Description.
This circuit can be used to test whether mains voltage is present or not without having electric contact with mains line. The CMOS IC CD4033 is the heart of this circuit. The CD4033 consists of a 5 stage decade Johnson counter and an output decoder for converting the Johnson code to a 7 segment decoded output for driving 7 segment LED display. A 10cm long insulated copper wire connected to the clock pin (pin1) of the IC serves as the sensor. The sensor wire has to be placed in the vicinity of the mains wire to be tested. When there is no voltage in the mains line, no voltage will be induced in the sensor wire and the display will show a random digit. When there is voltage in the mains line, a small voltage will be induced in the sensor wire due to electromagnetic induction and this voltage is sufficient enough to clock the CMOS IC CD4033. Now the display will count from zero to nine and repeat.

Circuit diagram.

Notes.

• The circuit can be assembled on a Vero board.
• Use 9V PP3 battery for powering the circuit.
• Use a 10cm insulated wire as the sensor.
• The IC must be mounted on a holder.
• Switch S1 can be a miniature ON/OFF switch.

All the testing methods below uses a multimeter and hence its How to test FET’s especially the JFET and MOSFET using a multimeter.

How to test an FET ?

FET testing method

FETs are checked by measuring different resistances by a multimeter. When resistance is checked between source and drain, it should be of the order of 10 kohms.

How to test a JFET ?

JFET testing methods

When a JFET is checked as a diode (gate-to-channel junction) multimeter should indicate low resistance between gate and source with one polarity and very high resistance between gate and source with meter polarity reversed.

Troubleshooting a JFET test

If the meter indicates high resistance with both the polarities, it means that the gate junction is open. On the other hand, if meter indicates low resistance with both polarities, it means that the gate junction is shorted.

How to test a Mosfet ?

Mosfet testing methods

While checking MOSFET, the resistance measured between gate and drain should be infinitely high in either polarity. Low resistance means faulty device.

Thermistors are of two types, NTC (negative temperature coefficient) and PTC (positive temperature coefficient types). As their name indicates the resistance of an NTC thermistor will decrease with temperature and the resistance of a PTC thermistor will increase with temperature. Both PTC as well as NTC thermistors can be roughly checked by using an analogue multimeter.

Keep the analogue multimeter in resistance mode. Connect the multimeter terminals to the thermistor leads. Polarity is not an issue here. Now heat the thermistor by moving your heated soldering iron tip to it. Now you can see the multimeter reading smoothly increases or decreases depending on whether the thermistor under test is PTC or NTC. This will happen only for a healthy thermistor.

For a faulty thermistor, following observations are possible.

• The change in reading will not be smooth or there will not be any change.
• For a short thermistor the meter reading will be always zero.
• For an open thermistor the meter reading will be always infinity.

This is only a rough test. For a perfect check up; you need some way to measure the temperature and the corresponding resistance reading must be according to the thermistor’s temperature-resistance characteristics provided by the manufacturer.

This article shows you how to test a capacitor using analogue multimeter. Firstly short the capacitor leads for discharging it completely. Set the multimeter to high resistance mode. Connect the multimeter terminals to the capacitor leads. For electrolytic capacitors the positive terminal of multimeter must be connected to the positive lead of the capacitor and negative terminal of multimeter to the negative lead of capacitor. For other capacitor types, polarity is not an issue.

At the moment you connect the multimeter terminals to the capacitor leads, the multimeter needle will move to zero and then slowly move towards infinity and settle there. This will happen only if the capacitor under test is healthy.

• If the capacitor under test is short, the multimeter needle will go to zero and remain there.
• If the capacitor under test is open, the multimeter needle will not move (will remain at the infinity position which is the initial position for analogue multi meters).
• If the capacitor under test has leakage then the needle will first deflect to zero, and then slowly move towards infinity and will settle at a point before infinity.

This is only a rough test and for complete check up you need to varify the capacitor value using a capacitance meter.

LDR (light dependent resistance) can be very easily tested by using a digital multimeter. We all know that the resistance of an LDR varies according to the light falling on it. At bright light, the LDR resistance will be around 500Ohms and at darkness the resistance will be around 200K. For a proper diagnosis we need to measure the resistance of the LDR at bright light and at darkness.

Test1.
Keep the multimeter at Ohms mode. The LDR must be subjected to a bright light source (day light is enough).Connect the LDR leads to the multimeter terminals as shown in the figure. Now the multimeter will show a low resistance reading around 500 Ohms.

Test2.
Keep the multimeter at Ohms mode. The LDR must be subjected to darkness by covering it with an opaque paper. Connect the LDR leads to the multimeter terminals as shown in the figure. Now the multimeter will show a high resistance reading around 200K.

If both of the above tests are positive then we can conclude that the LDR is healthy.