Wednesday, July 15, 2015

2. The schematics / circuit diagram

I have managed to finish drawing the schematics for the soldering station. The circuit diagram has 2 parts, one is the power board and the other is the logic board.

The inputs for the power board are:
  • The 3 outputs from the transformer: 12-0-12v AC
  • The "heater command" line. When this signal is high, the optocoupler will turn the BT137 triac on, letting current flow to the heater coil
 The outputs are the following:
  • GND - this is the common ground
  • +5 volts for the microcontroller and the LCD screen
  • +9 volts - the positive rail of the OP07 op-amp
  • -9 volts - the negative rail of the OP07 op-amp
  • The 2 connections for the soldering iron coil: HT1 and HT2
Diagram 1: The power board

The second diagram is for the logic board which contains the microcontroller and the TC amplifier. It has the following inputs:

  • The GND, +5, +9 and -9v DC signals from the power board
  • The 4 buttons that will be used for adjustment: Stand-by, Up, Down and Preset
  • The 2 wires from the thermocouple installed in the tip of the soldering iron

 and the following outputs:
  • Heater command signal (HCM)
  • Piezo buzzer signal (BUZ)
  • Status led signal
  • The LCD power and data signals

Diagram 2: The logic board

A few details regarding the LCD

Most 16 x 2 LCDs sold today have a LED backlight which must be powered by the user. The problem is that they are produced by hundreds of manufacturers.

As a result, some LCDs have different power requirements, some consuming more than others. To avoid destroying them (replacing the led is a pain) you will need to consult the datasheet of your LCD and find out the correct voltage and current for the backlight. After finding the values, you can use the formula below to calculate the backlight current-limiting resistor:

ResistorOhms = (5 - LedForwardVoltage) / (LedRatedCurrent x 0.8)

For example, the screen I bought (PC1602A) has a forward voltage of 4.2 volts and a maximum forward current of 195 mA according to its datasheet. Replacing the numbers in the formula gives us (5 - 4.2) / (0.195 x 0.8) which is equal to 5.1 ohms (a standard resistor value, otherwise rounding to the nearest value would be necessary).

Instead of this simple way of powering the backlight, PWM could be employed, but that would just add complexity and no real benefit.

In conclusion, everything looks good (at least on paper). The voltage regulators don't get hot at all, neither does the triac. Generally speaking, the circuit is stable. I'll keep testing it and if everything is OK, a PCB will be created and presented in the next post.


  1. HYE, can i know what is the function of the both variable resistor?

    1. Hi. RA is for getting 99 kilo ohm with higher accuracy. This is used for the thermocouple amplifier, to get 100 gain. The formula is: (RA + RB) / RB which equals to 99k + 1k / 1k which results in gain 100.

      R4 is used for setting the contrast for the LCD screen.

  2. Hey, You could tell me how to get temperature from ADC