The easiest headlamp
You will need:
SD card case
Li-Pol cell
5R6 resistor
small piece of adhesive tape
rubber band
+ half an hour of work :)


Behringer Graphic Equalizer EQ700 - modified to my own requirements

(always EQ present, increase gain after rock the pedal)


Electronic bass drum - better woof sound, no feedback issues

Built-in digital effect processor for electric banjo
Note: Metal on the bridge creates steelguitar sound

Input circuit for Winscope
There are few good freewares usable as audio signal oscilloscope. One of them is Oscilloscope 2.51 which uses PC soundcard input as signal sampler.
Disadvantage of commercial souncards is AC coupled input, but there is not so important for measuring AC signals (music and sine without DC offset). Important thing at repairement of an audio equipment is right measuring the AC voltages (Vp-p) to set right gain etc. I designed easy input circuit with voltage reference, all supplied from USB.

One switch ON/OFF control
There is lot of schematics about control relays, lights, etc. by one momentary switch (or pushbutton), but these are the simplest with 555 timer IC and 4069 (or 7404) hex inverter.

Another one is direct powered from 230V AC, always OFF after power restart. Resistors 120k and 4k7 have not exact values, they must be set according to ON-OFF characteristics of used relay.

LED driver
Because the LEDs have not linear current-voltage characteristics and also their voltage is depended on temperature, they must be powered by constant current source. Ordinary constant voltage source (like battery) can damage the LED. I know, the serial resistor can be simple solution, but only for higher ratio Source voltage to LED voltage (but this is not effective because of too much power loss across the resistor).
I designed (and also tested) simple current source for one, two, three or  more LEDs. Just put this circuit between +12V and anode and ~22mA current will be flowing through the LEDs and it won't be too depended on source voltage, temperature or number of LEDs.

12V LED bulb for car
After I changed (third time in one year) the bulbs in my rear lights (because of compulsory day-lighting in our country), I build very simple LED bulbs which have intensity of light comparable to original 10W tungsten bulbs. I know efficiency is under 50% because of balancing resistors, but total current consumption is about 50mA at 12V. Much lower than 840mA with original bulb. I used two SMD LEDs SPMWHT5606 (5200 mcd, 120°, 3x20mA) built in old bulb base.

POWER CONTROLLER for motors, bulbs, etc.
Once I have bought cheap RC boat without speed control.
I found that it has 4-channel RC system, but only 3 channels were used (for left motor, right motor and both motors).
The unused 4th channel inspired me to build power controller. Now I can select by remote one of 4 levels of power.
There is 555-based PWM controlled by voltage level set by counter/current summator/PNP transistor.
Input must be connected to output of decoder (logic levels can be 3.3-5V for High, and 0V for Low)
You must disconnect original ground wire from motors and connect it to OUT. Power MOSFET will be switching to ground with selected duty cycle.

Here you can see the video how it is working.

Infrared to RF converter
If you need to increase the range of your IR controlled device, you can use 434 or 868 MHz hybrid RF modules.
I designed and tested following circuit and it works fine.