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This circuit is intended for alerting purposes after a certain time is elapsed. It is suitable for table games requiring a fixed time to answer a question, or to move a piece etc. In this view it is a modern substitute for the old sandglass. Useful also for time control when children are brushing teeth (at least two minutes!), or in the kitchen, and so on.... [read more]

The circuit of graphic equalizer, allocates ten adjusting potesometer , that each one from them affects in a predetermined area of frequencies, the central frequency of which abstains a octave (double), from the central frequencies of her neighbouring regions. Each unit has common materials with remainder and it differs only in the capacity of capacitors that constitutes the filter in each unit.... [read more]

While talking to a distant subscriber on telephone, quite often we feel frustrated when the voice of the distant subscriber is so faint that it is barely intelligible. To overcome the problem, circuit of an inexpensive amplifier is presented here. It can be assembled and tested easily. There is no extra power source needed to power up the circuit, as it draws power from the telephone line itself. The amplifier will provide fairly good volume for the telephone conversation to be properly heard in a living room. A volume control is included to adjust the volume as desired.... [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 simple circuit generates narrow pulses at about 700-800Hz frequency. The pulses, containing harmonics up to the MHz region, can be injected into audio or radio-frequency stages of amplifiers, receivers and the like for testing purposes. A high-pitched tone can be heard from the speaker of the device under test when all is working properly. The clip must be connected to the ground of the device under test, touching with the probe the different stages of the circuit, starting from the last stage and going up towards the first. When the tone is no longer heard, the defective stage has been found.... [read more]

Here are two examples of bistable flip flops which can be toggled between states with a single push button. When the button is pressed, the capacitor connected to the base of the conducting transistor will charge to a slightly higher voltage. When the button is released, the same capacitor will discharge back to the previous voltage causing the transistor to turn off.... [read more]