Quick and Dirty Assembly Instructions:
- Check your power supply, it must be able to supply 5V. If it is not within .2V of 5 (i.e. 4.8V-5.2V) you cant use it
- Assemble the first 555 timer circuit according to the final schematic. The first timer circuit is at the top of the schematic, the second is in the middle, and the third is at the bottom.
- Test the circuit (see tips below for testing information)
- Assemble the 2nd 555 according to the schematic; it is not the same as the first timer circuit!
- Test 555 circuit #2
- Assemble 555 circuit 3
- Test 555 circuit 3
- Prepare the logic ic's by supplying power and grounds to each. Power needs to be connected to pin 14 of both ic's, and ground must be connected to pin 7. Refer to the pinouts and number convention pic for more details.
- Connect the output of timer 1 and the output of timer 2 to the inputs of the first XOR gate
- Connect the output of timer 2 and the output of timer 3 to the inputs of the second XOR gate
- Connect the outputs of the XOR gates to the inputs of the AND gate
- The output of the AND gate is your final output: the output is the positive terminal to drive a LED. I strongly recommend putting a 220 ohm resistor in series with the LED to prevent device failure! The negative terminal is ground.
- Attach another resistor to the positive power. This is the positive lead for your power LED. The negative terminal is ground.
- Attach power supply.
Note: see pictures spread throughout this guide for clarification of steps.
Click to Enlarge
Tips for building and troubleshooting:
Build in sections, and, if possible, build a prototype that uses no solder first. The worst thing that can happen after you have just spent three hours soldering a project together is that when you turn it on it does absolutely nothing. Even if you cant do this you can probably test as you go. Start by building a 555 circuit. Once just that circuit is complete (use the information on the 555 link given earlier if you're not sure what is the timer circuit) use a LED hooked in series with a 220-ohm resistor to watch the output. If the LED is on solid, either you used the wrong values for resistors and capacitors and the frequency is too fast for the LED to respond to it.
Unfortunately, another possibility if it stays on constantly is that you have miswired something or that you have fried your 555 (they are static sensitive, so treat it like any other computer component). If it doesn't seem to light, make sure you give it at least two minutes before you give up. It is possible that you used the wrong values again, but this time the frequency is really small (i.e. large period). Again, there is also the possibility that you have miswired something or that you fried the 555. If nothing lights, the first thing you should check is if the power is truly on (best way to do this is with a voltmeter- ground the black lead and probe pin 8. It should be at your supply voltage, which if you follow this guide exactly, is 5V) and that it is connected to both pins 8 and 4. Also, make sure pin 1 is connected to ground.
Once all the 555s work, it's on to the logic gates. Keep in mind that these ICs are also static sensitive, and will be destroyed if you shuffle across your rug and zap them. If by this time you have built up a good bit of confidence, I recommend you test the logic gates to make sure that a.) they're not fried and b.) you have power and ground hooked up properly. Otherwise, just be very careful and make sure you hook the correct pins to ground and power (pin 14 should go to +5V and pin 7 should go to ground). The logic circuit part is actually really easy- just hook the outputs of the 555s to the inputs on the XOR gates (make sure that the fast timer is pared with the medium timer on one, and the medium timer is paired with the slow timer on another) and then hook up the outputs of the XOR gates to the inputs of the AND gate. Once you've done that, presto! the output of the and gate is your final signal. Just hook up your light and go! Make sure that if you're using a LED (i.e. a case light) that you add a 220-ohm resistor in series to limit current. If you don't, the LED will probably start to feel hot after a couple of minutes, and then *poof* the smoke will escape and the LED wont be worth the silicon it's made of. If at this stage your LED isn't blinking at all, and you are sure that your 555 circuits are working right (i.e. you tested them, you didn't just assume they work), and you tested your gates to make sure they function as they should*, all I can really say is that you should check to make sure your power supply is still working and that everything in your circuit is still powered and recheck all your wires between the 555s and the logic and the wires between the gates. Except for the few things that I pointed out in this, I cant really say ahead of time what to expect. Troubleshooting really just requires a little experience so you know where to look, and patience so you actually think of all the possibilities and check them out.
For example, I had a miswiring problem when I first prototyped the circuit it wasn't flickering like I thought it should. It just stayed on, turned off, turned on, ect. It was *kinda* working, but not quite. I first considered that the 7486 couldn't respond fast enough to register the flicker, but ruled this out by directly connecting the 555 to an input of the 7486, setting the other to 1, and putting a led at the output. It turned out that I had just accidentally put one wire in the wrong place, and once I moved it, the circuit worked fine.
You should make sure all of the pots are set to the middle of their range before you even worry about making adjustments. If you built it right, this adjustment should give a fairly convincing HDD flicker. With adjustment it can get better though.
You'll be able to tell your circuit is working properly if when it's all built your LED flickers for 1-3 seconds, then turns off for 1-2 seconds, then turns on for 1-3 seconds, ect. Like I said, it's not completely random, but it definitely could be passed off as hard disk activity. Basically, you need to adjust the pots so that the frequencies of the 555s are best matched to create a random signal. It took me an hour or two to come up with a combo that worked well in simulation (in case you're curious, the ratios I used in designing were 8:1 and 4:1, the first ratio being fast:medium and the second being medium:slow)and I designed the circuit with two things in mind: the frequency ratios I discovered in simulation, and the fact that real components are not ideal and will almost always vary a little bit from their stated value. Also, I'm sure that I didn't exactly hit the "sweet spot" for randomness (it seemed like the "randomness" changed on kind of a bell curve- if the frequency ratio was too small, it was less random, and if the ratio was too big it was also less random). At the least you should try different combinations of adjustments, keeping in mind that the ratio between medium and slow is the most important. If you want to fiddle with the resistors and caps, by all means do it. Use the data on the 555 page I linked to earlier to improve on my design, for it is far from perfect- its just pretty good. Just be sure you don't do anything permanent with an experimental design. ; )
*To test your logic ICS, just hook up the power and ground leads to the IC, and then hook a LED in series with a resistor and hook that up to the output of the gate. Then apply all of the combos of high and low (there's only four permutations, so don't sweat it) and make sure the gate is acting like it should. If you couldn't figure it out from my description above, there are truth tables for each gate on the data sheet for the IC.
The final product:
Doesn't look like much does it? Well it sure fools everyone that walks into my room… they think its just another computer!