Some we need a negative resistor in the circuit we can take advantage from Field effect transistors(FET).source

here is a design for a “Telephone Ring Generator” which when connected to a suitable telephone will cause the bells to ring in the correct “cadence”- for what I might call the “British Post Office” type format.The ringing sequence used in the USA is 4 secs ring, 8 secs silent, 30Hz ringing frequency, then cycle [...]

This is short wave regenerative receiver.This circuit is use general electronic components except inductor.It is a nice performer and will tune from the 22 meter international broadcasting band down to the 11 meter band.source

Here is DC-DC converter using LM2576 circuits.It performs same function as the commonly known voltage regulator LM7805 from National Semiconductor. The 7805 voltage regulator dissipates a lot heat. The higher input voltage, the more heat is generated. The extra input energy is converted to heat, keeping the output voltage regulated at 5V.source

This device uses two photodetectors: the “signal” detector looks for an interruption in the light beam, and the “reference” detector measures the background light. The circuit subtracts one signal from the other to determine the ball’s position. source