Fun with UJTs

Discussion in 'Homebrew and Kit Projects' started by KL7AJ, May 4, 2017.

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  1. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    I just discovered a small stash of unijunction transistors. I haven't seen a UJT circuit in ages, but they are pretty cool. You can make a code practice oscillator with one device, or use it for other kinds of timing applications. Anyone else here ever played with these?
  2. K8ERV

    K8ERV Ham Member QRZ Page

    I grew up with UJTs, and remember a rep showing me the first 555 that eventually took their place in many applications. Story goes that GE was trying to make a FET but it wound up being a UJT.

    I think the UJT is the only semi device I never burned out.

    TOM K8ERV Montrose Colo
  3. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    When I was working at a radio shop in Inglewood, we had these devices called PUTs also (Programmable Unijunction transistor) which worked like a backwards SCR. We used them for low battery voltage detectors; when the gate dropped below a certain threshold, the anode fired and set off an alert. I see they're still available on Mouser.
  4. K9STH

    K9STH Ham Member Volunteer Moderator QRZ Page

    In the 1970s, with their top of the line two-way radio communications consoles, Motorola used plug-in modules using UJT circuits for all sorts of control functions. These circuits involved several transistor stages, including at least 1 UJT, to accomplish what could be done with a simple mechanical switch!

    Those modules were a constant source of problems with component failures happening on a very regular basis. Why Motorola just didn't us a mechanical switch is a very good question.

    General Electric, in their communications consoles, did something just as stupid! To avoid having to have a separate voltage source for pilot lamps in the switches, they just put the lamp in series with the control voltage. Then, when the pilot lamp burned out, nothing happened when the switch was activated!

    Glen, K9STH
  5. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    Well, as I'm sure you recall, the Motorola Vibrosponders and Vibrosenders were electromechanical nightmares. Maybe they wanted to get away from anything resembling a relay. :)
  6. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    By the way, in the aforementioned radio shop, we had three guys who smoked pipes. I never smoked, but this was the only time I ever thought I'd like to try. There was some really great smelling pipe smoke there. So, I inhaled but never lit up. :)
    W4KJG likes this.
  7. AI0K

    AI0K Ham Member QRZ Page

    Ah, yes, I haven't thought about ujt's for probably 40 years. But I had a lot of fun with them back in the late 60's and early 70'. Not only were they good for CPO's, you could vary the tone by holding a magnet near the UJT (hall effect). It was fun to palm a small magnet in your hand and show others how you could change the tone, and then challenge them to do the same. Of course they knew nothing about the magnet... :)
  8. K9STH

    K9STH Ham Member Volunteer Moderator QRZ Page


    Actually, Motorola did not invent the reed type CTCSS systems. The thing was that, in those days, reeds were the only thing that could reliably hold the very close frequency tolerances required for reliable CTCSS operation.

    There were other systems that used the coil / capacitor tuned circuits. Unfortunately, for receiving, those circuits did not have anywhere near as close the narrow frequency bandwidths of reeds and often "falsed" on CTCSS frequencies on either side. For transmitting, the frequency often drifted so much that reed type receivers would not be activated.

    Next came the solid-state systems that used multi-turn variable resistors for tuning and those were closer in tolerance to the reed systems. Finally, digital generating systems have replaced the other forms of CTCSS encoding, and decoding, and those are, usually, even closer in tolerance than the old reed systems.

    I don't know if you are familiar with the early open type reed CTCSS and paging reeds. Those were really interesting and you could actually see the coil vibrating and making contact with the external system. Those reeds were very common in the early solid-state pagers.

    The early tube type CTCSS systems also used the open type CTCSS reeds. Then, with Motorola, the TU-217 and TU-333 enclosed reeds came into vogue. Next, the TLN-8381 and TLN-6907 small reeds came into use before the digital systems took over.

    Glen, K9STH
  9. N1OOQ

    N1OOQ Ham Member QRZ Page

    UJT sidetone osc in old Tentec rigs.
  10. WA7PRC

    WA7PRC Ham Member QRZ Page

    Keep trying. :p

    "If it ain't broke, you're not tryin' hard enough!"
    -Steve Smith aka "Red Green"
    The Red Green Show

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