Wednesday, January 20, 2010

Chapter 2 - Exercise 11 Step 1

I'm breaking up Exercise 11 into 4 Steps... this post will cover Step 1 that is covered on page 85. Be sure to read up on the PUT (programmable unijunction transistor). Apparently this transistor is losing favor and you might find it difficult to track down locally... I was lucky to grab some at my local ACK Supply...

I read over the Essentials sidebar a few times on page 83 just to make sure I absolutely understood how the PUT works. Very similar to a standard transistor, but its inserted differently and its 3 connectors are named differently and work slightly different.

Now, for Step 1, unless you bought a big bundle of resistors, you might not have the values this experiment calls for - I've substituted resistors in earlier exercises, but not knowing a lot about how much this new circuit could handle (or require), I felt it necessary to get the proper resistors this time. I bought a huge bundle of 500 resistors in just about every value imagineable... $12 at Radio Shack. The only problem with this many is finding the exact one you need in the big bundles... I converted 27K, for example, to red-violet-orange and it was easy enough to locate, but I still used my multimeter to confirm its value... ditto for the 15k, and 470k resistors required. While I was at Radio Shack, I picked up a 2.2 microfarad and a 22 microfarad (99 cents each). I'm fortunate my Radio Shack is about half a mile from my house... gave me an excuse to go out for lunch today.

So, I wired up the circuit, plugged it in... and the LED began blinking fairly quickly. I've got a video below showing the 2.2 microfarad circuit. After unplugging, it still blinked (capacitor discharging, including any capacitance in the AC Adapter) and then finally died out.

Next, I substituted the 22 microfarad... it took a bit for the LED to begin blinking, but it did eventually blink. I've included a video of it, but I'm having trouble seeing the small green LED blink... but it does blink... I think my camera's built-in video recorder is very low resolution... sorry. Also, notice in the picture that I've used patch wires to connect the 22 microfarad - its metal "legs" are too short to insert into the breadboard where one is connected to the Anode and the other end connects to the negative voltage column. I used 3" lengths of stripped wire to assist with completing the circuit.

As the exercise states... we now have an oscillator. If I leave the AC Adapter plugged in, it'll just keep going and going... now on to Step 2 where we'll add some more to the circuit.

NOTE: Be sure to change your AC Adapter to 6V if you had it previous set to 12V.


  1. I bought the big bundle of resistors too. Then I bought a fishing tackle box to sort them into. I sorted them using the color code and then testing them with my DMM just to see if I was any good at reading the color code. I wasn't at first.

    After I measured each group of resistors, I wrote the nominal and measured resistance on the tape strips. Then I sorted them into the tackle box according to the third color band. It was kind of a boring task, but I just did a couple of groups at a time over several days and it was not too onerous.

  2. I wired up this circuit this morning, and it worked as described -- blinked quickly with the 2.2uF capacitor and slowly with the 22uF capacitor. I even clamped my multimeter leads to the capacitor and was able to observe the build-up and discharge.

    Then I tore the circuit apart and had my son rebuild it. Worked as before EXCEPT we can't get the capacitor to discharge when the multimeter is connected -- builds up to about 4.2V and then just sits there. As soon as I disconnect the multimeter, the LED flashes and all returns to normal.

    Best I can figure is that the multimeter is eating up a little voltage and not allowing the capacitor to build up as much as it should to exceed the PUT's gate threshold.

    Tried switching the power to a higher voltage, but got a similar result (I'd assume because the higher voltage is causing the PUT's gate threshold to proportionately increase).

    I tore apart the circuit and rebuilt it, and also validated the values of the resistors.

    Once again, we got the point of the exercise and will thus probably just move on to the next one.