How to build Touch Activated Light Description SK34

The circuits below light a 20 watt lamp when the contacts are touched and the skin resistance is about 2 Megs or less. The circuit on the left uses a power MOSFET which turns on when the voltage between the source and gate is around 6 volts. The gate of the MOSFET draws no current so the voltage on the gate will be half the supply voltage or 6 volts when the resistance across the touch contacts is equal to the fixed resistance (2 Megs) between the source and gate.
The circuit on the right uses three bipolar transistors to accomplish the same result with the touch contact referenced to the negative or ground end of the supply. Since the base of a bipolar transistor draws current and the current gain is usually less than 200, three transistors are needed to raise the microamp current level through the touch contacts to a couple amps needed by the light. For additional current, the lamp could be replaced with a 12 volt relay and diode across the coil.

SK34 IC

Through Hole Devices Are Becoming Hard To Get 2SK170

I had no problems getting the 2SK170 JFETs from Futurlec.  They were on tape.  I don’t know what manufacturer, but they are not Toshiba.

Right now, I think a lot of the major semi makers are abandoning through hole parts, and going to surface mount only.  To get a through hole part, one may have to buy a large amount (a whole reel) or get the part from a maker such as KEC (Korea Electronics), BEL (Bharat Elec) or Rohm.

You can get the National Semi Discrete Data Book 1978 Edition (PDF) from Archive.org.  When you look up a transistor, it tells you the process that it was from.  In other words, the chip inside the transistor is sorted from the chips made by a certain process.  Some have higher specifications, for instance higher gain.  Thes may be sold as 2N3904.  The lower gain chips may be sold as 2N3903.  But they are the same chip, and if the circuit is not critical, you may be able to substitute the 2N3904 for the 2N3903.

 In the Pro Electron Series, you may find that the older transistors such as the BC107 were packaged in the TO-106 package, which is no longer made.  But you may be able to find an equivalent transistor, such as the BC547, using the same process in the TO-92 package, which is common and still being made.

Regarding the 2SK170.  It may not be recommended for new designs, but the replacement, the 2SK880, is not through hole, it’s surface mount, and that would mean redesigning the PC board.  So the assembler must use the 2SK170 until his supply of PC boards has been used up.  The alternative would be to try to find a substitute JFET.  If the JFET is in the Databook, you could find out what process is used, and then find other JFETs that use that process and try to find one that would substitute for the 2SK170.

I also got some 2N7000 MOSFETs.  They also came on tape, but the tape has the side toward the MOSFETs coated with aluminum foil.  This is to prevent damage from static electricity.  The MOSFETs are held onto the the tape with masking tape.  Normally the leads are cut off above the tape as the part is put into the PC board.  The masking tape is never disturbed.  But in my case, I don’t have a machine so I usually pull the part off the tape, and sometimes the masking tape pulls of with the part, sometimes it stays on the tape and the part leads pull free.  The problem here with the MOSFETs is they are more sensitive than JFETs to static.  That’s why there’s the aluminum foil.  And the masking tape is triboelectric.  This means when the tape is pulled off, it generates static electricity.  So by pulling off the MOSFETs I could be causing them to burn out.

What to do?  I could try to duplicate the machine and just cut the leads off above the tape.  But then the leads are shortened and obviously not the same as when they were made.  My thoughts are that if I squeeze the 3 leads between my fingers and pull the MOSFET off the tape, the static from the masking tape will go through my fingers and not damage the MOSFET.  Or should I try to cut the masking tape off before I pull the MOSFET off the foil?  I’ll have to experiment a bit to find out how each works.