When measuring the resistance between two points in a circuit, you have to watch the resistance meter while ensuring that the probes are in good contact with both the test points.This circuit allows you to concentrate on one action, i.e., placing the probes at the right points, while providing an audible assessment of the resistance. The resistance-checker provides uneasy way to determine one of the following three resistance levels between its test points: Short-circuit. The two points are connected, or there is a very low resistance between them. Resistance. There is resistance between the two points. Open circuit. There is no electrical connection between the two points. The first and the third levels are thrones that we normally look for.
When wiring a circuit, you need to ensure that all the components that should be connected to the power rail are actually connected to it. For example, if a circuit has several logic integrated circuits(ICs), it is to be ensured that all the V CC pins are connected to the positive rail. Using this checker, one probe can be placed on the positive supply input terminal. You can then run through all the ICs, touching V pins with the other probe. If these are connected to the power rail, a high-pitched tone is heard every time. The connections to CC the negative or ground rail can be checked in the same manner. The continuity test can also be applied to check undesirable short circuits. A short-circuit between the positive and ground lines may lead to over-heating, burning smell, discharging of batteries, and too-low (or zero) voltage on the positive rail. To find this kind of short-circuit, first disconnect the circuit from the power supply. Connect the probes to positive and ground terminals. A high-pitched tone of around 925 Hz will be heard. Similarly, around 15Hz buzzing sound is heard when checking for open-circuits (open wires, diodes or transistors with open junction or dry solder).
At the heart of the circuit is IC 4046 phase-locked loop (PLL), which consists of a voltage-controlled oscillator (VCO), a source follower and two phase comparators. Only VCO pin has been used in this circuit. The frequency of the square-wave output at pin 4 of ICI depends on the voltage applied to its control input pin 9 and the values of capacitor C1 and resistor R1. If the voltage is equal to half of the supply voltage (+3V in this case), the output frequency is given by: f =1/ (R1.C1) With the values given here, f = 455 Hz If the input is reduced to 0V, the frequency too falls to zero. If the input is increased to the supply voltage, the frequency will be 2f, which in this case is 910 Hz. To avoid complete silence when the input is 0V, resistor R2 offsets the frequency range. A high value of R2 means that there is only a small offset. So 0V produces a low-pitched 15Hz buzz, which is just enough to confirm that the circuit is actually operating. At the other end of the range, 6V produces a note of 910 Hz.
The resistance being checked is connected between the positive rail and one of the many resistors. Range switch S1 lets you select which of the resistors will be in the circuit. On any range, an open circuit between the probes allows the switched-resistor to pull input pin 9 of IC1 down to 0V resulting in a low-pitched buzz. Also on any range, a short-circuit between the probes pulls the input pin 9 to 6V and a high-pitched tone is heard. To understand what happens when an intermediate resistance is placed between the probes, assume that S1 is turned to ground resistor R5 (10-kilo-ohm). If the resistance between the probes is close to half the supply level, a 470Hz note is generated, which is easily distinguishable from 910 Hz. When the resistance between the probes is greater than 10 kilo-ohms, the pitch is lower. As the resistance approaches 100 kilo-ohms, the pitch falls to the buzzing level. If S1 is switched now to bring R6 (100-kilo-ohm) into the circuit, the input rises again to half the supply level and the pitch rises to 470 Hz. Summing up, if S1 is switched to a given range and a medium-pitched note is heard, the resistance between the probes is of the same order as the selected resistor. If a buzz is heard, switch to a higher range and vice-verse.Switching range is necessary only if you want a rough measure of the resistance. If you are interested only in finding a short-circuit or open circuit, you need not bother about setting the range. The output of IC1 at pin 4 is fed to transistor T1, causing it to rapidly turn on and turn off through resistor R3. The current pulsing through the loudspeaker produces the buzzing sound or tones. This circuit can also be used for checking diodes and transistors. If the anode of the diode under test is connected to positive probe 1 and the cathode to probe 2, the diode becomes forward-biased. Current flows through it and the input voltage to the IC rises producing a medium-pitched note.
When the diode is the other way round, no current flows through it. The input is pulled down to 0V and a low buzzing sound is heard. You can check bipolar junction transistors (normal transistors) in the same way, treating the junctions as diodes. For the base-emitter junction of an npn transistor, the base is anode and the emitter is cathode. For the base-collector junction, the base is anode and the collector is cathode. The pnp transistors are much the same; only the anodes and cathodes are reversed. As a quick test, checking either type of transistors with the probes to the collector and emitter should give a buzzing sound. Assemble the circuit on a general purpose PCB and enclose in a suitable case. S1 is a single-pole, four-way switch. In place of S1, a four-way, dual in-line miniature switch can also be used. Connect its four pins along one side to the ground and the remaining four pins on the other side to the resistors. Mount switch S1 on the front side of the case with its common terminal connected to the ground. Use a pair of mini test clips, which are generally easier to attach to the pins of ICs. Banana plugs can also be used.
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