Multiple Continuity Tester

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Multiple Continuity TesterThe continuity tester is a handy adjunct to an ohmmeter. The unit or component whose continuity is to be checked is connected between terminals E1 and E2 (which may be probes or croc clips). The test current then flowing through the unit/component on test causes a potential drop across resistor R2, which is applied to the non-inverting input of buffer IC2. The output of the op amp is applied to transistor T1, in the emitter circuit of which there are a number of parallel-connected light-emitting diodes. Each LED is in series with a zener diodes and a resistor. The zener diodes have dissimilar zener voltages as shown in the diagram. When the drop across R2 exceeds the sum of base-emitter voltage of T1, a zener voltage, and the threshold voltage of the LED in series with that zener diode, the relevant LED lights.

The diagram shows at which resistance value of the unit/component on test a particular LED lights. Bear in mind, however, that these values depend to some extent on the type of LED, and also that the zener voltages are subject to tolerances. Serious deviations may be corrected by the addition of a standard diode or a Schottky diode. It is also possible to add branches to individual requirements, or to use a bar display instead of LEDs. It is important that the op amp used has a rail-to-rail output since the input voltages as well as the output may rise to the peak supply voltage. This requirement is met by the MAX4322 as used in the prototype.

Continuity Tester Circuit DiagramHaving good contacts is important not only in your daily life, but also in electronics. In contrast to social contacts, the reliability of electrical contacts can be checked quickly and easily. Various types of continuity testers are commercially available for this purpose. Most multimeters also have a continuity test function for electrical connections. A simple beep helps you tell good contacts from bad ones. However, in some cases the tester doesnt produce a beep because it wont accept contact resistances that are somewhat higher than usual. Also, poorly conducting (and thus bad) connections are sometimes indicated to be good. Here e-trix comes to your aid with a design for a DIY continuity tester that helps you separate the wheat from the chaff. Circuit diagram:

Continuity Tester Circuit Schematic Circuit description: Many multimeters have a built-in continuity test function. However, in many cases the resistance necessary to activate the beeper when you are looking for bad connections is just a bit too high. It can also happen that the beeper sounds even though the resistance of the connection is unacceptably high. This circuit lets you adjust the threshold between bad and good contacts to suit your needs. The circuit is built around an operational amplifier (IC1) wired as a comparator. The opamp compares the voltage on its inverting input (pin 2) with the voltage on its non-

inverting input (pin 3). The voltage on pin 3 can be set using potentiometer P1, so you can set the threshold between good and bad connections. When test probes TP1 and TP2 are placed on either side of a connection or contact to be tested, a voltage is generated across the probes by the current growing though resistors R1 and R3, and it appears on pin 2 of the opamp. This voltage depends on the resistance between the probe tips. If the voltage on pin 2 is lower than the reference voltage on pin 3, the difference is amplified so strongly by the opamp that its output (pin 6) is practically the same as the supply voltage. This causes transistor T1 to conduct, which in turn causes DC buzzer BZ1 to sound. This means that the resistance of the connection being tested is less than the threshold value set by P1, and thus that the connection is OK. By contrast, a bad connection will cause the relationship between the voltages on the inputs of the opamp to be the opposite, with the result that its output will be at ground level. The transistor will not conduct, and the buzzer will remain still. To ensure that the opamp toggles properly (which means that its output goes to ground level or the supply voltage level) when the difference voltage is sufficiently large and does not oscillate during the transition interval due to small fluctuations in the difference voltage produced by interference, its output is coupled back to its non-inverting input (pin 3) by resistor R4. This causes any change on the output to be passed back to this input in amplified form, with the result that the detected difference voltage is amplified (and thus boosted). Diodes D1, D2 and D3 protect the circuit against excessive positive and negative input voltages that may come from the connections or contacts being tested. They also ensure that the continuity tester does not inject excessively high voltages into the item under test. Capacitor C1 suppresses high-frequency interference. The circuit draws only a small supply current, so it can easily be powered by a 9-V battery.

