Rapid Fire Generator

Those who need to reproduce the sound of rapid gun fire for a toy, might be interested to consider the present device. You can hear different gun sounds on www.soundbible.com and realize that a gun sound is composed of a ‘bang’ followed by a ‘hiss’ (at least, such was my impression). The ‘bang’ is created by the high-pressure gases suddenly released from the barrel, and the ‘hiss’ – by the bullet moving in the air. My device reproduces both components fairly well for a toy (I would insist on this definition because it wasn’t my intention to replicate the sound), and is simple, consisting of 4 transistors, one IC and some passive elements. The video will show you the result.

Step 1: Circuit Explained

The circuit is shown in the pictures attached. The astable multivibrator built with Q1 and Q2 produces a square wave, the period T of which is calculated as

T = 0.7*(C1*R2 + C2*R3)

A detailed description of how an astable multivibrator works can be found here: www.learnabout-electronics.org/Oscillators/osc41....

The mark- to- space ratio* is chosen to be 1:1, then C1 = C2, R2 = R3, and the wave frequency is calculated as

f= 1/1.4*CR

I chose the frequency equal to 12 Hz, which gives 720 ‘shots’ per minute, and the capacitance equal to 1 microfarad (uF). The resistance is calculated then as

R = 1/1.4*fC

The calculated value is 59524 Ohm, I used 56K resistors because they were the nearest available. The frequency in this case will be 12.76 Hz (765 ‘shots’ per minute).

*The ratio of the duration of the positive amplitude part of a square wave to the duration of the negative amplitude part.

The multivibrator has two outputs: Out 1 and Out 2. When the Out 1 is HIGH, the Out 2 is LOW. The mark-to-space ratio being 1:1, the duration of ‘bangs’ and ‘hisses’ is equal; however, the circuit could be modified to change both this ratio and the period of the wave to modify the sound as you like. Following the above link, you’ll find those modified circuits.

The signal from the Out 1 is fed into the base of T4 (preamplifier) through a voltage divider composed of R8, R9 (trimmer) and R10. This feature allows you to modify the strength of the ‘bangs’ to find the most ‘natural’ (in your opinion) sound. You can also replace these resistors with a 470K trimmer to be able to modify the sound at any time as you wish. In this case, before you apply voltage to the circuit for the first time, you might consider turning the trimmer’s axis to the middle position because it’s quite near to the position that gives ‘natural’ sound.

From the collector of T4 the signal comes to the input of the final amplifier built with an IC LM386; the amplified signal comes to the loudspeaker.

The signal from the Out 2 comes to the emitter of T3. This is an NPN transistor; however, a positive voltage is applied to the base-emitter junction of the transistor. When this reverse voltage exceeds the value called ‘breakdown voltage’ (6V for a 2N3904, the emitter current being 10uA), a phenomenon called ‘avalanche breakdown’ happens: free electrons accelerate, collide with atoms, release other electrons, and an avalanche of electrons is formed. This avalanche produces a signal that has equal intensity at various frequencies (avalanche noise). You will find more details in the Wikipedia articles ‘Electron avalanche’ and ‘Avalanche breakdown’. This noise plays the role of ‘hisses’ in my device.

The emitter current of T3 can be regulated with the trimmer R5 to compensate for the drop of battery voltage with time. However, if the battery voltage drops below the breakdown voltage (6V), the avalanche noise won’t happen. You can also replace R5 and R6 with a 150K trimmer. (I didn’t have one readily available, that’s why I used a combined resistor). In this case, before you apply voltage to the circuit for the first time, you should turn the trimmer’s axis to the position corresponding to the maximum resistance to avoid excessive current through the emitter of T3.

From the emitter of T3 the signal comes to the input of the final amplifier built with an IC LM386; the amplified signal comes to the loudspeaker.

Step 2: List of Components and Tools

Q1, Q2, Q3, Q4 = 2N3904

IC1 = LM386

R1, R4, R11 = 2.2K

R2, R3 = 56K

R5 = 47K (trimmer)

R6, R10 = 68K

R7 = 1M

R8 = 330K

R9 = 10K (trimmer)

C1, C2, C6 = 1 uF (microfarad), electrolytic

C3, C4 = 0.1 uF, ceramic

C5, C8 = 100 uF, electrolytic

C7 = 10 uF, electrolytic

C9 = 220 uF, electrolytic

LS1 = a 1W loudspeaker, 8Ohm

SW1 = a momentary switch, for example, a pushbutton

B1 = a 9V battery


1) Power ratings of all resistors are 0.125W

2) Voltages of all capacitors are at least 10V

3) R5 and R6 could be replaced with a 150K trimmer

4) R8, R9 and R10 could be replaced with a 470K trimmer

The circuit is built on a piece of circuit board 65x45 mm, connections are made by wires. To build the circuit you’ll need a soldering gun, solder, wires, a wire-cutter, a pair of tweezers. To power the circuit during experiments I used a DC adapter.

Step 3: Physical Arrangement

The circuit board, the loudspeaker and the battery could be placed in a drum, the size of which should be proportional to the overall size of the toy. In this case, the size and the shape of the circuit board must be such that the board fit into the drum. This solution is convenient if you already have a toy representing a drum-fed submachine gun, say, a ‘Tommy’ shown in many projects on this site.

It’s also possible to place the board into the main body of the toy, especially when you make a model of a modern assault rifle with a rectangular feeder. In this case, a small loudspeaker could be put into the ‘sub-barrel grenade launcher’ of the ‘gun’. Obviously, the switch SW1 should be put where the trigger of a real gun is situated.

Step 4: Actual Presentation

What you see in the video and the pictures is not a real toy, it’s just a way to better show you my device in action. The sound is also better when the loudspeaker is situated in an enclosure. Therefore, I downloaded a picture of a ‘Tommy’, printed it, glued it on a piece of cardboard, cut it out, fabricated a small drum for the loudspeaker. I made the drum’s front and back sides of 4-mm thick plywood; to make the lateral surface, I used thin strips of plywood soaked and formed on a cylinder of appropriate diameter.



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