Usually resistor decade boxes are mechanical – a rotary switch comutates resistors while turning it around. But if you decide to build one, you may face problem by selecting this switch. They seems to be quite expensive and quite old fashioned. Stynus has been planing to build one of those, but mechanical switching handle didn’t look very attractive, so he decided to switch resistors electronically through microcontroller.
He ended up by building three PCBs where one is a control board carrying LCD, PIC 16F648A microcontroller, rotary encoder and push button. Other two boards are resistor boards – one for high value resistors and another for low value. Each resistor board caries 16 small relays that are controlled through shift registers. Then only annoying thing with electronically controlled resistor box is that you need a power supply for it where mechanical boxes are passive. But seeing resistor value on screen probably pays off.
There are many uses of countdown timer. It can be used for PCB development, in a kitchen or during workout. All you need that timer was easy to set up and start. Umetronics have build simple PIC16F886 based countdown timer with four bright 7-segment displays.
LED displays are driven using TLC5916 LED driver and couple dual P-channel MOSFETs. Time can be selected with rotary encoder which comes with button for select. When timer runs out there are two options for notifying – one is display flashing and another micro-switch selectable sound option along with green LED indicator. Sometimes sound might be annoying, so you can always turn it off. Timer is two AAA battery powered. So it is small portable timer for any use.
It is said: “There are 10 types of people in this world, those who understand binary and those who don’t”. If you choose understanding side, then you might want to wear binary wristwatch.
Building such watch is fairly simple and easy, because you only need few components and even simpler code. Elias watch is based on PIC microcontroller which comes in SSOP-20 package. With few difficulties on soldering it to small PCB he had it running. Watch is powered from coin cell battery CR2032. With smart software solution combined with sleep modes this watch can last long. But the main point of wearing this watch to be identified as nerd. I’m not suggesting to replicate this one, but build your own version.
There is always a dilemma on how to build an indicator for your next project. There are many options like LCDs, LEDs, VFD. Sometimes one or another is enough, but eventually you want something eye catching and obvious that could be seen from distance. For instance for water meter a dial based display probably is better than LCD. IT can be seen from distance and is informative to tell how mutch water is in tank.
John simply built a LED ring display out of 16 single color LEDs. He needed to use shift register, but he thought that popular 74xx595 chip is very current limited. Total current draw shouldn’t exceed 50mA which is like 2 more powerful LEDs at a time. So he looked for more proper shift register and came op with better one – TLC59282 16-bit shift register which is capable of sinking 30mA on each pin. Neat feature of this chip is that it comes with current setting pin, meaning that you only need single resistor for setting current for all LEDs. This saves space on the board. He wrote a nice demo for PIC microcontroller which goes through several display effects. Once you’ve done playing you can use code snippets for displaying info on it like water level, RPM, speed or what ever you want.