This kit has been around for years, using a BA1404 single IC to produce a weak stereo FM signal. Just noticed that Jaycar still sell these with a revised PCB in a runout sale. So if you want one better get it now.

It doesn't have a PLL or any kind of crystal locking, so it tends to drift a little, especially if the antenna loading changes.

Originally it comes powered by a single AA battery, which produces poor modulation, poor battery life, and a very weak signal. Changing to a 5 Volt mains-powered source fixes all these issues.

I found amongst my junk an already-completed kit, but not attached to its box. So I drilled holes in the box for 2 RCA sockets for the audio, and a TV PAL connector for the FM signal output. Mounted the PCB in the box and soldered up the sockets.

Next was to construct a power supply. Again from the junk box I found a metal box with a power cord attached. Inside the box was a transformer, 2 diodes, and a 1000μF capacitor, producing 7.9 volts . I found a 7805A plastic-tab regulator, so wired this in together with a 470μF electro on the output. The meter couldn't decide if the output was 4.99 or 5.00 volts, close enough.

Wired the power supply to the transmitter, and now there is plenty of power in the house, and excellent modulation. Outside though the signal quickly drops away, not reaching any of the boundary fences.

It makes good stereo sound, which is the main thing. I would have liked proper frequency locking, but one can't have everything.

https://www.jaycar.com.au/short-circuits-three-project-minimitter-fm-transmitter/p/KJ8114