Those of you who like the telephone hell louder may find this circuit of interest. It uses the telephone bell signal to actuate an oscillator, which in turn drives a relay that operates a standard door gong. The oscillator is an RC type based on a 4093 Schmitt trigger. Both the charging and discharge times[…]
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Real-time switch debounce
Most debounce circuits shorten or lengthen the time a switch is pressed to obviate interference pulses. The present circuit does not affect that time: the pulse at the output is exactly in accordance with the time elapsed between the depressing and the releasing of the switch. When switch S1 is pressed. monostable IC1a is triggered[…]
Read moreTelephone extension
Telephone extension: In many countries, unlike. most of the UK, it is still not possible to parallel two or more telephones easily. The circuit described here may either be connected in parallel to an existing telephone or be used as a stand-alone unit, when it will energize a relay on receipt of an incoming call.[…]
Read moreFault signalling circuit
Fault signalling circuit: The present circuit was developed to make it possible for different sensors to be added to an existing alarm installation. These sensors may be gas or smoke detectors, door switches, 1 infra-red detectors, and others. In quiescent operation, the level at all 44 inputs must be zero. When the relevant sensor is[…]
Read moreStorage oscilloscope tester
The tester generates two different ” signals for checking a digital storage oscilloscope. The first is a stepped voltage on to which glitches are superimposed. The second is a rectangular 2 kHz signal on to which a 15 Hz analog signal is superimposed The first signal quickly shows whether the oscilloscope eliminates the glitches during[…]
Read moreTriangular-signal generator
Pulse duration modulators 19%; a perfect triangular signal, that is, one that is absolutely symmetrical with straight sides. Any deviation from this results in distortion. The design is the usual rectangular-triangular oscillator. A Schmitt trigger converts a triangular wave into a rectangular one. which is used to continuously charge and discharge a capacitor via current[…]
Read moreInfra-red a.f. receiver
This infra-red receiver is the counterpart of the infra-red transmitter. The output from the infra-red diode D1 is amplified by IC1. The diode is a Type BPW41N which works well with the transmitter and is fairly fast 200 ns). The output pulses of IC1 are taken to comparator IC2, which separates them from the ambient[…]
Read moreVideo demodulator
The demodulator is based on a Type TDA8341 chip, which is the successor of the well-known Types TDA2541 and TDA3541. A part from a demodulator, the chip contains AGC (automatic gain control) section for tuners whose AGC voltage is directly proportional to the gain and an AFC (automatic frequency control) facility. In the present design,[…]
Read moreWideband active telescopic antenna
Wideband active telescopic antenna: Most VHF/UHF amplifiers for use with asymmetrical antenna (such as an open or closed dipole) have a balun at the input. A balun (short for balanced to unbalanced) is an inductive device that converts the symmetrical (balanced) RF signal into an asymmetrical (unbalanced) that can be applied to the base of[…]
Read moreField-strength meter
Field-strength meter: The unit discussed here indicates, by means of a chain of LEDs, in logarithmetic ratios the strength of an electric field surrounding the unit. The inputs signal across the conducting discs is applied to the non-inverting input of IC1. The amplification of IC1, A = R4/P1. In the prototype, P1 was set at[…]
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