Monday, 22 April 2013

Hot Wire Polystyrene Cutter–Part 4

Finally! I got the time to install the 12V AC Adaptor into the polystyrene cutter.

I also put in a 5mm red LED and a 470 Ω resistor between the switch and the NiChrome wire. I did this because my switch doesn’t really tell me when the device is on. I simply soldered the Anode leg of the LED to the switch, the resistor to the cathode leg and a short length of wire between the resistor and the cutter “head”.

I cut the adaptor coupler off the end of the adaptor wire, stripped and tinned the wire and connected it to the terminal block.

Using a ready made power adaptor makes this a very simple circuit.

Polystyrene Cutter_schem

A possible enhancement would be to add a 10K pot between the resistor and the NiChrome wire.

I powered the cutter up and, presto, the NiChrome wire got good and hot. Hot enough to cut through the polystyrene with very little resistance. The polystyrene leaves some residue on the wire, but this can be picked off without any trouble.

I will make a smaller, hand-held version of the polystyrene cutter using a 9V battery as the power supply. The main reasons for making a smaller version are:

  1. I can use the smaller pieces of NiChrome wire when it breaks (and it does break, so be prepared for it);
  2. A hand held cutter can be useful for smaller more complex cuts.

The way that this works is fairly simple. The NiChrome wire acts as a resistor, the energy has nowhere else to go in the circuit so it is converted into heat … the end. I am bleeding off a tiny amount of electrical energy with the LED, but it is a trivial amount overall.

My youngest thinks that the cutter is cool and wanted to cut some shapes using it, she had no trouble after I gave her instruction. Then I got to explain the physics of the project … she thought that bit was boring (hahaha)

Monday, 15 April 2013

Hot Wire Polystyrene Cutter–Part 3

I’ve been doing some tinkering and going through old AC Adaptors to see what I had hanging around (mostly from old mobile phone chargers).

I connected a 9V adaptor to the styrene cutter and I still didn’t get enough current through to heat the Nichrome wire. It seems that my wire is too long to be able to generate enough resistance in a short enough space to produce the heat that I need to cut the polystyrene.

I have a couple of options:

  1. Reduce the length of the Nichrome wire. This isn’t a particularly attractive option as it means that I have to do some structural remodelling of the table and arm;
  2. Reduce the gauge of the Nichrome wire. This option doesn’t seem practical as I have pretty much exhausted my supply here in Hobart. I could harvest some Nichrome wire from an old soldering iron (that’s what’s used to heat the iron) … this is possible, but I’ll use this as a fall-back option; and
  3. Increase the voltage of the adaptor that I’m using. This is probably the best next step in the design evolution of the polystyrene cutter.

I tried connecting my 12V car battery charger to the cutter to see if increasing the voltage to 12V would have the desired effect. The cutter certainly made some clean cuts, but the internal fuse in the recharger kept on tripping, so I can’t really use that in the long term.

Cleaning out some old electronic equipment, I found a 12V adaptor that used to power a TV antenna (with boost/gain). The antenna is now obsolete since analogue TV has now been phased out. The adaptor is 12V 600ma, so that should deliver the power that I need.

I will probably also need to remove the 5V fuse from the circuit … I’d also like to put in a light (such as a LED) so that I can see when the power is on. At the moment, the toggle switch doesn’t give any indication that the power is on … so I am also guessing when the circuit is actually live.

I’ve made power connection a little bit easier for myself by including a screw terminal block.

Changing the power should be a fairly quick operation, I just need to cut the connector from the end of the adaptor cable, tin the wire and then connect the cable ends to the screw terminal block. Fixing in the LED is going to be a little bit harder … but not too hard.

Steampunk Electronics–Buzzer

Another part of the internal electronics that I want to incorporate into the Steampunk gun, is a buzzer that sounds when the trigger us pulled.

I found a simple schematic using a 555 IC that flashes an LED and makes a kind of buzzing sound through an 8 ohm speaker.

This schematic seems to be pretty widely available on the Internet, so I thought that I’d give it a go.

The circuit uses a small number of parts and, all together, it cost me about $1.25 using components sourced from eBay.

  • J1 – 2 pin (2.54mm pitch) right angle pin header
  • VCC1– 2 pin (2.54mm pitch) right angle pin header
  • C1 – 1μF non-polarised capacitor
  • C2 – 10μF electrolytic polarised capacitor
  • R1 – 220kΩ resistor
  • R2 – 470kΩ resistor
  • U1 – 8pin DIL socket and NE555 IC
  • LED1 – generic blue LED

J1 connects to the 8Ω speaker and VCC1 connects to the 9V battery.

I have added DuPont female connectors and housing to the wires soldered on to the speaker and 9V snap to make it easier to connect and disconnect them.


I’ve used Fritzing to do the layout of the board (as shown above). This board is approximately 4cm x 3cm, so it would fit nicely inside the prop.

First, I laid the components out on a prototype board and set up the jumper wires to test the arrangement. Once I was happy with the circuit, I used Fritzing to lay out the design (and make the schematic). Then I printed the circuit out onto some photographic paper and cut it to size.

My first attempt to print was pretty poor … I heated the PCB too much and the copper blistered away from the board. This may have happened because the board was a bit scratched from a crappy cut that I made previously.

When I had successfully transferred the design to the board, I then went over the transfer and fixed any breaks with some enamel paint and a fine paintbrush, then … into the acid bath.

I cleaned the board using acetone to remove the resist and drilled the holes with a pin vice.

I then went over the board on the silk side and marked what each of the components and their orientation should be and then populated the board with the components and soldered them up.

I put DuPont connectors onto the speaker wire and the 9V snap so that I could more easily attach/detach them (an so use them on other projects later).

The circuit is described, on the Internet, as a “Laser” sound … frankly, you would need a pretty good imagination to believe that. It’s more of a Geiger Counter sound … but that may be the way that I constructed the circuit. It probably needs a higher hertz to fix that.

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