This simple circuit imitates the sound of a machine gun and using a larger capacitor also produces sound of a repeating pistol. Simple to assemble it uses few components, some of which obtained from discarded devices. Among the applications we can be given toys, sound effects in mechatronic assemblies and robots.
The operating principle is the following: when you press S1 the capacitor charges up but instantly opens contacts of the relay interrupting the supply for the attraction coil. With the interruption the relay contacts not immediately return to its normal position but take few second which is enough time the capacitor needs to be discharged again by the coil.
With the return of the contacts to the initial condition we have new charge and the phenomenon is repeated. This opens and closes of the relay produces current pulses through the speaker producing a noise similar to the weapon.
The speed of shots depends on both the value of the capacitor as the type of relay used (resistance of its coil). The reader should to experience in order to obtain the value that give better effect.
In Figure 1 we have the complete diagram of the device.
In Figure 2 we have the arrangement of components that can be installed in a box to improve the acoustics.
The relay may be any type coil with 6 V and a maximum current 100 mA.
General purpose types can be used.
The capacitor may have values between 100 µF and 470 µF.
The switch S1 is a push button and power supply is formed by 4 of any size batteries. The speaker can be of any type or size with power from 1 W.
To test the device simply press S1. If the shots are too slow decrease the capacitor C1.
B1 - 6 V - 4 standard batteries
S1 – Push Button NO
K1 - 6V sensitive relay - see text
C1 - 100 µF to 470 µF x 6 V - Electrolytic capacitor - see text
FTE - 4 or 8 Ω - small speaker
Battery holder, box for mounting, wires, welding, etc.