The Sci-Fi Grenade [02062007 1713]

I was mulling over the idea when the RPF Forum came up with a contest for a Sci-Fi Grenade. As it so happens, my pepper mill broke. It started when I ran out of my favourite black pepper and decided to use the white pepper that was give by an in-law of mine. Just one twist to the top and it broke. It seemed the plastic parts at the bottom used to grind the pepper, sort of disintegrated.

So, I am now left with a broken pepper mill that I was about to throw away until I saw the Forum. And after having a second look at the mill, it CAN be a grenade of some sort. After taking the pepper mill apart, I started to have a rough idea how I wanted it to be. Its not about the prize of USD25 (RM84.00) but I was itching to make something that WILL be finished after starting so many projects that got stuck halfway for years. And I have to make sure all the parts does not exceed the cost of USD25 too. So, here I go, rummaging into my box of electronic spare parts.

Some of the parts for the grenade. The broken pepper mill, a PVC pipe,

a microswitch, a LED torchlight's battery holder, a rocker switch with a

different red push-button switch.

Version 1.0

The idea is to put in some LEDs, let it flash in a sequence or two before it "explodes". So, the sequence is:

1) Switch on the grenade using the slide switch at the bottom, the white LEDs will form a rotating triangle

2) Flip open the protective cover, press the "activate" button and the LEDs will blank out for a fraction of a second

3) First countdown. The LEDs then light up one by one in sequence, with the each beep when the last LED is on. The sequence starts from slow to fast.

4) Last countdown. The LEDs flash very fast. The beeps increases speed.

5) The grenade "explodes" with all 5 LEDs lit up briefly. You'll have to reset the grenade by switching the slide switch off and on again. The delay for the grenade (from the moment you push the push-button) before exploding is about 5 seconds.

This is the idea for the first version. Please ignore the top part (left) of the grenade as I am not an expert on using Google's SketchUp software yet.

So the 3D drawing is not that accurate


And here is the short video of the LED sequence

Version 2.0

Then my friend mentioned something about the thermal detonator in "Return of the Jedi". Although I have not seen the movie for decades, I roughly remembered that when activated, the yellow lights blink in a sequence and with a menacing clicking sound. So, I decided to change my design a little. At the same time, I was happy to locate some rare 3mm bi-colour LED (red/green) which, previously, only comes in the more common 5mm version. Then I found a few 5mm white LEDs and used them instead. So, the design is now this:

1) Flip open the rocker switch and this will activate the grenade.

2) The white LEDs will run in a sequence of two LEDs at one time. The beep is now more frequent. The bi-colour LED is green to show "activated" stage


3) First countdown. When the red push-button is depressed, the bi-colour LED turns orange and the white LEDs blanks a bit before going into countdown mode.



4) Last countdown. As the countdown gets to its fastest speed, the bi-colour LED turns red where you have about 1-2 seconds before the grenade explodes


5) The grenade "explodes" with all 5 White LEDs lit briefly. You ave to reset the grenade by closing and opening the rocker switch cover again. The delay for the grenade (from the moment you push the push-button) before exploding is about 8 seconds.


This is the second version. I have now taken out the rear black disk and used a micro switch instead of the slide switch. And I might

have to fit the electronics without making a circular circuit board. Its extremely difficult to cut a fibreglass PCB.


Here is the video of the Version 2.0 LED sequence

Design notes

I intend to make the grenade as simple as possible. It would use minimum cutting since I do not have any Dremels nor cutting tools. And so, within the first week, I encountered the first problem: What type of power source is small enough to fit into the bottom of the mill without enlarging the opening? Since the microprocessor chip have an operation voltage of up to 6.5 volts, I decided to use batteries that gives out voltages lower than that so that I do not have to add a voltage regulator which takes up space in there casing. I decided to get one of those cheap LED torchlights that were made in China. For a reasonable price, I would not only get the small battery holder but also an imitation Luxeon LED to boot. But I am not out of the woods yet. The height of the 3xAAA batteries would take up most of the space in the casing and so, at the area where the push-button is inserted, it might clash with the batteries.

