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    This is simple circuit that illustrates the function of the programmable unijunction transistor. It may be quickly wired on a proto-board.

    PUT Flasher Specifications

    • Flash rate: approx 1hZ
    • Load current: approx 300uA
    • Minimum voltage: approx 4V

    PUT Flasher Circuit Schematic

    put flasher

    How it works

    The programmable unijunction transistor remains dormant until the voltage across C1 exceeds the gate voltage of Q1 by one diode drop (0.6V) or in this circuit about 6.8V in reference to circuit common. At this point, current starts to flow into the anode of Q1. When the current exceeds the “peak current” threshold (about 1.25uA), the transistor triggers and shorts all three terminals together until the anode current drops below the “valley current” (about 100uA in this case), and the transistor resets itself. When the transistor triggers, it dumps capacitor C1 across the LED. Peak current is limited by the LED internal resistance.

    By varying (programming) the gate voltage, the voltage at which the transistor fires varies—in this case, it varies the flash rate.


    This cute little device was developed by General Electric about 50years ago. The original GE part numbers were D13K1 and D13K2 (now 2N6027 and 2N6028). The 2N6028 has tighter specifications for more precision applications. On Semiconductor is now the sole manufacturer, but NTE has a relatively expensive equivalent device.

    PUT Applications

    There are many, many applications for this device, but it is mainly useful for triggering thyristors. Other than for thyristor triggering, the ubiquitous 555 timer has taken over.


    Proto-board circuit layout

    For the Future

    Use of this device in a thyristor switch circuit

    Bill of Materials

    • Q1 On Semiconductor 2N6027 (comes only in T0-92 package)
    • R1 100K
    • R2 10K
    • R3 22K
    • C1 10uF, 25V aluminum electrolytic
    • D1 LED Red, High Brightness
    • Battery Standard 9V

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    7 Responses to "Programmable Unijunction Transistor Flasher"

    1. This is an elegant circuit.

      thank you very much.

    2. Its a very useful circuit.

    3. Thanks for this. I was working through the Make: Electronics book and it wasn’t working for me. The values were a bit different and that helped me to find my mistake. I had substituted a 470 resistor for a 470k on the anode. I went back and changed it out on their circuit and it also began to flash. Again: THANKS!

    4. can i use the design for 2n 6028 also????

    5. moonbriyahoo-com says: on February 20, 2016 at 4:49 am

      Having trouble getting it to flash. I did not have a 10uf capacitor so substituted 47 uf. The anode charges to 7.8 volts and gate is at 5.4? Capacitor value should change cycle rate only, correct?

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