Sonic Grenades

"Sonic Grenades?"

Yup. Instead of exploding, they emit a piercing alarm shriek.

Pull a pin, and after a few seconds these "grenades" emit a very loud and annoying alarm - instead of shrapnel. This can be used just for fun (toss one into a sleeping roommate's bedroom, for example) or used on the honor-system to enhance any wargames you might play - like laser tag, Airsoft, Nerf, or even Paintball. (If one "goes off" next to you, you're out.)

I modified some personal alarms to make these, and if you're somewhat experienced with electronics you can make them too - I'll describe what I did below.

First, I will describe the units in more detail. Then I will provide parts and schematics for two different models (Basic and Deluxe). Finally I will describe the general steps I took for converting a personal alarm into a sonic grenade (applicable to both models).

The Basic Model uses common parts and is simple in function. The Deluxe Model uses a PIC 12F629 (inexpensive but requires a chip programmer) for enhanced operation.

The Basic Model is more useful as a novelty or annoyance device because it keeps going off until the pin is replaced.

The Deluxe Model is more useful as a "grenade simulator" since it has a working spoon (handle) and after an initial alarm sound it quiets down.

What They Are

Fundamentally, that diagram about covers it.

As manufactured, the personal alarms (pictured above, left) emit a piercing shriek when the pin is pulled out of the unit. The alarm stops when the pin is replaced. I present two methods of enhancing these so that when the pin is pulled, the alarm will only go off after a short time delay (simulating a grenade's fuse - allowing time for the user to throw it and take cover before it goes off).

One method is basic (simple parts only) and one method is deluxe (enhanced operation, but uses a 12F629 PIC). The basic one is useful for anything, the deluxe has some features that make it more useful in wargames.

Limitations

This project will only explore modifying the kinds of personal alarms I got my hands on for 1$ each at the Dollar Store. Other personal alarms should be similar, but you're on your own for adapting my design to them.

Also, you're on your own for re-packaging the modified units. You can enclose the alarm for a more muffled sound, or you can make sure the alarm is open to the air for maximum volume.

Basic Model - Parts and Fuse Schematic

Operation: You pull the pin on the grenade, and an LED goes on to confirm that the unit is armed and "live". After approximately 3 seconds, the alarm will turn on and the LED will go out. The alarm turns off when the pin is replaced.

You will need:
- Personal Alarm
- 74HC14 Schmitt-Trigger Hex Inverter
- 3.3uF capacitor (x2)
- 1 Megaohm resistor
- 10k resistor
- 470 ohm resistor
- red LED
- 1N4148 (or other small-signal diode)
- ZVN2106A N-Channel MOSFET (though any N-Channel MOSFET is probably OK)

Q: Why not use a 555 timer IC?
A: Good idea, but I didn't have one in my parts bin.

Deluxe Model - Parts and Fuse Schematic

Operation: You push in the SAFETY switch, and while holding it down, you pull the pin. The LED lights solid. Nothing happens until you release the SAFETY switch (such as when let go to toss the grenade). Once the SAFETY switch is released, the LED begins to flash and 5 seconds later, the alarm sounds and the LED turns off.

If the pin is pulled while the SAFETY is not pressed, the LED begins flashing immediately and the 5 second countdown begins right away. When the grenade "goes off" after the 5 second fuse, the alarm sounds for 5 seconds after which the alarm turns off. After 60 seconds (and every 10 seconds thereafter) the LED will blink and the alarm will "chirp" briefly until the pin is replaced.

The reason for this is to make the deluxe model more suited to being a grenade simulator. Not only does it work more like an actual grenade (by way of the safety), but the alarm only stays on long enough to clearly indicate it has "exploded".

After that, the grenade turns off so as not to be annoying, and if left alone, it will "chirp" and flash periodically as a reminder to make sure you don't forget or lose it.

You will need:
- Personal Alarm
- PIC 12F629 (8-pin microcontroller) and the means to program it.
- 0.1uF capacitor (if you don't have one it's not critical, just preferred)
- 3.3uF capacitor (actually anything bigger than 1uF should be OK)
- 10k resistor (x2)
- red LED
- Momentary switch
- ZVN2106A N-Channel MOSFET (though any N-Channel MOSFET is probably OK)

PIC Code for the 12F629

12f629-sonic-grenade.hex : .HEX file for programming the chip
12f629-sonic-grenade.bas : PICBasic source code

Procedure for Modifications

I used the same procedure for both versions.

