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Engagement
  • Author Author: jc2048
  • Date Created: 19 Jun 2018 4:16 PM Date Created
  • Views 660 views
  • Likes 8 likes
  • Comments 5 comments
  • unijunction
  • analog design
  • transistors
  • music
Related
Recommended

Uni-Tunes

jc2048
jc2048
19 Jun 2018

Introduction

 

In the course of doing the Simple Music Maker I experimented with a few other odds and ends, both as real prototypes on a

breadboard and messing around with the simulator. Here's one of them. I'll blog about a couple of the others, too. These

aren't complete projects, just a bit of tinkering and trying things out, but they may be of interest for all that.

 

When I was in my teens and first developed an interest in electronics, one of the projects I built back then was a simple

electronic organ. The oscillator was based on an unusual kind of transistor called a unijunction transistor. It was unusual

because it had a single junction, with an emitter, two base connections and no collector. Nowadays, you can't seem to buy

new unijunction transistors, and old ones sell for a lot of money, but you can buy a replacement called a 'programmable

unijunction transistor' so I'm gong to try using one of those. This modern part isn't really a single-junction device at

all, instead it's a 4-layer structure like an SCR with two external resistors needed to define the trigger point (hence the

'programmable' in the name), but it should behave in a similar way and allow me to design an oscillator that will run at

audio frequencies.

 

 

The Device

 

The device I'm going to use is the 2N6027 from On Semi. This is readily available and not too expensive. Here is how it is

arranged to mimic the kind of unijunction device I used back then.

 

image

 

The characteristic of the device that allows for oscillation is a region of negative resistance. This is how it is shown in

an old book for a real unijunction part [1]

 

 

image

 

Once the trigger point is reached (by the emitter), the voltage decreases as the current increases. This characteristic is

very similar to that of a neon bulb (which can also be the basis of a simple oscillator if combined with a resistor and

capacitor), though the physics is obviously somewhat different.

 

If we connect the device to an RC circuit, the capacitor will charge to the trigger voltage, discharge until there's no

longer enough current to keep the device active, and repeat the cycle. [In practice, it's slightly more complicated than

that, because the dynamic load-line doesn't meekly follow the curve down, rather the capacitor holds the voltage up and the

dynamic load-line swings out and back again.] 

 

 

Practical Experiment

 

So let's try that on a breadboard.

 

Here's the circuit and the constructed circuit:

 

image

 

image

 

This is the voltage across the capacitor (yellow trace). The frequency is approximately 360Hz:

 

image

 

Here's the voltage across the 47R (blue trace) when it's discharging the capacitor:

 

image

 

 

Simple Music Maker?

 

So the next question was: how can I turn this into the simplest possible instrument? I wondered about adding a follower, to

buffer the capacitor voltage, but was there anywhere in the circuit that might drive a 40 Ohm loudspeaker? The timing

circuit was out, but the resistance that I'd used from the cathode to ground was 47R, so why not try there? It's a stupid

idea really - the discharge only lasts for a microsecond - so I had to try it and it actually works; dumping the energy

from the capacitor into the loudspeaker coil at an audio repetition rate sounds a note. It's quiet, but it did mean I could

make a very, very simple electronic organ with just one active device (hurrah!).

 

I did, briefly, have a go at doing that, with a selection of resistors to give different notes. But I gave up after I got

to five notes - in something that might, possibly, have approximated a pentatonic scale - because it was such a hassle

determining the values. Next up would have been a buffer and an amplifier for the speaker, but I'd gone back to the Music

Box at that stage and didn't get any further, so it remains a curiosity to blog about and not a project. Which suits me

nicely.

 

And, since I called this Uni-Tunes, I had better end by saying: That's All Folks!

 

[1] Handbook of Linear Integrated Electronics for Research. T.D.S. Hamilton. McGraw-Hill 1977.

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Top Comments

  • jc2048
    jc2048 over 5 years ago in reply to balearicdynamics +5
    The interesting signal - the one that really is an audio waveform - is the charging voltage (it's a nice sawtooth that will be harmonically rich), so if I had wanted to do more with the circuit I would…
  • DAB
    DAB over 5 years ago +4
    Nice post. Brings back a lot of memories of testing circuits in the lab to see what they do. DAB
  • balearicdynamics
    balearicdynamics over 5 years ago +4
    The principle sounds very interesting and you explained me a bit of "vintage" electronics that I don't know. But - as the output is obviously very low in terms of Audio signal, does it has sense so send…
  • jw0752
    jw0752 over 5 years ago

    Hi Jon,

    I just checked my stock and I have several of the 2N1671 ujt transistors. Since I haven't played with the UJTs recently it will be fun to check them out again. Thanks for your post.

    John

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  • jc2048
    jc2048 over 5 years ago in reply to balearicdynamics

    The interesting signal - the one that really is an audio waveform -  is the charging voltage (it's a nice sawtooth that will be harmonically rich), so if I had wanted to do more with the circuit I would have added a buffer (a follower of some kind) to that. It requires a high impedance - even the 10M scope probe loads it and changes the frequency a little - so it would need to be a Darlington pair, or a FET, or a MOSFET. That would then give a signal that could drive a preamp, or even an amplifier directly. It's 6V pk/pk so it would need a 10:1 attenuator to get it down to line levels.

     

    This blog is really just a bit of fun to introduce an old component to anyone who hasn't come across it before. In passing, it also introduces the idea of a negative resistance, an idea that will appear again in a future blog that's on my 'to do' list. It isn't an audio oscillator that anyone should build, unless they want to experiment with unijunction transistors.

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  • balearicdynamics
    balearicdynamics over 5 years ago

    The principle sounds very interesting and you explained me a bit of "vintage" electronics that I don't know. But - as the output is obviously very low in terms of Audio signal, does it has sense so send tit to a preamplifier session?

     

    Enrico

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  • DAB
    DAB over 5 years ago

    Nice post.

     

    Brings back a lot of memories of testing circuits in the lab to see what they do.

     

    DAB

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  • genebren
    genebren over 5 years ago

    Jon,

    I too remember the unijunction transistor.  It was a strange but fun device.  I had built several variants of oscillators as part of class experiments, but now it is distant memory covered in deep piles of dust.

    Thanks for the trip down memory lane. (Cute sign off)

    Gene

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