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A simple test circuit to fault find audio and radio equipment. Can be used to inject a square wave signal, rich in harmonics, or used with headphones as an audio tracer.... [read more]

This circuit could be used for replacing your manual volume control in a stereo amplifier. In this circuit, push-to-on switch SW1 controls the forward (volume increase) operation of both channels while a similar switch SW2 controls reverse (volume decrease) operation of both channels. A readily available IC from Dallas semiconductor, DS1669 is used here.... [read more]

This is a simple servo tester which will comprehensively test the capabilities of almost any modern servo. It has two pushbuttons, CENTRE and SWEEP and a potentiometer which works as follows: - CENTRE Does exactly that, centers the servo, afterwards the potentiometer determines position. - SWEEP Sweeps the servo back and forth at a rate determined by the potentiometer setting.... [read more]

This circuit shows how to use an ordinary LED as a light sensor. It makes use of the photovoltaic voltage developed across the LED when it is exposed to light. LEDs are cheaper than photodiodes and come with a built-in filter, which is useful when the application involves colour discrimination. The photo-voltage of a red LED (its bandgap voltage) is typically about 2V. The source impedance of this voltage is about 800MΩ in daylight, rising to infinity in darkness. A TL071 JFET input op amp is used to amplify and buffer this extremely high impedance signal.... [read more]

Firecrackers are normally ignited by using a matchstick or a candle. You have to run away quickly after igniting the fuse of the firecracker. This method of igniting firecracker is unsafe, because the danger of the firecracker bursting before you reach a safe distance is always there. The device described here uses remote control, usually used with TV receivers or CD players, to burst the fire-cracker. Thus the firecracker can be ignited from a safe distance using the circuit described below in conjunction with the remote control.... [read more]

The purpose of this circuit was to create a ring in which LEDs or Lamps illuminate sequentially. Its main feature is a high versatility: you can build a loop containing any number of LEDs or Lamps, as each illuminating device has its own small circuit.... [read more]