Having fun with discrete transistors - Page 1
Forum section: electronics / projects

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 10, 2019, 09:16:14 pm
content: It was cold and raining outside, a miserable Sunday afternoon. I had just finished a book, and did not want to binge-watch another Netflix series. Bored to death, I headed to my lab to figure out something, anything, to do.Rummaging thru my parts bin, I noticed some 2N2646 unijunction transistors. When was the last time I actually built something employinga metal-case transistor? Eons ago. So I decided that this was it.I quickly downloaded the 2N2646 datasheet, and quickly breadboarded a circuit, and in no time I had a oscillator running consisting of only 3 resistors, 1 capacitor and a single transistor.Could this simple circuit be enhanced? The Base1 pulse is kind of wimpy, and the emitter waveform is the classic non linear RC-charging ramp.The first thing is to replace the capacitor charging resistor with a CC source. There are dozens of CC schematics, but in the spirit of discrete transistors, it was replaced with a J175 P-chan JFET found in my parts bin. This transistor has as very healthy IDSS of up to -60 mA. Therefore it has to be throttled down with a source resistor. If a trimpot is inserted in series with the resistor, the current and therefore the frequency can be adjusted.To amplify the output pulse to a rail to rail level, the B1 pulse was applied to the gate of an 2N7000 Mosfet, also from the parts bin.Now that I have a nice looking pulse, what should I do with it? Drive a transistor bi-stable multivibrator, of course. Classic circuitry, I initially simulated in LTSpice to find workable values. Once that I became satisfied, I breadboarded the circuit, and connected it to the pulse.The final schematic is attached, and the results are shown in the waveforms image. Amazing that with such simple circuitry, using only discrete semiconductors, many things can be achieved.BONUS POINTS: And it was done with the four main transistor families: UJT, JFET, MOSFET and BJT. Hurray!By now, my wife is calling me. Supper is ready.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: January 10, 2019, 09:22:26 pm
content: Thanks for posting your circuit. Where in Mexico do you get miserable, cold and rainy afternoons in January?

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 10, 2019, 09:36:35 pm
content: Rigo Tovar's hometown.Not cold, cold, like our fiends in Canada or Sweden. But very high humidity makes the weather dreary.

author: jmelson
author_rank: Super Contributor
timestamp: January 10, 2019, 10:58:43 pm
content: Quote from: schmitt trigger on January 10, 2019, 09:16:14 pmYikes! A diode-steered, capacitor-coupled FF! What a huge blast from the past. I built one of these when I was in high school, from the GE transistor data book.I really didn't have the test gear to know if it ever worked or not (1967 or so). A couple years ago I whipped one up on a solderless breadboard and it worked quite nicely.Jon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 11, 2019, 12:28:22 am
content: NiceOne of my favorites, a pile of transistors to realize a fully symmetric master-slave flip-flop, hard wired as a type T. Forget if I have a circuit of it yet... can't seem to find anything. Did I mention, realized in ECL/CML, differential I/O? Forget how many transistors it was, 16? 32? Ran up to 50MHz I think, just from 2N3904s.Tim

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 11, 2019, 02:45:55 pm
content: The key to run bipolar transistors that fast was to keep them out of the saturation region.....consumed a lot of power though!

author: soldar
author_rank: Super Contributor
timestamp: January 11, 2019, 03:10:34 pm
content: Quote from: schmitt trigger on January 10, 2019, 09:16:14 pmGosh, I still have a few which I have kept over the decades... I believe I have not used one in decades nor seen any new designs using them. Are they even made any more? I doubt it. They are really of very limited use and 555 chips or others can be substituted with advantage.

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 11, 2019, 09:15:36 pm
content: The 2N2646, having a metal can, has long been obsolete.The plastic versions 2N4870/ 2N4871 are still available. Though not widely available.

author: james_s
author_rank: Super Contributor
timestamp: January 11, 2019, 09:17:00 pm
content: I like UJTs, I remember using them to build lamp flashers, it can be done very easily.I have an old Heathkit wind speed/direction display that uses a UJT based oscillator to clock the display latches.

author: Circlotron
author_rank: Super Contributor
timestamp: January 11, 2019, 09:27:57 pm
content: Next, how about an oscillator using an SCR and a resonant tank? These kinds of circuit techniques are becoming lost in the sands of time. And what about a tunnel diode doing something?