Thursday, 31 March 2011

MAKE Electronics: Experiment 11

This is quite a long experiment so I will be writing it in steps.

Step 1: Flashing LED with a PUT:
A PUT, Programmable Unijunction Transistor is similar in function to a normal transistor in the sense that it acts as a switch. When voltage is applied to the anode it either is blocked or allowed to flow depending on the value that the gate is set at. The voltage at the gate determines how high the voltage at the anode has to be for current to flow.

The first part of the circuit is setup to explain this theory. The 2 resistors at the gate determine the voltage at the gate (and thus the voltage barrier at the anode). A resistor at the anode protects the PUT from excess current. When the circuit is set up, the capacitor attached to the anode charges and thus the voltage increases until it reaches the voltage set by the gate and current flows through and lights the LED between the cathode and ground.

Step 2: Making a metronome with a speaker:
This is very much the same circuit as above however the LED is replaced with a speaker, there is a resistor in series with the speaker, I assume this is to protect the speaker or to stop it from distorting. The sound is only faintly audible because of the very low current. A metronome can be produced by using a capacitor with a larger capacitance, this slows the oscillating of the speaker, whereas using a lower capacitance results in a faster oscillation. In this circuit a 0.0047 uf capacitor is used, therefore the speaker oscillates fairly frequently.

Step 3: Adding an amplifier:
In this step, a 2N222 transistor is added to amplify the current to the speaker so it makes a more audible tone. The collector of the transistor is protected by a resistor, the output of the PUT (cathode) is attached to the base of the transistor and then the speaker is put in series with the emitter and ground.
When the current flows through the PUT it reaches the base of the transistor which switches the current from the collector to the emitter. Because more current is flowing through the speaker it sounds louder.

Step 4: Adding another amplifier:
To give the speaker one final boost we add another 2N222, this time the speaker is hooked up between the positive voltage rail and anode, I assume it could also be placed in between the emitter and ground, the speaker only operates when the switch is open/the transistor is oscillating.

From MAKE Electronics: Videos
(at first the speaker is only going through 1 transistor, the second time it is being driven by 2, for some reason it is not much louder but at a slightly different pitch, I do not understand why this is!)

Step 5: Adding step 1 (the slow oscillator) to the fast oscillator driving the speaker:
The output from the cathode of the PUT in the top circuit is joined to the gate of the PUT driving the speaker, I assume this varies the gate voltage which means that when the PUT in the top circuit is passing current, the the capacitor in the speaker PUT circuit takes less time to charge and therefore the speaker oscillates more quickly, thus producing a higher tone, then when the slow oscillating PUT is not passing current the capacitor charges slower (as it is having to reach a higher voltage) in the speaker circuit producing a lower tone in the speaker.

Tuesday, 29 March 2011

ATX Bench-top Power Supply

I made this on the weekend. It's a converted ATX PSU, I cut the cables off and added binding posts to use as a bench-top power supply, I could have simply bought one of those cheap ones on ebay from china for ~£40 however I had this lying about and it cost me about £6 in parts.
First I took apart the PSU and took the PCB out after marking the top heights of the components on the case so I could figure out how to arrange the binding posts & fuses. I then drilled and sanded the holes (time to buy a hand reamer).

The wires colours correspond to the voltages:

Yellow - 12V
Yellow/Black - -12V
Orange - 3.3V
Red - 5V
Black - ground

I desoldered a bunch of these which I didn't need clogging up the case, and then cut the others down to a reasonable length and heat-shrinked them (so far no shorts). I then soldered the leads corresponding to the voltages I wanted; 3.3V, 5V, 12V and ground to their corresponding fuse holders and then from the fuse holder I soldered wire with crimped connectors on the end (to be used with the binding posts), hooked them up and voila, all I had left to do was to attach a switch from ground to the thin green wire which turns the power supply on and off when it is plugged in.

I mounted the fan the other way around as the binding post obstructed it from being in it's original position, I also put the fan guard on underneath as I didn't want the fan and the wires to hit each other. I used 5A quick blow fuses as these things can output ~20A and I'd rather not see my projects go up in flames if I can help it.

If you're looking for a 'how-to' vs. an 'account' I suggest looking at these links, which I used when making this power supply, I see no need to saturate the web with another set of instructions! (I'd like to note I do not use a load resistor, I didn't buy one and the supply seems to function ok without it)

One final note, if you do undertake this project, be very careful of the power caps inside, I left my supply off for a week before touching it, I'd rather be on the safe side!

Sunday, 27 March 2011

MAKE: Electronics: Project 7 - 10

I have been working through the book "Make: Electronics" By Charles Platt. I have been inspired to document my progress by James Floyd Kelly's blog regarding the book, It has been invaluable when carrying out the projects, check it out here - "" I like the idea of documenting your troubles and how you overcome them, it seems most people just document their success rather than failure, I think the latter is far more important when it comes to learning.

I've written a bit about project 7 - 10 on Xournal and uploaded the PDFs to Google Docs so people can have a look at what I learnt, I find writing down what I did right/wrong helps me understand the circuits and theory better and improves my retention of the content.

Project 7 & 8:

Hello All

Hello everyone,
I am hoping to document some of the more interesting aspects of my life on this blog, probably ranging from interesting science articles to electronics projects. I want to improve my ability to describe things as currently I find that I'm hindered by my lack of ability to be concise in what I am saying, this should be a good exercise in improving my writing skills!