[Alex] decided to build a solar charger for his car battery. He had an 18V solar panel able to provide up to 83mA. You cannot connect panel directly to battery because charging voltage cannot exceed allowed safe limit and also solar panel may become as load for battery in dark time and this way discharge it. So he ended up with simple circuit utilizing LM317 and couple resistors setting voltage so that battery would be charged at recommended 13.2V. In order to prevent back supply a Shottky was used. Of course it adds some voltage drop (0.7V worst case). This was taken in account while calculating voltage adjust resistor divider. As a test [Alex] left solar charger for three days connected to his battery and it charged up to 12.35V which is about 75% of capacity. Not bad at all.
Power down feature is very handy on battery operated devices. For instance most of multimeters already have this feature allowing them to self turn off when there is no activity for some time. Check out this simple solution which uses pair of FETs and resistor. It was originally made for Atmega328 project because author didn’t want to bother with AVR power down modes. Instead he used this simple circuit. It can be used to turn device on with switch this is where P-FET part plays role and turn off with N-FET. Turn Off signal wire comes from AVR pin market as 5. While device is powered N-FET has to be on with high gate signal. When N-FET gate signal goes low device turns itself off. This can be used on any device where possible to get such signal conditions. Or simply use in microcontroller circuit by adding some delay count…
When designing an electronics device you want to make sure that it won’t go in smoke under unexpected circumstances. You never know what may go wrong during life time of device especially if it is powered from mains. Jon Chandler made a great overview of common and pretty simple power protection circuits that may help to avoid lots of trouble. Small circuit between your battery and device may protect it from mistakes like reverse power polarity. His lists starts with simple series diode which is cheap but may help protect from reverse polarity. Like any other it has a downside – voltage drop (0.7V) that may be a pain for low voltage devices. The list continues with more smart solutions like Crowbar diode and fuse which avoids voltage drop but ends up with fuse blow on reverse voltage. Using MOSFET is smarter way of protecting circuits. It gives a way…
When building AVR DDS2 signal generator there were lots of discussions about signal conditioning in analog part of device. First argument was that LM358 wasn’t the best choice for this purpose. Another one pointed to sine wave that weren’t smooth enough. As you can see there are some dents on it. Other waveforms also are distorted especially when higher voltages are selected. This definitely asks for better analog part. Some people suggested to replace LM358 with OPA2134, but it seems to be quite expensive choice. In my opinion low noise general purpose op-amp can be great too. I’m gonna give a try to Texas Instruments TL074 low noise op-amp. It is low power, high slew rate (13V/us) IC – almost five times faster than LM358 and for same reasonable price.
Multivibrator circuit is one of the first projects you start learning electronics. It is really wonderful circuit widely used for educational purposes and even in end projects as waveform generators. Lots of hobbyists simply grab a microcontroller/Arduino to blink LEDs. But using basic circuits like multivibrator may be cheaper, faster and even fun. [Ray] decided to go through multivibrator theory and explain its working in detail step by step. Circuit itself consists of two transistors, two capacitors and four resistors. When powered, circuit generates square wave signal that can be used to flash LEDs, or clock other circuits. You will get an intuition on what causes multivibrator continuously generate. Formulas allow to calculate resistor and capacitor values for particular frequency. Especially if you are starter in electronics build one on a breadboard and do some experiments, why not to start wit Christmas lights.
I mentioned on a previous post that there are few projects that involves using LEDs as light detectors – well I searched in google and well “I take that statement back!”. This doesn’t actually really involve a microcontroller – it explains how to use LEDs as both emitters and detectors of light thus by using two LEDs we can build a two way opto isolator. By using a LED as a detector and a 4011 NAND gate to drive another indicator LED this tutorial takes you to the concept behind LED based communications – the 4011 has schmitt trigger inputs thus eliminating the undefined regions in the response of the detector LED. It also pulls down to ground one of the pins of a LED indicator through a current limiting resistor to indicate that the detector received data from the sending LED.
There are quite a lot of digital logic families that differ in their I/O voltage levels. Mainly we use 5V and 3.3V levels in our projects. And there is always dilemma to communicate 5V with 3.3V devices. In many cases a 3.3V devices come with 5V tolerant pins but it is better to know than guess. Here is a nice concentrated chart that provides visual information on how these logic level thresholds differ among technologies. Chart include CMOS, TTL, mixed TTL/CMOS, ETL, BTL, GT, low voltage glue logic families. FYI most CMOS families are tolerant to 5V input and probably you won’t, need a logic level converter in many cases. This is OK in hobby level, but if you are designing end product to market then it is better to place a level converter to ensure reliable operation.