This is really low budget frequency counter as it uses only microcontroller and standard LCD indicator.

It can be used for calibrating of other devices like frequency generators or simply measuring unknown frequencies. Assembly routine can be downloaded here.

There may be some situations when it is hard to use a tachometer only because it is impossible to read revolutions per minute mechanically. The only way is to read RPM without touching rotating shaft.

This great design was constructed by Ibrahim Kamal who used an IR signal to transmit and receive it reflected from reflective stripe fixed to rotating shaft. Device is controlled by AT89S52 microcontroller which counts reflected pulses and displays value on LCD. RPM counting algorithm uses last three times between pulses and this way constantly gives instant RPM value. RPM meter consists of two parts: microcontroller board and IR proximity sensor connected to microcontroller board via simple 4 wire cable. Project files are available for download in a single zip.

Matrix keypads are common devices where human has to interact with embedded system. Most common are scanning matrix keypads where rows or columns are constantly scanned to detect which button were pressed.

This nice course takes goes through theoretical explanation to practical implementation on AVR and 8051 microcontrollers. Each schematic is followed by ASM and C example programs. This is really practical tutorial especially for those who are new to microcontrollers.

The idea of this programmer was to get rid of Parallel port which is common for ISP programmers. One reason is that parallel ports are disappearing from computers(I think COMs are also). Second reason is that most ISP programmers aren’t safe - especially without buffering circuits or simple resistors. Anyway these things are a matter of discussions. There is a simple ISP programmer that runs from COM port.

One shortcoming is that programmer uses another microcontroller AT89C2051 which communicates between PC and target MCU. But this option gives flexibility for future upgrades. As author promises it will also going to support AVR microcontrollers. For now it has following features:

• Supported devices - AT89S51, AT89S52, AT89S8252, AT89S8253;

• Read and write of Intel Hex files.

• Chip Erase.

• Verify.

• Lock.

• Read Device Signature.

• Advanced Device Specific Functions.

Windows GUI and firmware are available for download. But keep track for newer versions.

These things are very popular among radio amateurs because they look cool and unusual. This one is developed by Ivica Novakovic. He didn’t use any motor to move Stick of LEDs but he used spring like steel plate which allows to vibrate at high frequency. This way vibrationscreates a virtual display with resolution of 30×7 LED dots.

Because of vibration method it is not easy to determine the position of LEDs in order to synchronize displayed information. So there is a piezo element used, which detects mechanical movement and via amplification circuit it gives a synch signals which guide AT89C2051 microcontrollers in lighting LEDs. There are four buttons for time setting which also include alarm setting. When alarm is triggered, LEDs are playing effects. Program and PCB are available for download.