Here the low cost and easy build circuit of ticking bomb sound generator. This circuit generates a sound matching to a loud clicking clock. The frequency of the tick is altered through the 220k potensiometer part.
The circuit gets going by charging the 2.2uF and when 0.65v is on the base of the NPN transistor, it begins to turn on. This activates the BC557 and the voltage on the collector increases. This forces the small charge on the 2.2uF into the base of the BC547 to turn it on more.
This continues when the negative end of the 2.2uF is higher than 0.65v and now the electro begins to charge in the opposite direction until the two transistors are completely activated. The BC547 receives less current into the base and it starts to de-activate. The two transistors de-activate very quickly and the cycle starts repeatedly.
The circuit is powered using 9V battery or general purpose power supply.