Step 1:

Disassemble the personal alarm carefully and desolder the pull-pin switch. This switch is normally open, and closes once the pin is pulled. It connects a trace on the alarm PCB to (-) to trigger the alarm. Set aside the pull switch unit, we will use it later. If you didn't disconnect the power before starting you'll probably get a good surprise when the alarm goes off if you short a couple of these contacts!

Step 2:

Identify the holes/traces vacated by the switch. There should be a GND (-) and one we will call /ALARM. When /ALARM is connected to GND, the alarm is activated.

(On my alarm, the switch has 3 pins and there 3 holes, but two of them are to the (-) of the battery.)

Step 3:

Connect the MOSFET. Drain goes to /ALARM, Source goes to GND (-).

Then connect the 10k resistor between the Gate and GND (-).

(Resistor not shown above - this shows the placement of the MOSFET only.)

Step 4:

Test it so far. Connect the batteries if you disconnected them, and connect the Gate to (+). The alarm should sound.

Disconnect the gate - the alarm should stop. If so, then all is well and we can continue.

Step 5:

Build either the Basic or Deluxe trigger circuit. Use the pull-switch salvaged from the alarm unit as indicated. Refer to the schematics - as usual it is advisable to build the circuit on a breadboard as a test first.

Step 6:

You should be left with three connections to make to the alarm unit:

• +V to the (+) battery terminal

• GND to the (-) battery terminal

• Signal out to the Gate of the MOSFET

Make those connections.

Step 7:

Test! Pull the pull-switch - this connects your circuit to the batteries. The LED should light up (or flash, depending on whether you made the Basic or Deluxe version). Then the alarm should go off.

Assuming all is working as it should, you can build the fuse circuit and then re-connect the halves of the personal alarm (I think it's easier this way - the case is also the battery compartment) leaving some wires hanging out. Then connect the wires to your fuse circuit, and seek a more suitable case to fit the whole thing into.

Here is what I used:

For the basic model, I put it into an empty Pain-Relief pill bottle. Is your hungover roommate in bed with a pounding head? If so, relief is definitely NOT what is contained in this bottle...

For the deluxe model, I had the good fortune to have a hollow plastic fake grenade around. I sawed it in half to insert the modified unit, drilled a hole for the pull-pin switch, and put the safety switch where it would be held down if the spoon was pressed (the spoon is the "handle" on the grenade). Then I anchored things with hot glue and taped it back up.

In both of these, the alarm volume is muffled by virtue of being encased in something. If I wanted the full 110db of goodness, I would simply cut out the bottom (or some similar modification) to allow the sound to escape.

Usage and Video

These can be used for many things, but of course the simplest uses are just for fun (toss one into a sleeping roommate's bedroom, for example). Alternately you could use them on the honor-system to enhance any wargames you might play - like laser tag, Airsoft, Nerf, or even Paintball. (If one "goes off" next to you, you're out.)

Note that the pull-ring makes these excellent candidates for tripwire boobytraps. Wedge the grenade somewhere, tie a string or wire to the ring, and attach the other end of the wire to something solid. When someone walks across it, the tripwire will pull out the pull-ring and trigger the grenade.

Possible Improvements

Integrating a flash bulb (for example) could result in a useful "flash" effect that could even simulate a flashbang in a dim environment. At the very least it might be neat to have the added visual effect.

(Added 2006-07-11) Using a 555 Timer IC

For those of you who are electronically-inclined enough to try a 555-based basic grenade, but need a starting point: try this. Take a look at http://555-timer.clarkson-uk.com/operation/frames3.html. There are 4 things to do to use this instead of my Basic design.

1. Use the page's form to pick part values to make a 3 second delay (or 5 if you wish, or whatever you want).

2. The 555 design from the link mentioned completely replaces the 74HC14 circuit from my design.

3. Use the pull-ring as a switch that provides power to the 555 (the pull-ring is used in the same way in my design).

4. The output of the 555 is pin 3. This connects to "IN FROM TIMER" on the MOSFET in my design.