# What is the difference between a PUT and a normal transistor?

What is the difference between a Programmable Unijunction Transistor and a normal transistor? I am reading the book Make:Electronics and it says I need one of those. Can somebody explain to me in somewhat non-tech terms what it is?

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Normal transistors like BJTs can be found from a variety of sources like Mouser, DigiKey, Newark/Farnell and many others. PUTs are found in museums. –  Olin Lathrop Dec 3 '11 at 21:32
Newark appear to have 6000+ of the 2N6027G in stock. newark.com/on-semiconductor/2n6027g/programmable-ujt-1a-to-92/… –  mindcrime Dec 3 '11 at 22:00

As far as I know these are completely different beasts (and a simple google search agrees).

The built-in negative resistance of the unijunction transistors is what makes them attractive (or maybe "cute" is a better word) in "simple" oscillators but IMHO you will be better off building a 3xRC ($180^\circ$ phase shift network) oscillator on one normal bipolar transistor or a simple oscillator on an op amp. Almost nobody uses negative resistance devices nowadays.

Negative resistance is fairly common in nature (a humble spark gap has this property) so this may be the reason why historically they were used for intro level oscillators. But there also were times when people notoriously made their own rectifying diodes out of some kind of rock and a piece of wire so they could build a long-wave radio receiver, so... :P

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According to Wikipedia:

Unijunction transistor circuits were popular in hobbyist electronics circuits in the 1960s and 1970s because they allowed simple oscillators to be built using just one active device. … Later, as integrated circuits became more popular, oscillators such as the 555 timer IC became more commonly used.

The 555 Timer is probably the easiest way to build an oscillator, programmable or not. It can be bought at Radio Shack, or anywhere chips are sold. Instructions can be found in Wikipedia, various datasheets published by its many manufacturers, or hobbyist websites. It's old as rocks. Calculate the waveform and frequency you want and smooth with a resistor + capacitor if necessary.

I remember when I was a 10-year-old electronic hobbyist and Mac programmer. I guess some things don't change! Have tons of fun.

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He wants to know the difference between a PUT and a BJT! –  Federico Russo Apr 26 '11 at 8:04
@Federico: He is following directions in a recipe book. He is 10 years old. There are no "non-tech terms" for the difference between a BJT (assuming that is a "normal transistor," which is not true nowadays anyway) and a PUT, which barely qualifies as a transistor at all. –  Potatoswatter Apr 26 '11 at 8:12
I think, if he really wanted a comparison of discrete semiconductor devices, he would have specified BJT or FET. –  Potatoswatter Apr 26 '11 at 8:14