The push-button switch is already very small. It has already fit the requirement that it must be short enough so that the rocker cover does not hit it. But then, internal-wise, it might hit the batteries. Initially, I wanted a push-button that has a small LED inside it and if I could replace that LED with a bi-colour version, it would be very nice. However, I think I would have to drill on the casing to fit this LED instead. Or maybe, on the black metal panel of the rocker cover.

The next problem would be the electronics. I gave up on the idea of making a round circuit board. It is not easy nor economical to do one because of the limited time frame and also, I do not have the tools to make such circuit boards even for a small quantity. So, I would have to just wire it like I did with the Medical Scanner and hope there is no shorting.

Using a PVC pipe to house the batteries and making sure it does not clash with the push-button, I will try to insert the electronics into the empty space between the PVC pipe and the casing. Then, maybe I will get something to fill in that gap. I thought of using expanding foam but I think better not. There will be no access to the grenade once I glue the two halves of the pepper mill. The only access would be for the batteries at the bottom of the mill, secured with two screws.

To make the White LEDs brighter, I will use aluminum tapes as reflectors. This might show up nicely on each the 3-holes at the cover. I thought of removing the three holes so that I could pop the white LEDs on the top and secure it with a transparent LED holder but I decided against it since I do not have a drill that is kind to plastics.

Some LED covers for the White LEDs

Construction notes

The grenade also needs to have some kind of warning label and after looking at a few and also testing the Warning Label Generator, I decided to make my own. I think I'll just print it on some photo paper which I have some left. The label reads Standard Hole-In-The-wall Equipment (Take out the "w" and you have a SHIT bomb). Actually, I wanted it to be Super Highly Irradiated Thermalnuclear Emitter (at least it becomes SHITE) but it sounded so boring. There are other weird stuff I've written in there but you'd have to in the right frame of mind to appreciate the humour.

As the pepper mill is about to be cut, my friend mentioned that it is not easy to fit the micro switch into the pepper mill since it might be jutting out. Also, if someone were to lob the grenade (assuming it IS a working grenade), the enemy catches it and closes the rocker cover, it will be deactivated. So, this means I would have to use back the slide switch to turn the grenade on. Bummer. But lets forget about this minor detail first. Ha ha ha ha.

Anyway, after the pepper mill was returned to me, I discovered another problem. Where to put the micro switch? It must be placed next to the rocker cover so that when it is flipped open, it will close the micro switch via the metal tab which then activates the whole circuit.


The top picture shows how the rocker switch will be placed and the bottom part shows the Dremel-ed area.

During the Dremeling, my friend had to cut it very carefully as the top part of the mill is made from

soft plastic while the bottom part is tougher.


Printed Circuit Board

OK, finally, I decided to make a PCB (Printed Circuit Board) out of this circuit. This will be part of my exercise into outsourcing local companies to some dangerous jobs for me. In this case, they're to fabricate some PCBs. After some small talks, they can only process PCB designs that are output in Protel file formats, which is an expensive solution considering the obscene four or five-figures prices these types of professional PCB software commands. But there is a low-tech solution: They will accept laser printouts of my circuit board. Now, all I need to do is to get a reasonable PCB software to start from. Once done, I will try to see them again soon. But chances are, there would need me to put a minimum order of 10 or so boards.

The board measures about 1.1 inch by 2.1 inches and it is the smallest board I could design wihout having to go into using SMT (Surface Mount) components. I have no problem designing it for SMT but this would also mean I would have to get a special SMT adaptor for the microprocessor chip so that it can be connected and be programmed by my current microprocessor programmer. There is another alternative in programming the chip while it already soldered on the circuit board but I have no experience in this area yet. All the main components would be soldered on to the board with the exception of the batteries, LEDs and switches which are to be connected via wires soldered to the board. If I take the piezo buzzer out of the board, I could save another quarter inch. But then, it would make the whole PCB having too much wires in the end. I found a little bit more space and so I added in a LED to indicate there is power flowing into the PCB board. But this will not be a normal LED as I am trying to find that flashing Blue LED just like the one I used for my Ligthsaber. Failing that, I will just have to get a flashing red one instead. Or maybe a tri-colour flashing LED. Or maybe none at all since it won't show up well as the whole board is inside the pepper mill. Maybe I will drill those standard 5-hole patterns (looks like the Olympic symbol) seen in mecha Japanese designs.

