Charging CAP to open PUT

[Jeldage]

I am following along with projects in the MAKE: Electronics book, and have been experimenting with using a Programmable Unijunction Transistor to blink an LED. Using the books schematic, it works fine, but when I change the resistor and cap values, it behaves differently.

It takes roughly a second for the LED to turn on, but it never turns off as it does in the book's circuit. What is it about my circruit that is causing the PUT to never close the gate?

FYI: The book's original circuit has the PUT blocking current until the capacitor charges enough to match the voltage at the gate and promptly discharges through the PUT and LED dropping the voltage back down and ceasing flow through the PUT, and repeats.

You can compare my circuit with the book's

 

[(*steve*)]

if the resistor is low enough, the PUT may not be able to discharge the capacitor faster than the resistor charges it up. So it will turn on, but never turn off.

Those two 120 ohm resistors in your circuit are just wasting power. Make them larger (you'll see they are much larger in the original circuit)

 

[Jeldage]

if the resistor is low enough, the PUT may not be able to discharge the capacitor faster than the resistor charges it up. So it will turn on, but never turn off.

Those two 120 ohm resistors in your circuit are just wasting power. Make them larger (you'll see they are much larger in the original circuit)

That did it! Switched the CAP's resistor out for some higher values and it works. I hadn't considered the resistance through the PUT and LED. It also never dawned on me that the low value resistors were just dumping a lot of current. Much appreciated!