I was reading a post from a school teacher about wanting a subject for the class project. Bingo, I thought of something that had not really come up before: “How much does it cost to make a Joule Thief?”
I guess it would be best to start with a BOM, or Bill of Materials. The prices are based on retail at an electronics store, but somet parts such as wire and batteries are not available in single quantities, you may have to buy a larger quantity.
Qty — Item Name ——————————- Cost $ (retail / large qty)
1 — LED, white or Blue ———————– 2.00 / 0.15*
1 —- Transistor, NPN PN2222A ———– 2.30 / 0.05*
1 —- Toroid, Fair-Rite 267324002 ——– 1.00 / 0.12*
2 – Wire, 24 AWG, 10 in. or 25 cm long — 1.00 / 0.30
1 — Resistor, 1000 ohms, 1/4W, 5% ——– 1.30 / 0.02*
1 — Battery holder, AA cell, with leads — 0.99 / 0.70
1 — Battery, AA cell, Alkaline pkg of 4 — 2.00 / 0.20
*The LED, transistor, resistor, toroid can be purchased in a quantity of 100 with a steep discount. For instance, if the transistor is purchased from a retail store, it may cost a dollar or more. A ‘bargain pack’ of 15 may cost $2.30 US. But if 100 are ordered from a distributor, the price per transistor may be only 5 cents or even less. The same applies to resistors and toroids.
The Coil – The coil is the more expensive part of the JT. Larger cores can cost a dollar or more apiece, but can be reused, The cores from a PC power supply can also be used, so it´s a lot cheaper if you scrounge some coils from old equipment, especially dead CFL lights. I gave the part number of a toroid available from Mouser.com. Other similar toroids are available from surplussales.com.
Wire The wire for the toroid can be bought from a home improvement / hardware store for less than a dollar a foot, but make sure it is small enough – 24 AWG or 0.5mm – so you can get about ten turns through the small core. There is speaker wire that is thin, and bell wire or thermostat wire. I’ve found that solid conductor telephone wire makes good JT coil wire. Another source for small wire is keyboard cable or similar cable – the price being free. That brings up the point that there is wire all around your. I took the bobbin out of an old table fan, and it has a lot of 28 AWG wire, useful for winding coils. The windings in motors can overheat, so make sure the wire´s enamel insulation is in good condition and does not crack when the wire is bent.
The LEDs don’t get discounted as much, but an easy way to buy a lot of bright white LEDs is to get the four pack of 9-LED flashlights on sale for under five dollars (I’ve seen them for less than four dollars US – a dollar each). That gives you 36 LEDs for about ten or fifteen cents apiece. The only problem is that they have short leads, and have to be removed from the flashlights and unsoldered from the PC board. The whole PC board with 9 LEDs can be used as it comes from the light, but it takes a lot of current to light all 9 LEDs up brightly.
The battery holder is about 70 cents apiece if ordered from a distributor in quantities of 50 or more. However some improvising can be done. I recently designed a circuit board a bit longer than AA cell, and put holes in each end to accommodate a loop of steel cut off of a paper clip. These act as a battery holder, and can be adjusted a bit for a tight fit.
Another way to hold the wires to the battery is with a magnet on each end. I soldered a small steel screw to each end of the positive and negative wires, and put some heat shrink tubing over the joint to prevent the wire from breaking. A small 1/4 inch or 6mm diameter neo magnet holds the screw to the ends of the AA cell. I obtained the small disk magnets, 1/4″ diam by 1/16″ thick from Amazingmagnets.com for $13.00 for fifty magnets (see note below).
Very strong magnets can be obtained from defective hard disk drives. They are too large for a battery but they can be broken and the pieces used. NOTE: The magnet material is very sharp, almost like glass so it poses some safety hazard. Also these magnets are very powerful and can hurt you if you get your skin between them. And if you let two magnets pull together violently, there is a good chance that one or the other will break (I’ve already had this happen).
Conclusion? When I started this blog I thought I could pin down the price pretty easily – not The problem is that there are many ways the experimenter can save, and save a lot of money. If the parts buyer is purchasing enough parts for a hundred JTs (say for a classroom project), the total price of the parts could be reduced to a fraction of retail prices. The price of 30 AWG wire is about 10 to 12 dollars US for a quarter pound roll – that will make hundreds of JTs. But just about anyone can get wire free by removing some old telephone wire from the baseboard in a room, and strip off the jacket, leaving wire that looks like new. Or take apart an old motor and unwind the wire.
The one rule that every experimenter should obey is before starting a project, obtain all of the parts. Before you pick up any tool, you should already have all of the parts.
Back to experimenting…