Programmable Uni-junction Transistor (PUT) Flasher

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

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

PUT Flasher Circuit Schematic

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.
 Background
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

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