Unijunction Transistor Flasher

Unijunction Transistor Flasher

Using the rather dated 2N4871 unijunction transistor it is possible to make a simple, but very bright, daylight visible LED flasher. This is a good project for the beginner and will enable the experimenter to get a basic understanding of unijunction transistor operation. Numerous unijunction transistors are available and they all work about the same. However, they are all headed toward obsolescence, so get a few of these interesting devices while they are easily available –most electronic distributors have already discontinued the 2N4871, but they are easily available on eBay.

The circuit is similar to D. Mohankumar’s Mock Flasher LED Circuit with the exception of the LED placement and the LED type. It is also similar to one that I previously posted: Programmable Unijunction Transistor Flasher. Then there is also the 555 Door Adjar Flasher or the Firefly Lights Circuit that also uses the 555 timer.

Transistor Flasher Schematic

Unijunction Transistor Flasher

Key to the brightness is the LED. I selected a Cree device –Cree is well known for ultrabright white LEDs.

Cree white LED matrix copied from the DigiKey search engine, and edited –numerous devices are available.

Circuit Operation

Operation is very simple –C1 charges via R1 & R2. When the voltage across C1 exceeds Vcc x the intrinsic standoff ratio (a fancy name for about 2/3 Vcc), the unijunction fires and discharges C1 though the LED. Per the oscillograph, the peak LED current is about 50mA when Vcc = 12V, or 70mA when Vcc = 24V. Under ideal conditions, the 2N4871 is rated for a peak discharge current of 1.5A, but the LED is rated for only 100mA –the dynamic resistance of the LED at 100mA is approximately 20Ω and this is what ultimately limits the LED current. The 1Ω resistor is a shunt for monitoring LED current –it is not required for circuit operation.

One of the specifications for the unijunction transistor is the valley current. In this case it ranges from about to 4mA (min) to 7mA (typ) to (no maximum indicated) –very sloppy spec. If R2 is adjusted too low, the unijunction transistor will not reset itself and the LED will light continuously –try it to see what happens.

R4 is not absolutely necessary, but it tends to reduce peak battery current when the device fires.

Advantages and disadvantages of the various technologies

The unijunction (topic of this discussion) is perhaps the easiest to use because the high valley current tends to guarantee that the device will oscillate over a wide range of conditions. The PUT (programmable unijunction) (2N6027) seems to be more critical in that it has less range due to the low valley current –it is, however, more efficient and offers low battery drain.

The TLC555 is perhaps the most efficient of all due to the very low battery current –the standard bipolar LM555 device is not nearly as efficient.




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  • mrdav1e

    maybe a stupid question but it doesn’t look like the pot (r2) is connected right? i wouldn’t think the top of the pot would be connected unless you’re counting on the flow following the path of least resistance.

    • mrdav1e

      Thank you.

    • Jim Keith

      R2 is a potentiometer wired as a rheostat. In this case, connecting the top of the pot to the arm is optional and makes n0 difference. The reason why I use the shown connection is in the case of an intermittent arm (common failure mode)–this type of failure will not completely open the circuit.

  • Robert Benson

    For the unijunction transistor flasher ,what wattage resistors and what voltage led are required. Thank you for your help.

    • Jim Keith

      0.25W resistors will do. Any single junction LED (garden variety) will also do, but the best will be the super bright and ultra bright types with a 30mA current rating.

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