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AUDIOHM A nifty audible continuity tester Most continuity testers beep at you when the circuit being tested is good but not this one. It gives a tone which varies from a low note (a few hundred Hertz) for a low resistance to just above audibility for an open circuit. This feature prev­ents the AudiOhm from driving you mad while you are not actually measuring anything. By RICK WALTERS One of the things we frequently do in the pursuit of our hobby is to check for continuity or bridged tracks in the pro­jects we build. While some digital multimeters have a buzzer for this function, most do not. Sure, you can use a multimeter to measure continuity of circuits. Just switch it to a low Ohms range and you are in business. Trouble is, you have to keep looking at the multimeter to see if anything has registered each time you put the prods on the cir­cuit. This is where an audible indication is pretty handy. As well, the AudiOhm can test diode and transistor junc­tions and it will even give a relative indication of the ca­ pacitance and leakage of electrolytic capacitors. When using the low range, you can discriminate between a short and a resistance of 72  Silicon Chip Fig.1: the circuit is based on FET input op amp IC1 and phase locked loop IC2. The DC output from IC1 is proportional to the resistance across the probes and this signal is used to control the frequency generated by the VCO in IC2. The output from IC2 drives a loudspeaker via complementary output pair Q1 & Q2. 50Ω and on the high range a 4.7MΩ resistor will register. It can also readily differentiate between, say, a 56Ω and a 560Ω resistor – handy when you are building a project and find the colour codes hard to read. How it works As you can see from the circuit in Fig.1, only two ICs are used. IC2 is a 4046 phase locked loop but we are using only its VCO (voltage controlled oscillator) section. IC1 is a FET input op amp and its DC output, which is proportional to the resistance across the probes, is used to control IC2. Let’s look at IC2 in more detail. Its oscillator output frequency at pin 4 is controlled by the DC voltage applied to pin 9; 0V gives the lowest frequency and +9V gives the highest. The lowest frequency is set by the capacitor between pins 6 & 7 of IC2 and the resistance at pin 12. The highest frequency (with +9V applied to pin 9) is determined by both these former values and the resistance at pin 11. These frequency setting resistors have been made adjustable with trimpots VR1 & VR2 to allow you to set the tones to your particular preference, as well as to compensate for variations in ICs from different manufacturers. The maximum frequency is set by VR1 and the minimum by VR2. IC2’s oscillator output at pin 4 is connected to a pair of complementary emitter followers, Q1 & Q2. These provide sufficient current gain to drive the speaker. We have used a fairly large resistor in series with the speaker to keep the volume down to a reasonable level and also to reduce the current drawn from the battery. Our unit drew only 18mA so the battery should last for a long time. Op amp IC1 is used to monitor the voltage across the probes and amplify it a level sufficient to give the full audio range at the speaker. On the high resistance range, as selected by toggle switch S2, a high impedance voltage divider (one 4.7MΩ and two 1MΩ resistors) sets the voltage across the probes. As you can see from the voltages on the circuit of Fig.1, there is about 1.34V across the probes when no external resistance is present. Note that while we have quoted fairly precise values here, the actual values will depend on the battery voltage and resistor toleranc­es. This voltage of 1.34V is amplified by IC1 to give about 7.6V at its output (pin 6) and this is fed directly to pin 9 of IC2, to set the highest frequency. When an external resistance is present between the probes, the voltage between the input pins of IC1 will be less than 1.34V; if a short circuit is present, there will be virtually no voltage between pins 2 & 3 and so the output voltage at pin 6 will be the same as the voltage on pin 2; ie, +1.34V or close to it. This sets the minimum frequency from IC2. PARTS LIST 1 plastic case, 130 x 68 x 25 (Altronics H0342 or equival­ ent) 1 PC board, code 04103971, 57 x 55mm 1 miniature 8Ω loudspeaker (Altronics C0606 or equiv.) 1 SPST toggle switch (S1) 1 DPST toggle switch (S2) 1 9V battery 1 battery clip 1 set of test leads (Altronics P0403 or equivalent) 1 16-pin IC socket 1 8-pin IC socket 1 5kΩ PC mount trimpot (VR1) 1 500kΩ PC mount trimpot (VR2) Semiconductors 1 CA3160E op amp (IC1) 1 4046 phase locked loop (IC2) 1 BC338 or BC548 NPN transistor (Q1) 1 BC328 or BC558 PNP transistor (Q2) Capacitors 2 100µF 16VW electrolytic 2 0.1µF MKT polyester 1 .047µF MKT polyester 1 .022µF MKT polyester Resistors (0.25W, 1%) 2 4.7MΩ 2 10kΩ 3 1MΩ 1 2.7kΩ 1 150kΩ 1 56Ω 1 47kΩ Miscellaneous Hookup wire, solder. March 1997  73 When the low range is selected with switch S2, a lower impedance voltage divider is switched in parallel with the high range divider. This keeps the voltage applied to the probes the same, but allows them to sense lower values of resistance due to the increased current. Without this range