## Wednesday, March 18, 2015

### Simple Circuit Unijunction Transistor Flasher

Robometricschool. - Here is very simple circuit for Unijunction Transistor Flasher using 2N4871 like in figure 1 below. 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.

Circuit Schematic
 Figure 1. Simple Circuit Unijunction Transistor Flasher(Source: electroschematics)

Description

Circuit schematic like in figure 1 above is Simple Circuit Unijunction Transistor Flasher. The 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.

The original article : "Unijunction Transistor Flasher"