Arduino ISP Bootloader Module

The Arduino ISP Boodloader Module (ABM)is based on the bread board version mentioned here. There are others PCBs that are available that are are not very easy to use compared to this, unlike others this booard does not require an external power supply. The ABM has female header pins that supply power and connects to an Arduino Uno board as well or anything similar with the advantage of avoiding wire clutter and components on a bread board.


The Working/use is made simple. Connect the ABM board to the Arduino Board with the jumper wires and follow the below instruction also mentioned in the previous link.

Burning the Bootloader

If you have a new ATmega328 (or ATmega168), you’ll need to burn the bootloader onto it. You can do this using an Arduino board as an in-system program (ISP).

To burn the bootloader, follow these steps:

  1. Upload the ArduinoISP sketch onto your Arduino board. (You’ll need to select the board and serial port from the Tools menu that correspond to your board.)
  2. Wire up the Arduino board and ABM as shown in the diagram to the right (yet to be uploaded).
  3. Select “Arduino Duemilanove or Nano w/ ATmega328” from the Tools > Board menu.
  4. Run Tools > Burn Bootloader > w/ Arduino as ISP.
  5. Once the bootloader is burned to the new ATmega328 on the ABM the Rx and Tx would blink once.

You should only need to burn the bootloader once.


DIY Arduino Spot Welder

The ATmega328 based Spot Welder Microcontroller

Earlier when i did mini projects it was hard to solder the 18650 batteries that i had salvaged from old laptops. Not recommended by the manufacturers either. not all the cells in a laptop battery pack were bad, so why not take them out and make use of them to power some DIY stuffs like a power bank for example. They cost a lot when purchasing new ones from online stores. So the best way electrically connect them was to spot weld them using nickel strips.

One way to get it done is to salvage an old Microwave Oven Transformer (MOT) and  modify it to get approximately 400 to 600amps (the turns ratio is something you’ll have to figure out). Here the current has to be kept low. Generally reducing the voltage will also reduce the current here. I’ve used 3 turns at secondary with 10mm width flexible cable. That would give enough amps to weld. I’ve set the weld time from the pot to 60ms.


  • More turns with thicker cable would give more current
  • Less turns would decrease current.

The components used here are cheapest those that are readily available at any local store.Then comes a Microcontroller built on the ATMEL ATmega328 chip. This controls how long the weld is done. The weld time are measured in milliseconds. When spot welding it is common as well as important that there be two welds, Why ? because the first weld clears the strip of all impurities that maybe stuck to it after production. The Second weld is when the strips melt and gets welded to the cell. The chip is powered by a regulated 5v supply from an on-board PCB transformer.


  1. Surge Protection
  2. Inbuilt Power Supply
  3. Dual Pulse Welding
  4. Dual Surge protection
  5. Thermal fuse at the secondary
  6. Zero Cross Peak detection
  7. Variable timed pulse (indicated by LED from 100ms to 450ms)
  8. Dual function weld
    1. Manual Weld
    2. Continuous Weld (upon holding  weld button for more than 800ms)
  9. TVS protection diode at the dc side
  10. Snubber Circuit
  11. TRIAC to control AC load
  12. Audible Weld alert


PCB with soldered components

The Spot Welder Microcontroller is powered by a PCB transformer rated 2 x 9v 177mA. The PCB is protected by a TVS diode D13. This protects all the underlying components from surge without which would burn the components during a surge such as lightning. here the TVS diode suppress the spike in voltage when the threshold voltage is reached.

PCB powered on

Once switched on all the timing LEDs blinks once. The READY led indicates that it is ready for welding. The ATmega gets its clean power from the regulator IC LM7805 filtered by capacitors C9, C7 and C5. The ATmega continuously monitors the AC zero crossing signal from the Optoisolator H11AA1. The optoisolator sends out a pulse for every zero crossing of the AC signal and when the weld button is press, the ATmega sends out dual pulse to another optocoupler triggering the gate of the TRIAC BTA26.

The first Pulse duration is 500ms which prepares the tab for welding, the second pulse is when the actual weld happens. The TRIAC by nature switch’s off at every zero cross. Here a little late after zero crossing but that would not make a difference as it a few micro seconds. Since the load will be an MOT and thus highly inductive, it is very important that load is switched ON at the peak of the sine wave and not at zero. When the Weld button is pressed and held down for a period of 800ms, this helps when you have to weld several cells at a time to built a battery pack. The TRIAC conducts 6 times with a two second delay between each weld. The LEDs 50ms to 120ms reflects the value set by the potentiometer.

Working video can be viewed here.

Scope View

scope view

The above scope shows the following, respective to the numbers

  1. AC reference voltage (YELLOW)
  2. Zero Crossing ( LIGHT BLUE)
  3. Second pulse send from the Arduino to trigger the TRIAC ( PURPLE)
  4. Secondary wave from the MOT (BLUE)

With a little modification or as a complete replacement, this pcb can used for the cheap Chinese Models like the SUNKO 788H



I have the PCB’s on sale here. 

Sunko 708A Replacement:

One of the readers Giordano Cantori had been very nice to send me the pictures of his finished Sunko 708A Spot Welder after the stock board had failed. He had replaced it with the PCB described here with his own idea and design.




Giordano Cantori’s successful manual welds

Here is the built in action