solder the NPN transistors, the header pins, the buzzer, the terminal connectors, the potentiometer, and the electrolytic capacitors. Ensure that the electrolytic capacitors are soldered with the correct polarity.
This is very important! If the heatsnik touches the pcb it could short the L7809 output with ground.
- Screw the heatsinks on the IRLZ44 and the L7809 before soldering them.
- Use a spacer (like a coin) to have some clearance between the heatsink and the pcb. You could add a piece of electrical tape below the heatsink to make sure it won’t short the regulator’s output.
- Make sure that there are no shorts on your board.
- Use a multimeter in continuity mode to verify everything has been soldered correctly.
- Connect your power supply (18-24V) on J1 and measure the voltage on Vout pin. It should be very close to 9V.
Do not continue if you don’t have 9V on Vout!
- Put a jumper between Vout and 7-12V and measure the voltage on the top right of the arduino pins.
You should read the same voltage.
- Remove the power supply and solder the arduino board.
A good practice is to solder it a little higher. This will allow you to remove it easily using a cutter instead of desoldering each pin, in case it got damaged.
- Connect again the power supply.
You should see the green led on the arduino to be on.
- Measure the voltage on 5V line (for example pin 8 of opamp).
It should be close to 5V. Do not continue if it’s not.
- Remove the power supply again and solder the opamp. Optionally add a ceramic capacitor between pins 8-4 on opamp.
- Prepare the LCD for soldering. It’s a good idea to add some electrical tape behind the LCD module. Screw 4 M3 screws with 2 nuts as spacers behind the LCD and solder the LCD module on the board.
Be carefull, the LCD is soldered on the other side of the board with the rotary encoder.
How power distribution works:
Right behind the U1 pads there are 3 pads to solder another terminal connector (J3). They are very close so you won’t be able to solder both. This terminal is optional. The reason is to be able to select how you will power the 5V components on the board (arduino, LCD, opamp, buzzer). Using a terminal connector on J3 you can use an external 5V power supply or a step down module instead of the L7809.
By soldering a L7809 you get 9V on Vout and powering the Arduino with 9V on Vin line. The arduino’s 5V voltage regulator powers up all the components need 5V (LCD, opamp, buzzer).
To achieve this you have to put a jumper on “Voltage Select”. The middle pin (Vout) is the output of the voltage regulator (L7809) you soldered on U1 or the power source on J3. If you used a L7809 put the jumper between Vout and 7-12V. That connects the Vout (9V) with arduino’s Vin line. Arduino’s Vin cannot exceed 12V so be carefull not to exceed that when you put the jumper. If you chose to use a L7805 on U1 (not recommended) or a 5V power supply on J3, put the jumper between Vout and 5V. That connects the Vout with 5V line on all the components (arduino, LCD, opamp, buzzer) and arduino’s regulator is not used anymore.
Flashing the firmware
- Remove the usb cable/programmer and connect the jumper again.
- Connect the power supply. The LCD should turn on but probably you won’t see any characters.
- Start to rotate the potentiometer to adjust the contrast until you see characters on the screen.
Connecting the iron with the board
- Connect the heater wires on IRON + and – (J2).
- Connect the thermistor wires on PTC and GND (J4)
- On REED (J4) you can connect a reed switch that gets grounded when the iron is on the base. Read below.
Add a switch that gets grounded when you put your iron on its stand. That could be a physical or a magnetic switch. In my example I will show you how to connect a reed switch inside your iron.
You need a switch that could fit in your iron. Solder it between ground and another wire that’s not used by your iron.
Usually the 907 irons have 5 wires. If your iron doesn’t have any other wires available for the reed switch you could free up one by using a single wire for “GND” & “thermistor GND”. Don’t mix any of the heater wires with others. That way the 5 wires in your iron will be connected like this:
1. heater +
2. heater –
3. thermistor – & reed switch GND & GND
4. thermistor +
5. reed switch
That switch is connected on the pcb on REED pin. That pin is pulled high when the switch is open and gets grounded when the switch is closed (near a magnet).
Put a strong magnet on your stand that’s close to the reed switch when you leave your iron on it. The REED pin should read LOW when you leave your iron on the stand. That way the station knows when you don’t use it.