2N3906Analog
Dual LED blinker with PNP astable multivibrator
Astable multivibrator built with two 2N3906 PNP transistors that take turns driving the two anodes of a common-cathode bicolor LED, making it blink alternately between red and green.
Analog
2N3904Analog
Astable multivibrator with transistors
Classic astable multivibrator built with two 2N3904 transistors that switch alternately to blink two LEDs. The circuit generates a square wave with no integrated circuit at all.
LOC110Analog
Linear isolation amplifier for an isolated analog input
Isolation amplifier built around the LOC110 linear optocoupler that copies a 0 to 5 V analog voltage across a galvanic barrier. The trick is the servo loop on the input side, which cancels the LED non-linearity and leaves the gain set by nothing but two resistors.
LM324Analog
Square wave oscillator
How a square wave oscillator works using an LM324 operational amplifier with positive feedback and a timing capacitor.