The difference between this circuit and the previous one is that it uses PNP transistors. The circuit is from Radio Electronics magazine from July 1968. The frequency depends on R1 and C1.
This circuit is from a July 1970 Radio Electronics magazine. The sensors are metal probes. The 18 V...
This commercial medium and short-wave radio was manufactured in Japan in the 60s. The circuit has...
General Electric suggests this circuit in an 80's documentation. The circuit makes use of PUTs...