The 70-year-old, whose house is separated by just a narrow strip of land from the shop where the blast occurred, said she tried to warn workers of the imminent danger, but was ignored.
The workers proceeded to cut apart the bomb with a blowtorch, triggering a massive explosion with destroyed the scrap warehouse on Soi Lat Plakhao 72 in Bang Khen district.
"When I saw those men carrying that object, I immediately knew that it was a bomb. You could definitely see that it was a bomb," Mrs Shomchin told the Bangkok Post.
She said she saw four construction workers deliver a huge metal object to the scrap shop on the back of a pickup truck.
When she approached the truck to take a closer look at the item, she saw it was a 230-kilogramme bomb.
Mrs Shomchin said she told the men not to tamper with the object and to report it to police. But her warnings went unheeded, so she left the scene as quickly as possible.
She raced back to her house to inform her older sister, Pen Yingthong, 80, that a bomb was being carried into the junk shop next door. She asked her sister to immediately call the police.
But before Mrs Pen could reach for the phone, the bomb exploded.
Seven people were originally reported killed in the blast, but the discovery of the body of a three-year-old girl Thursday took the death toll to eight. The explosion also injured 20 and left surrounding homes badly damaged.
Investigators said the bomb dated back to World War II, when Allied forces flew bombing missions over Bangkok after Thailand's alliance with Japan.
"If I had not rushed to tell my elder sister, or if I had lingered around for three extra minutes, I would not be standing here right now," Mrs Shomchin said.
She said the worker who had cut the bomb was the husband of the junk shop owner. Both he and his son were killed in the blast.
The husband was a Cambodian immigrant, as were several of the other junk shop workers who also died.
The owner was not at the shop at the time of the blast, and has yet to be located.
Mrs Shomchin also said two of the construction workers who brought the bomb to the scrap shop died in the explosion.
"The site manager and another worker were standing near their pickup truck when the junk shop workers tried to cut the bomb open. The pickup was destroyed and the two bodies were found right next to it. Two other workers were lucky, because they walked over to my house to buy drinks and survived the explosion," she said.
Mrs Pen, known to locals as "Pa Toi", Thursday went to file a damage report at tents set up by Bang Khen police and district office officials to provide help to residents.
She had to take pictures of the damaged areas of her house and present them to police before she could file a damage report at the district office tent.
She said that as well as her ceiling being heavily damaged, all her windows had been shattered, and her mother, aged 99, had been injured by flying debris.
Another witness, Chuensuk Supattano, 70, said she would most likely have been killed had she been on the second floor of her house, next to the explosion site. The blast ripped apart much of her home's upper storey, but the ground level where she was staying at the time of the blast was left mostly undamaged.
She said she had lodged multiple complaints with authorities against the junk shop owner over pollution, but her complaints had never been investigated.