Servo - September 2010

Toner Transfer

Toner transfer is a method (common in hobby PCB making) of printing your circuit board on paper, then using heat and pressure to de-laminate the plastic toner onto the clean, bare copper untreated (and cheap) PCB.

It can be done with ink jet photo paper and a clothes iron, but you get more consistent results if you use a laminator and glossy paper. I use Hammermill Laser Gloss, but others have had good results with the gloss paper used in magazine pages.

You print the resist pattern on the paper, use the iron or laminator to transfer it to the copper, then you soak the paper in water to make it easy to remove. The plastic toner stays on the copper, and makes an excellent etch resist.

It takes a little practice to get the technique down, but once you have it you can knock out good prototype boards for essentially the cost of the copper board.

see if I could build a better charger (with two smart charger circuits) that would fit in exactly the same slot as the current printed circuit board (PCB); see Figure 1.

Cramming all these components in this tiny space meant I would have to use surface-mount components. That’s right up my alley anyway, as I use them exclusively for all the circuit boards I make. This has the drawback that I can’t prototype things as easily as with through-hole components and a breadboard, but there is the advantage that when I have a working product, I don’t have to convert it to the final form. I just have to stick it in a case and move on. I do my prototyping using software design and simulation tools, specifically NI Multisim (a descendant of Electronic Workbench), and minimize my breadboard costs by buying in bulk and making my own PCBs. SMD components are much less expensive than through-hole parts.

There are basically two methods of smart-charging NiCd and NiMH batteries: one method detects the fall in voltage that happens when the battery is fully charged and is called delta V; the other method detects the rise in temperature of the fully charged battery and is called delta T. I decided to forgo the temperature change method of determining that the batteries are charged, and instead used the delta V method. This way, I didn’t have to fiddle with trying to sense the temperature of the batteries in the stock Versapak battery holder. Sensing the temperature of the batteries would require putting thermistors in physical contact with the batteries, and I didn’t want to have to make the modifications to the holder that this would entail.

There are quite a few manufacturers of smart charger chips, but Mouser (my favorite electronics supplier) carries the MC33340DG made by ON Semiconductor which happens to use the delta V method for primary charge termination. It also supports elapsed time or even thermistor temperature sensing as secondary methods for terminating charging. And at $1.45, each they are fairly inexpensive.