This is Crystal tester circuit using NPN transistors as main. LED1 is light up when a crystal is good and simple oscillator circuit works.
This Simple remote control tester circuit so use this photo transistor is light receiver from remote control.If it is nice to have bias of transistor BC558
High performance LM3914/LM3915 VU meter circuit projects that widely popular can display with 20 LEDs on stereo or mono with 10 LEDs for all audio system. [...]
Here is cheap Stereo audio vu meter circuit with PCB. using KA2284 x2 and 10 LED or 5 LEDs per channel, it is simple to builds and few parts. put projects
LED VU meter using transistor is analog circuit to show level of power audio signal. You can build 5 LED to 20 LED display with PCB.
This simple moisture meter circuit uses for level moistness temple of the earth. Fine the value R2 give Meter read the value has 1mA. When the moistness [...]
This is simple Solar plant watering alarm circuit. It will sound alarms when the soil is dry. The trees have died. It the solar sell is 6V DC power supply