Another great crystal oscillator circuit with BB139, a 2-nd harmonic quartz crystal and one BF214 transistor. So, the Q quartz is half of the desired transmitter frequency, L1 has 24 turns, 0.08mm Ø ...

The project described in this article is a ready-made item from a $2.00 shop! And it is made in China! It is a wireless doorbell with a cost of about $10.00. It is so cheap, you could not build it ...

Fig 1. This Pierce-series resonant oscillator can reliably start at supply voltages as low as 2.4 V. It also draws very low current, ensuring long battery life. Fig 2. An oscilloscope photo shows the ...