This circuit keeps the neon lamp on for a moment when it receives a pulse. The turn on time depends on C. The transistor is of the old type, from the 60's, when the circuit was published.
The sensor for this alarm is an LDR and it consists of an oscillator that goes into action when...
This sensor triggers an oscillator emitting sound when a magnet is approached by a reed switch....
This interesting circuit of a one-transistor receiver was found ina march 1961 Radio Electronics...