The PCB would roughly look like this (without wires and the dark blue pattern)

Oh-oh. I think the PCB might have a fitting problem, especially the capacitor and the piezo buzzer which is too high.

11 Days later

Anyway, I got the PCB and I was impressed with the quality. Any mistakes I noticed was actually my own and luckily, its only the silkscreen and not the copper tracks or else the whole PCB is useless. The company allows a minimal quantity, which in my case, is five. Compared to the cost of having it made overseas and having to wait for the mail to arrive, I think its pretty much justified as I am quite impatient for this project. After seeing their quality, I can safely say, I could have shrunk the board size to less than 1 inch by 2 inches.

(Back L-R) Copper mask, Solder mask and silkscreen mask

(Front L-R) PCBs viewed from the top and one on solder side


Although it is small, it was too bad that they do not have those 0.8mm PCBs as this one

is 1.6mm thick, which is like holding a chewing gum



This is the section where I will attempt to put everything together. I will also explain the problems I encountered at this stage as well.


Although I have taken eveything into consideration, there was one minor detail which I suspect could turn out to be a major problem. ANd I was right. The local PCB company uses normal 1.6mm fibreglass boards instead of the 0.8mm ones I needed. When I test fitted the PCB into the pepper mill, sure enough, there was limited space inside. So, this means some components such as the on-board LED and the piezo buzzer would have to be relocated elsewhere off the board. I cannot even use the IC socket as this would increase the PCB board's height so I had to solder the microprocessor chip directly. Since there are no more code revisiosn, I am OK with that, as long as I do not feed it the wrong voltage. But somehow, I thik I should have explored about programming the chip while it is on the PCB.

The PCB partially completed with most of the components


Sigh. This is another big problem for me. The switch is too tall to fit into the pepper mill and too short for the nut to hold it from inside. The sponge is just to make sure the tube is in place for the moment. If you look carefully in the picture, you can see how tight the PCB board is inside it. I should have gone for SMT (Surface Mount Technology) where I believe, the overall size of the PCB should be 1inch by 1 inch. But the cost would be more than 10 times and longer to obtain.

Oh-oh. The switch is the problem here. If I cut away the black part, it won't work anymore......


The only solution I can come up with would be to use another switch. And since the piezo buzzer needs to be relocated, I decided to make it as the top part of a small push-button switch. I glue the switch to the inner cylinder, then glue the piezo on top of the switch. So, now its just a matter to make sure the piezo buzzer would not pop out of the pepper mill unintentionally. By now, you would realise that with a small Dremel-like tool I just found, even a blind-folded monkey can do better. Yeah, the results are damaging.


I am a horrible monster with any hand-tools. I destroy more than I create. Sigh.


After breaking a 3mm drill bit, I realised that the first hole was off the mark by 2mm, so now, the switch is a bit out of place and it looks a bit bent. And after breaking that drill bit, I got a new sharp one which can really drill through metal. (OK, so the first bit was meant for wood. I didn't know that). Just to make sure, I placed two pieces of metal underneath (inside) the pepper mill to reinforce the whole thing. This was because I am quite worried as the spring from the rocker switch is quite strong and everytime it snaps back to its place, it hits onto the black metal plate and then on to the plastic. But when it comes to the final stage, I will have to use some acrylic double-sided tape between the switch and the plastic so that it can absorb the extra kinetic energy. OK, so now I am babbling about.


The switch now looks "stronger" with the two hex bolts.


This is roughly what it looked like when closed. You can see the two metal strips underneath the

rocker plate. But now, where to put the small 3mm red/green LED?


If I had more a little more time (like a year), I would:

1) Do a R&D on playing different sound samples, controlled by a chip. But this costs money, especially for the programmer.

2) Modify the grenade to switch between White and other colour LEDs, maybe during earlier countdown stage and then to RED during the last few seconds. But this would make the grenade act like a portable disco light instead.

3) Put in Luxeon LEDs and blind everyone who looks at it.

4) Buy more black pepper mills. Yeah and see the Doctor for sore throat after eating all those pepper.

5) Make some thick stripes to go with the grenade like this:

But then, it would look more like those M203 Grenades