2011-04-18 2011 Apr 18 JT Transistor Switch
I thought that it would be a really great idea to put several transistors on a switch so that I could switch between transistors and compare their performance.
I only have to connect the base and collector leads to the switch. The emitter lead is almost always connected to the negative or common or ground or earth connection, so it does not need to be switched. This means I need a 2 pole, multiple throw or multiple position switch. I figured that 5 or 6 positions would handle most of the common transistors used in a JT.
I got on the web and looked up a 2P5T switch and came across some at SurplusSales. As I figured, the solution would be a rotary switch. But I got a bit of sticker shock when I found they wanted up to $45 apiece for a rotary switch.
I started thinking about an alternate method of switching. The first thing that popped up in my mind was to use some jumper strips. These are like the ones that are (or were) on the back of hard disk drives, to allow you to select master, slave or cable select. I could put two of these on a PC board, one for the base and one for the collector. I just pull the jumpers and move them to a different transistor. It’s a bit slow and tedious, but it would probably work okay.
Then I thought about the setup I made years ago for two rooms that we used at different times of the year. One room was used during the peak periods, and then we moved across the hall to a smaller room during the last few weeks. Every time I moved the telephones, I had to pull off the jumper wires and move them to the other room.
This got really old really fast. So I took 2 male and 1 female DB-25 connectors like the ones used on the PC end of the parallel printer cable. I needed to move four phones or 8 wires, so I could have the female connector wired so that both rooms connected to it, along with the feeds from the central office. One of the male connectors was configured for routing the wires to the first room, and the other male connector was for routing the wired to the second room. I just pulled off one connector and put the other one on, and Bingo! The phones were moved to the other room.
This gave me a brainstorm. I have a lot of DB-9 connectors from old mouse extension cables. I can put a female 9 pin on the Joule Thief circuit, and take several male connectors and solder a transistor to each one. I really only need three of the 9 pins, but I don’t have three pin connectors. I can solder up as many transistors to the connectors as I want, and swap a transistor in a matter of seconds. COOL!
Another thing this allows me to do. I can solder different resistors or coils to the pins and switch them too. I can use a completely separate connector setup for swapping the coils, which have four wires.
Suppose I didn’t have the 9 pin connectors. I have a ton of power cords that seem to accumulate over the years from old PCs, monitors, printers, etc. Each power cord as a three pin plug on the end, and I could cut it off with a few inches of wire and solder the transistor to the three wires. Then I could use a socket and solder the JT to it. Then I just unplug the transistor and plug another one in. It’s a bit harder to unplug than the 9 pin connector, but it has large contacts for very low resistance connections.
Back to experimenting…