Wire it up: A Modular Project
This experiment was the longest yet. The goal here is to build up a circuit that makes a pulsating style noise, like a burglar alarm. In the process I learned about programmable unijunction transistors (PUTs), and how to use bipolar transistors (in this case the 2N2222) to amplify a signal.
Making some notes to myself here about PUTs, as this is the major new component introduced in this experiment. The most common PUT is model 2N6027.
Like a bipolar transistor, a PUT has three leads. However, they are called the anode (input), gate, and cathode (output). The lead orientation of a PUT is opposite of a 2N2222. So if you have the flat side facing left and the round side facing right, this will put the anode at the top and cathode at the bottom.
There are two main differences between a PUT and a bipolar transistor. In a NPN style transistor, a small amount of positive voltage will activate the gate lead. In a PUT, the point that the gate enters the switch point can be programmed to a particular threshold. This is done by putting a resistor inline to the gate lead, which programs the voltage level. I’m still not 100% clear on how you know which resistor you chose to get which voltage level. Apparently you just guess via trial and error, based on the ranges specified in the PUT’s datasheet.
The second difference between a bipolar transistor and a PUT is that a bipolar transistor can amplify a signal. I believe it amplifies the current, not the voltage, though I could be wrong.
Now onto the experiment, which is broken up into four steps. The video above includes steps one, the second half of step three, and step four.
Step 1: Slow-Speed Oscillation
In step one you wire up a simple LED oscillator. This one is pretty basic. A resistor with a higher level of resistance (470K) feeds a PUT’s anode, while two lower resistance resistors (15K, 27K) feeds the gate wired to the positive and negatives sides of the circuit. An LED is wired to cathode of the PUT, and the into the negative side of the circuit.
The oscillation in this circuit is created by a capacitor (2.2uF) which is wired (along with the 470K resistor) to the anode of the PUT. The other side of the capacitor is wired to the negative side of the circuit.
The PUT starts in an off state, and so the LED is not lit. As electricity slowly flows through the 470K resistor it charges the capacitor. Once the capacitor gets close to 6 volts of charge, the gate threshold of the PUT is triggered, the capacitor discharges, and electricity from the anode begins to flow through to the cathode, lighting the LED.
The switch then closes as the 470K resistor is once again blocking the flow of electricity below 6V, and the capacitor slow charges, starting the cycle over again. This repeats about every half second.
Step 2: Beyond the Persistence of Vision
This is kind of interesting as now the author introduces the concept of modular circuits. As you create the second step of the circuit, you see it’s a self contained ‘chunk’. It reminds me of a function in programming. Like a function, it accepts an input and produce an output. As long as those two interfaces remain the same, you can screw around with the internals of the section and it should continue to interact with the circuit as a whole.
The point of this second part of the circuit is to create another oscillation, but this time much more rapid oscillation. This speed of this oscillation can’t be seen by the eye, but can be experienced by the ear as sound.
This circuit is virtually identical to the first circuit, accept in place of the the LED is a loud speaker. The loudspeaker has a 100ohm resistor in front of it to protect it from current overload.
Also very importantly, the capacitor used is a much smaller (.0047uF instead of 2.2uF). This way the charge gets full and dissipates much quicker, about 500 times faster. So this means a pulse about 1000 times second.
Step 3: Amplification
The problem with the circuit at this point is that you can barely hear the buzzing of the speaker. The signal is too weak, so you need to amplify it. Adding two 2N2222 bipolar transistors can amplify the signal. The 2N2222 amplifies at a 24:1 ratio. Putting two inline gives you more than a 500:1 amplification.
To reconfigure the circuit you wire the anode of the PUT to the base lead (gate) of a 2N2222 transistor, with a 1K (why 1K? I’m not sure) resistor between the two. The gate lead of the 2N2222 leads to yet another base lead on the next 2N2222, which is then wired into the loud speaker the same way the loudspeaker was originally wired into the PUT. The collector of the 2N222 is hooked into the negative side of the loudspeaker, and the positive side of the loudspeaker is still wired to that the 100ohm resistor, which in turn is hooked into the positive side of the circuit. Oh, and the emitter of the 2N222 is hooked into the negative side of the circuit.
The result of all this work is a much stronger audio signal. It sounds like a steady drone. In my video, I swapped out the 1″ loudspeaker with a 3″, so the sound is really loud.
Step 4: Pulsed Output
You have some kind of alarm at this point, but it’s not very ‘alarmy’. It doesn’t have that whooping siren tone that is so annoying. The second section of the circuit is where the sound generation begins, specifically where the PUT and the capacitor interact to oscillate 1000 times a second. What’s needed is to automatically change the voltage threshold on the gate of the PUT, resulting in different voltages getting through which will result in a small range of sounds making it to the loudspeaker. Different voltages seem to mean less different frequency of oscillation, which changes the sound.
The first section of the circuit which the LED is currently hooked up to is the perfect way to do this. Removing the LED, you replace it with two components. First, a 10K resistor runs from the cathode of the first PUT to the gate of the second PUT (the beginning of the sound generation circuit). Second, a 2.2uF capacitor also runs from the cathode of the first PUT, to the negative power side of the circuit.
Once you’ve wired up the circuit as described, the output of the first section feeds into the input of the sound generation circuit. The capacitor at the cathode of PUT 1 begins filling up at the beginning of the pulse and releases at the end. I’m a little foggy on the next part. I think what happens is the voltage goes through a range of levels as the capacitor on the cathode of PUT1 charges/discharges. The result is that the voltage threshold of the second PUT2 also goes through a range of threshold, which then causes the signal produced by PUT2 to change how often it oscillates.
The result is is a very fast ‘sweep’ sound which is like an alarm.
Okay I just did a little more experimentation. If I take the capacitor out (the one that hooks up to cathode of PUT1) I still get an oscillation, but it’s just pretty much two tones. Only putting it back in do I get that sweeping sound. The reason I took it out is I was wondering why I needed two capacitor in the first part of the circuit. Well, the first capacitor modifies the signal going into PUT 1, and the second capacitor modifies the signal going out of the PUT 1. I think of it like using two guitar stomp boxes in line, like two delay pedals. The second pedal adds more delay to the first one, much more delay sound than if you were applying the second delay pedal to just the guitar’s output alone.