How to test a relay

john February 16, 2010 13 Comments
This article shows you how to test a relay. A relay will usually have a coil, pole terminal and a set of contacts. The set of contacts that are open when the relay is not energized are called normally open (N/O) contacts and the set of contacts that are closed when the relay is not energized are called normally closed (N/C) contacts. The following steps can be used to perform the testing of the relay using a multimeter. Keep...
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Wind meter

john July 29, 2009 6 Comments
Description. Here is a very simple wind meter (anemometer) circuit. I can’t guarantee much on the accuracy of this circuit but it circuit works quite fine. You can measure wind speeds up to 75m/s using this circuit. The transistors Q1 and Q2 are used for sensing the wind. The relationship between thermal impedance of the transistor and the surrounding wind speed is utilized here. Transistors Q1 and Q2 are wired so that the Vce of Q1 is higher than...
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Zener diode tester

john September 21, 2009 6 Comments
Description. Here is a very simple circuit that can be used for testing Zener diodes. The IC1 NE555 is wired as an astable multivibrator and the output of the IC is stepped up to a high voltage AC using the transformer T1.The unloaded AC voltage will be around 120V. The AC voltage is rectified by the diode D1 and filtered by the capacitor C3.This voltage is applied across the Zener diode through the current limiting resistors R2 or R3...
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Transistor tester circuit

john March 23, 2011 7 Comments
This article discusses various circuits that can be used to test a Transistor, both NPN and PNP. We have structured this article into two circuits. If you have any doubts about any section please ask through comments. 1. Transistor Tester built using Transistors 2. Simple Transistor Tester (Contains both Circuit Diagram and PCB Lay Out) 3. LED Based Transistor Test 1. Transistor Test Circuit Description. Here is a very simple circuit that can be used to check the hfe...
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VHF field strength meter

john April 15, 2009 9 Comments
Description. This is a simple and low cost wide band VHF field strength meter. The field strength is measured by converting the radio signal to DC and measuring it. The RF signal will be picked up by the coil and rectified by the diode D1.Even a very small DC voltage is sufficient to alter the biasing of FET and it will be reflected in the meter as an indication of the presence of a radio signal. The meter can...
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Quartz crystal tester circuit

john April 15, 2009 No Comments
Description. This a very simple and inexpensive circuit that can be used to test quartz crystals.Here a Colpitts oscillator is wired used the transistor T1.When the crystal is connected between terminals A and B the circuit will produce high frequency oscillation.The oscillations will be generated only if the crystal is a good one.The diode OA91 will rectify these oscillations and it can be visually identified from the meter.The deflection of the meter will be directly proportional to the activity...
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High impedance DC voltmeter.

admin August 17, 2009 5 Comments
Description. A high impedance DC voltmeter using a uA741 IC is shown here. The operational amplifier is used as a non-inverting DC amplifier in which the negative feedback is through a DC meter requiring 1mA for full scale deflection. Since R6 is 100 Ohms, the meter will show full scale reading when the DC input voltage to pin3 is equal to the voltage drop across R6, viz 0.1 volts. Choice of R1 and R2 for getting different voltage ranges...
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Audio clipping indicator circuit

admin April 7, 2010 No Comments
Description. This circuit can be used to identify whether there is a clipping in a particular wave form. Clipping is a phenomenon in which the amplitude of a particular waveform drops before it reaches the expected limit. This circuit glows an LED as an indication if the signal under test has clipping. The circuit is very useful in sorting out distortion problems in amplifiers.   The circuit is based on a window comparator based on the two opamps inside...
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Signal tracer circuit.

admin April 7, 2010 13 Comments
Description. A high gain amplifier circuit that can be operated from a battery pack of 6 or 9 volt battery pack is shown here.The IC1 uA 741 is wired as a high gain non-inverting amplifier and its output is amplified by a pair of complementary transistors (Q1 & Q2) to drive the speaker.The negative feedback from the common emitter junction of the two transistor stabilizes the gain of the circuit.The resulting audio output is sufficient to serve as a...
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Schmitt trigger circuit using IC uA 741

admin February 20, 2014 5 Comments
Description. A Schmitt trigger circuit shows two distinct signal input levels for turning the circuit ON and OFF. The difference between the Von and Voff voltages called hysteresis Schmitt triggers are useful in converting slowly rising waveforms into fast rising ones and in relay like application. The circuit shown here uses a 741 with positive feedback via R5,R6 and R7 for fast switching.Voltage dividers R8 and R9 set the DC input voltage to the non-inverting input terminal to half...
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