12V Vehicle Electrical Wiring Tester Circuit

This little tester is useful for checking vehicle electrical circuits. Two LEDs indicate whether one of the clips is connected to the positive supply line (red) or to ground (green).

The unit is powered by the vehicle battery. It is advisable to terminate the unit into  two insulated heavy-duty crocodile clips. These enable connection to be made directly to the battery or to terminals on the fuse box. It is also possible to terminate it into a suitable connector that fits into the cigarette lighter socket.

IF a sharp needle is soldered to test point, it is possible to check insulated wiring, but only those that carry 12V. Although the needle pierces the insulation, it does not damage it.

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Infrared Remote Control Tester Circuit

This is a simple circuit you can build in order to test infrared remote controls. The circuit uses an IR photo-transistor (IR1) to detect a remote control’s IR output signal.

This circuit is designed to  demodulate amplitude-modulated (AM) IR light beams will drive a speaker.

LM741

LM386

LM741 is amplified the IR1 signal and LM386 is driven the speaker.

VR1 is used to control sensitivity and VR2 is used to control the volume of speaker (LS1)

The Speaker  will sound when any of the Remote-Control pushbuttons will be pressed.

This circuit tests consumer electronic hand held remote controls such as those used by TVs, VCRs, DVD players and more.

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In Circuit Transistor Tester Schematic

Here is a circuit that can indicate the condition of a transistor by using two LEDs. A good NPN transistor conducts during the positive half cycle (pulses are generated by 556 dual timer) and D5 is off while D6 flashes. With a good PNP transistor D6 is off and D5 Flashes.

Both LEDs flash if the transistor is open and will be off if it is shorted.

The tester checks the transistor even if in-circuit resistance across the transistor is as low as 50 ohms; below this the glow becomes dim. Diodes in the ensure that only low saturation voltage of a good transistor turns the LED off.

NE556 Dual timer IC

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SCR TESTER CIRCUIT

The Device under test cathode, anode and gate are connected to the unit’s CATHODE, ANODE and GATE terminals, respectively.

Pressing switch S1 feeds a gate current to the DUT (Device Under Test), which triggers it on. Resistor R1 limits the gate current to the appropriate level. Resistor R3 limits the current through the LED to about 20 mA which, with the  current through R2, results in a latching current of about 110 mA. The LED is used to monitor the latching current. IF the DUT is good, once the gate is triggered with S1, the LED will remain light, indicating that the device is conducting. To end the test, turn off the device by interrupting the latching current flow using switch S2. The LED should turn off and remain off. The preceding procedure will work SCRs and Triacs. To check LED and other diodes, connect the anode and cathode leads to the anode and cathode of the diode; LED should light. When the leads are reversed, the LED1 should remain off.

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Crystal Tester

We have crystals lying about, but don’t know whether these are still working all right. The crystal tester described here will quickly show whether a crystal can be used or should be discarded.

Transistor Q1 and the crystal under test from an oscillator. Capacitors C1 and C2 form a voltage divider in the oscillator circuit. If the crystal is in good order, the oscillator will work. Its output voltage is then rectified and smoothed by D1 and C4 respectively. The resulting direct voltage at the base of Q2 is sufficient to switch this transistor  on, so that the LED lights.

The circuit is suitable for use with crystals of a frequency between 100kHz and 30MHz. Current consumption is about 50mA.

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Zener Diode Tester Circuit

This tester helps to check the voltages of zener diodes. It is very inexpensive and  handy to find the voltages of small glass zeners, whose markings get rubbed-off very easily. The 741 op-amp has been used in differential mode.

The 47k linear potentiometer’s dial is calibrated in terms of voltages between 0 and 27V, either by using various known zeners in the range, or by applying a known voltage at pin 3 of IC 741. The position of the potentiometer where the LED gets turned off, gives the breakdown voltage of zener.

The power supply voltage can be between 9V and 30V. But zener to be tested, and should not be changed once the dial is calibrated.

You can also check the polarity of zener. The cathode side should be connected to the red clip.(i.e. pin 3 of IC 741) If you connect the zener the other way round, then it will just behave as a diode, and the LED will remain off at all positions of the potentiometer.

In case of a zener that is shorted from inside, the LED will remain off with zener connected either way ti the clips. In case of a zener that is open-circuit from inside, the LED keeps glowing at all positions, with zener connected either way to the clips.

You may even differentiate between a diode and a zener by their behaviour on this versatile tester.

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