Posted by Matthew
I recently found a doorknob touch alarm schematic while browsing Discover Circuits’ archives. The project was originally intended as a present for my brother’s dorm room, but a bad capacitor and the lack of a proper oscilloscope caused delays. It has not made it off the breadboard, and it probably will not until his next semester. The circuit contains a few basic elements, an flip-flop based oscillator, a set of delays, a flip-flop as a sensor, and the audible alarm.
Posted by Matthew
Depending on the application, charging batteries can be complex process. Charging methods range from constant voltage to pulsed and random charging. Once power is being delivered back into the battery, you have to know when to stop charging!
Once a battery is fully charged, the charging current has to be dissipated somehow. The result is the generation of heat and gasses both of which are bad for batteries. The essence of good charging is to be able to detect when the reconstitution of the active chemicals is complete and to stop the charging process before any damage is done.
Typically, common household batteries are charged with a current that is kept constant and relieved when the batteries reach a predetermined potential. However, solar cells typically generate a constant voltage of 0.5V and a varying current that depends on the amount of collected light. As such, a consant voltage charging model is easier to implement. I found two respectable tutorials on building your own charger:
The first solution uses a diode to stop the batteries from discharging when there is no sunlight. I highly reccomend including this protection. Unfortunately, neither project implements a charge limit. You have to remove the batteries and test their charge with a multimeter. A shunt regulator is the simplest way to regulate the upper limit.
Posted by Matthew
Ladyada has an interesting rant on her new blog. In linear circuit analysis, we tend to ignore the difference between the positive and negative inputs. However, in practice positive feedback results in a bistable circuit. (Also read up on negative feedback.)
Usually everyone ends up remembering this detail the hard the way. While designing a tape head preamplifier she was utilizing this circuit from an application note:
After a few hours of staring at the circuit and debugging and wondering “man why the hell is this railing?” I finally look back at the datasheet and realize: oh its in positive feedback, of course its railing.
Just a reminder that the textbook (or pdf) isn’t always right!
Posted by Matthew
Scouring the Internet for information on LEDs, I accidentally stumbled upon a PDF detailing a flashlight made from PVC. For the torch, he biases ultra-bright LEDs with ballast resistors as described in my LED lighting guide. This is a simple solution, but a with a slightly more complicated circuit we can extend battery life by over ten times!
Download PDF instructions for the inefficient version
Posted by Matthew
LED lighting is becoming increasingly popular in fish tanks, case mods, and even household lighting
LED lights are extremely efficient compared to standard incandescent lighting. No other lighting source outputs as many lumens per watt. They are particularly efficient at producing a single color of light. Other light sources have to produce the entire spectrum and optically filter out unwanted colors. However, LEDs can be manufactured to produce only one wavelength of light. This makes them particularly useful in stop lights.
Posted by Matthew
Lately I have been daydreaming a lot in my laser communication course. Surfing around Google after class, I managed to find an article on sending serial port data over a laser link. The article is rather old (1997), but was published in a prominent Australian magazine. This is way back when a good, red laser pointer was only $70.
All said, this is an interesting read. It would be nice to see this applied to more practical situations. Many computers do not even have serial ports anymore. We have been discussing the feasibility of using a laser to send video signals in the DIY Live Forums. The idea is great because running cable your projector is often a major problem in home theaters. Unfortunately, I have no home theater and my only only monitor is an old, huge CRT. I will have to find another excuse to play with lasers.
On a side note, it is theoretically possible to send terabits of data per second over one channel, because the optical frequency of a laser is extremely high (~460 terahertz for a red laser). This bandwidth is limited to mere gigabits by the response time of current optical detectors and circuitry, though.
Posted by Matthew
I recently discovered an interesting application note from National Semiconductor in October of 1999. The article details nine sine wave generation techniques. Some techniques can be accomplished several ways so there are more than nine circuits available. The note also tabulates each technique with its frequency range, typical distortion, and amplitude stability. Both fixed frequency and adjustable frequency oscillators are covered. The simple crystal oscillator also has one of lowest distortion rates.
Posted by Matthew
If you have ever wanted to build your own binary clock, then Building a binary clock is the best reference I have ever seen. The page lists complete schematics and explains their functionality in detail. In my opinion, it looks a lot geekier straight from the breadboard!
Don’t forget to check this diy binary clock out, too.