Three-State Continuity TesterThe continuity tester can distinguish between high-, medium-, and low-resistance connections. When there is a conductance between the inputs, which are linked to small probes, a current flows from the +9 V line to earth via R1 and R2. The consequent potential difference, p.d., across R2 is used to determine the transfer resistance. Operational amplifier IC1c amplifies the p.d. across R2 to a degree that is set with P1. A window comparator, IC1a and IC1b, likens the output of IC1c to the two levels set with potential divider R4R6. Depending on the state of the outputs of the two comparators, three light-emitting diodes (LEDs) are driven via the gates and inverters contained in IC3 and IC2 respectively in such a way that they indicate the transfer

resistance in three categories.

Circuit diagram: Three-State Continuity Tester Circuit Diagram When the resistance is high, green diode D3 lights; when it is of medium value, yellow diode D2 lights, and when it is low, red diode D1 lights. The levels at which the diodes light is set with P1, but note that in any case the minimum value depends on the p.d. across R2. It is possible to reduce the value of the p.d. to enable lower transfer resistances to be detected, but this would mean an increase in the test current through R2. With values as specified, the circuit in its quiescent state draws a current of about 17 mA, but in operation each LED adds about 10 mA to this. The LM324 (IC1) may be operated from a single supply line: R1 prevents the voltage at the input from reaching the level of the supply line (which is not permissible). The supply voltage may be 518 V. The LEDs are driven directly by the inverters in the 4049 (IC2), which can switch currents of up to 20 mA to earth.

Circuit Continuity Tester Using Op-Amps

The circuit was developed to produce a continuity tester with a low resistance mainly for checking the connections between soldered joints.

Operational Amplifier (Op-Amp) a DC coupled high gain electronic voltage amplifier with differential inputs and usually a single output Light Emitting Diode (LED) a semiconductor diode that is commonly a source of light when electric current pass through it 741 Op-Amp the most common and cheapest op-amp used in several circuits because of 1 MHz gain bandwidth product and is not prone to producing false oscillations due to tailored frequency response

The use of a 741 op-amp to fully function the circuit provides several features such as high input voltage range, excellent temperature stability, no latch up, short circuit protection, offset voltage null capability, and internal frequency compensation. As it operates in differential mode, it provides high input impedance and low noise amplification in the input stage. Specifically, the input being amplified comes from the voltage difference between the inverting and non-inverting inputs. The amplification is done by the full open loop gain of the op-amp which is developed when there is no feedback used in the circuit. However, in the presence of increasing frequency, the open loop gain of an operational amplifier falls very quickly. The addition of 470K ohm and 10K ohm resistors in the circuit is very essential since they are responsible for creating a minimal voltage difference to be applied to the inputs of the op-amp. Without these resistors, if both resistors connected to the op-amp inputs are ideally equal, the circuit would be balanced wherein the output from the probes would be zero thus producing zero voltage difference. On the other hand, as this voltage difference is amplified, there will be a swing to full supply by the op-amp output which will cause the LEDs to shed light. Before using the circuit for live testing, the probes should be connected initially to a resistor having a value between 0.22 ohm and 4 ohms. This is done to adjust the control until the LEDs give light while having this resistor across the probes. After the adjustment, the resistor must be removed while shorting the probes so that the LED light will disappear. The probes should be

kept clean and free from dirt to avoid the increase in resistance and the circuit not to function well because the circuit itself has an extremely low resistance value. I cases where the LEDs do not switch off, the 10K ohm preset resistor should be connected across the offset null terminals which is the Pin 1 and Pin 5 on the metal can package. These offset null terminals are responsible for eliminating the effects of internal component voltages on the output of the device. The wiper of the potentiometer or the control should be connected to the negative terminal of the battery terminal. This offset circuit will work at 0V and Vcc and will behave as a comparator where it compares two voltages or currents and switches its output for indication of which is higher. The 741


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