This circuit simulates the siren “Red Alert” – “Red alert” the TV show Star Trek. It is based on the integrated circuit timer 555, It uses two timers 555 and two common transistor in the circuit to make the sound of the siren.
The circuit is simple, but its result is quite identical to the original. The 555 on the right is attached as an alarm tone generator and the second timer 555 the left is connected as a non-symmetrical astable of 1,5 second, it generates a form of sawtooth wave.
This waveform is buffered by transistor and used to modulate the tone generator and make your frequency increase slowly over the parts falling from the sawtooth. Thus the output starts as low frequency, climbs 1,15 seconds to a high tone, shall end 0,35 seconds and, then, the cycle repeats.
Red Alert siren Movie Star Trek
The Red Alert Siren Movie Star Trek can be constructed in several forms of electronic assemblies, the transistors used in this project are the 2N3904, but any general purpose transistor can be used, As or BC546, BC547, BC548, BC549, etc.
You can play with sounds by changing the components of the oscillator and timer integrated circuit 555 and get very nice sound effects.
Source Randy Linscott