switching, it is harder to resolve lower resistance values. Putting it together We designed a small PC for the Au- the IC sockets, transistors and capacitors. Make sure that they are all correctly oriented, as shown in Fig.2. If you use PC stakes, now is the time to fit them. I prefer to poke each wire through the PC board and solder it, as it makes a neater looking connection. Wire the two switches, the battery and speaker leads next. Before fitting it all into the case, you should plug the ICs in and do a preliminary test of the circuit. Connect the battery, switch the unit on and vary the MAX pot VR1. You should be able to vary the frequency from about 7kHz or 8kHz at the low end, up to the limit of audibility (16kHz+). Now short the probe pads and check that the MIN pot, VR2, changes the low frequency. If all is OK, proceed with the assembly, otherwise you will have to find and fix Fig.2: the assembly details. the problem. Take care to ensure that the The plastic case we have semiconductors and 100µF specified has provision for capacitor are correctly the battery in a separate oriented & be careful not to compartment but with a lot get Q1 & Q2 mixed up. of effort you may be able to cram everything into a different case. Stick the label onto the diOhm. It measures 57 x 55mm and is case and drill the nine holes to let coded 04103971. It is fitted into a small the sound emanate from the speaker. plastic case and the two switches are While slide switches are nice, it is fitted at one end, as can be seen from much easier to mount toggle switches the photos. (just one round hole). Drill 2 x 6.5mm The component layout for the PC holes in the top of the case for the board and the other wiring is shown switches, 16mm either side of the in Fig.2. centre line. As usual, check the PC board for Fit the two switches, then mount etching faults and shorts, especially the speaker on the front of the case the track which goes between pins 13 with a couple of dobs of contact & 14 on IC2. Fit and solder the one link cement. You probably will not be and the resistors. Next fit and solder able to position it exactly behind the RESISTOR COLOUR CODES         No. 2 3 1 1 2 1 1 74  Silicon Chip Value 4.7MΩ 1MΩ 150kΩ 47kΩ 10kΩ 2.7kΩ 56Ω 4-Band Code (1%) yellow violet green brown brown black green brown brown green yellow brown yellow violet orange brown brown black orange brown red violet red brown green blue black brown 5-Band Code (1%) yellow violet black yellow brown brown black black yellow brown brown green black orange brown yellow violet black red brown brown black black red brown red violet black brown brown green blue black gold brown This is the view inside the completed prototype. The two trimpots at the bottom, right of the PC board are used to set the frequency range of the VCO. Note the holes files in the side of the case for the probe leads. speaker holes (this will depend on the switches used). File two small half-round holes in the top and bottom left side of the case with a needle file to let the probe leads out, but don’t make them too deep. Try to file them so the probe leads are actually clamped when the case is assembled. This prevents them being pulled out and possibly damaging the PC board. As you can see from Fig.2, they are also looped through the hole adja­cent to the pad before being soldered to the PC board. The PC board can now be secured in place with the two short self-tapping screws. Setup procedure Short the probe leads together and use VR2 to set the low frequency, then open circuit the leads and adjust VR1 until the whistle sound is just inaudible. There is quite a variation in 4046 ICs from different manu­ facturers. The one we used in our unit was a Motorola device. If you use a different brand you may have to change the values of the resistors in series with the trimpots to get the required range or in an extreme case alter the value of the capacitor between pins 6 & 7 (smaller value for higher frequency and vice versa). Fig.3: this full-size artwork can be photocopied and attached to the front panel of the Continuity Tester. Using the continuity tester Using the AudiOhm to check continuity is straightforward but as we mentioned at the beginning of this article, the unit can also be used to check semiconductor junctions and capacitors. When the red (positive) probe lead is connected to the anode of a diode, the AudiOhm should indicate a low resistance but not a short circuit. When the leads are reversed, the fre­ quency should be inaudible. A shorted diode will give the lowest tone in both directions. In a similar manner, base-emitter and base-collector junctions of NPN and PNP transistors can be test­ed. Finally, when a discharged capacitor (electro­lytics on the low range, others on the high range) is connected, the tone will initially be low (indicating a short circuit) and then increase as the capacitor charges. By comparing Fig.4: check your PC board carefully against this full-size etching pattern before installing any of the parts. the charge time of a known value of capacitor with that of an unknown value, an estimate of its capacity can be made. The final frequency gives an indication of the leakage current through the capacitor; the higher the SC frequency, the better. March 1997  75