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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 nifty sound level meter is a perfect one chip replacement for the standard analog meters. It is completely solid state and will never wear out. The whole circuit is based on the LM3915 audio level IC and uses only a few external components.... [read more]

This alarm circuit was designed to monitor a mains-powered smoke detector located in a shed (which is used to house dog kennels). It provides complete isolation from the mains so that low-voltage (12V) cabling could be run to the alarm circuit which is located inside the house. In operation, the alarm signal (I) from the smoke detector is rectified using bridge rectifier BR1 and then fed to optoiso lator OPTO1 via resistor R3. This in turn drives the gate of SCR1 which turns on and activates a piezo siren with inbuilt oscillator.... [read more]

This two-transistor white noise generator has a surprising feature – about 30dB more noise than the more traditional designs. Q1 and Q2 can be any small-signal transistors with a beta of up to 400. The reverse-biased emitter-base junction of Q1 provides the noise source, which is fed into the base of Q2. Q2 forms a simple amplifier with a gain of 45dB. The improved output level is due mainly to the inclusion of C1, which provides a low-impedance AC source to the noise source while not disturbing the DC bias of Q1.... [read more]

This is a small protection circuit from loudspeakers, from DC voltage that likely to exist after some damage in the power amplifier.... [read more]

This supply uses no heavy step down transformer and has an extremely low parts count. The circuit can be built very small and can supply small currents for small projects. The major downfall of this supply is that it is not isolated from the AC line and can only supply small currents.... [read more]