Christopher Dooley, 40, used neighbour Joanne Dixon as a shoulder to cry on when his girlfriend Christine Cooke, 31, died suddenly after collapsing with a blood clot – leaving him to raise eight children alone.
However, Miss Dixon became concerned Dooley was becoming obsessed with her when he showered her with expensive gifts, got a t-shirt made her photo on it and installed a tracking app on a mobile phone he had bought her.
Preston Crown Court heard that when Dooley discovered Miss Dixon had a new boyfriend, he stuffed nails and gunpowder from dismantled fireworks inside a plastic bottle, and left it in her garden in Ribbleton, Preston, in July.
The bomb was found by Miss Dixon’s three-year-old son Levigh who picked it up, not knowing it could have killed him.
Mum-of-eight Miss Dixon said: “I didn’t know what it was at first, until the powder came out. Then we noticed the tiny nails attached to it.
“It was the hottest day of the year, and we have been told if the sun had have hit it the heat may have been enough on its own to set it off.”
Dooley’s cleaner, Stacey Finch, said she had seen him pouring black powder out of fireworks in his kitchen the previous day.
Miss Dixon confronted Dooley who denied all knowledge of the device, saying he had been at home with the children all night.
But when she checked her phone Miss Dixon saw she had received a text message from Dooley at 3am, saying: “Just had a walk round to yours. Shame that.”
Explosives experts concluded the was a viable nail bomb and had the sun hit the device it could have exploded, killing Levigh instantly.
Dooley, of Slaidburn Road, Ribbleton, pleaded guilty to making an explosive and was jailed for three-and-a-half years.
Recorder of Preston Judge Anthony Russell QC told Dooley: “Whatever the relationship was, it was your connection with this lady and what has been described as an obsession with her that led you to commit this offence.
“Fortunately, the device did not explode. Had it done so the nails would have been propelled around and would have been likely to cause very serious injury, acting like shrapnel.
“It seems to me that your misuse of drugs and alcohol are a serious problem that you need to address and they must also have played a part in this offence.
“The fact that the device contained nails, that it was placed in the garden of a dwelling and that it was found by children are all factors which make this offence a serious one.”
Dooley told to the Evening Post in August 2011 that, following his partner’s death, his friends had been his support network. He said: “I nearly fell apart four months after she died, I was a bit of a mess but my friends were brilliant, they helped me round.
“I miss (Christine) every day but I have got some brilliant friends and my family have been fantastic.”