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Build a digital multimeter for only $30 How cheap can you get? This little digital multimeter has no less than 19 ranges, including resistance, transistor gain measurement & a 10 amp DC range, all for just under thirty dollars. You buy it as a kit & put it together in an hour or two. By LEO SIMPSON Back in November 1989, we featured a low cost analog multi­meter kit which cost just under $40. Now, with the inexorable march of technology, $30 will buy you a 3½-digit LCD multimet­ er with accuracy and resolution way beyond the reach of the old analog multimeter. The meter measures 125 x 70 x 27mm thick and has a liquid crystal display with 12mm high digits. The display reads up to 1,999 and has auto polarity; ie, it has a minus sign to 62  Silicon Chip indicate when the reading is negative. Of the 19 separate ranges, five are for DC voltage (200mV, 2V, 20V, 200V and 1kV); two are for AC voltage (200V and 750V); five are for DC current (200mA, 2mA, 20mA, 200mA and 10 amps); five are for resistance (200Ω, 2kΩ, 20kΩ, 200kΩ and 2MΩ); and the two remaining ranges are for diode test (forward resistance at 1.5mA maximum) and transistor gain (hFE with a base current of 10mA). Rated accuracy is ±0.25% of reading ±2 digits on the 200mV DC range and ±0.5% of reading ±2 digits on the other DC voltage ranges. On the remaining ranges, accuracy typically is around ±1% of reading. As noted above, this is considerably better than could be expected from a typical analog multimeter. The meter is powered by a standard 9V battery and rated battery life is 100 hours for carbon zinc and 200 hours for alkaline batteries. Intersil ICL7106 As with many basic digital multi­ meters, this kit is based on the Intersil ICL7106 IC. This chip is an analog-to-digital converter combined with a liquid crystal display driver. Apart from the 7106 and the liquid crystal display, there are only two other semiconductors: one diode and one transistor. Everything else is inside the chip. The ICL7106 has an on-board Fig.1: taken straight out of the instruction manual for the kit, the circuit of the multimeter doesn’t show much. Most of it is devoted to the 20-position rotary range switch. 24k 1 9V VR1 1k 100mV 36 35 32 1M INPUT The finished multimeter is compact & convenient to use. It is small enough to fit into your shirt pocket. 31 REF HI REF LO 33 0.1 34 IN HI IC1 ICL7106 IN LO C4 100pF 38 0.47 29 47k 28 CREF 0.22 27 E2 14 F2 13 A Y K K F E G D D3 B C D2 D1 G2 25 A3 23 OSC3 AUTO ZERO INT E1 8 F1 6 C2 10 D2 9 OSC2 BUFFER detail. Pins 2-25 provide the 7-segment drives for the display and the backplane signal which is common to all segments. Pins 38 to 40 are for the internal clock compon­ents. Pin 36 is the reference voltage input, pins 30 & 31 are the actual pins for measuring the input voltage. The other pins are for the auto-zero and dual slope integration components. C1 3 D1 2 G1 7 A2 12 B2 11 CREF 40 OSC1 R3 100k 39 Y 20 K 19 A1 5 B1 4 COMM .01 30 V+ B3 16 C3 24 D3 15 E3 18 F3 17 G3 22 VBP 26 21 Fig.2: this diagram shows the functions of the 7106 digital voltmeter chip in more detail. Pins 2-25 provide the 7-segment drives for the display & the backplane signal which is common to all segments. Pins 30 & 31 are the actual pins for measuring the input voltage. voltage reference which ensures its accuracy. The circuit of the meter is depicted in Fig.1 and is taken straight out of the instruction manual for the kit. Actually, as a circuit it doesn’t tell you much because most of it is devoted to the 20-position rotary range switch. Fig.2 shows the functions of the 7106 digital voltmeter chip in more Construction When you open the kit, you will find a bunch of parts in plastic bags. These will include meter test leads, battery and battery clip, all the tiny springs, ball bearings and screws to assemble the selector switch as well as the printed circuit board, the 7106 chip and the LCD display. The first step is to assemble the components on the printed circuit board. These are in one plastic bag with the 7106 IC. Refer to Fig.3 for the position of the components. Most resistors have to be mounted vertically. The easiest way to do this is to bend one lead over so it lies parallel to the resistor body and spaced so it fits into the PC board holes. The resistor end should be about 2mm from the board. C6 will have to be mounted parallel to the board. R9, the 0.01Ω shunt resistor June 1995  63 Above: the liquid crystal display (LCD) is loaded face side down (mirror side up) into the yellow plastic bezel which clips into the PC board. The rectangular insert holds the LCD in place, as well as providing channels for the elastomer contact strips. Their plac­ing is quite critical, otherwise some segments of the display may not light up. Right: most of the top side of the PC board is taken up with the con­tacts of the rotary switch. This board is complete, showing the assembled bezel for the LCD, the three banana jack sockets and the transistor socket. for the 10-amp range, is a piece of thick wire 60mm long with a 10mm bend at each end. It should be inserted so that the ends just come through the PC board, then soldered. Leave the 7106 till last and handle it carefully as it is a CMOS device. The battery clip and fuse clips, along with other compon­ents, are in a separate plastic bag and should now be fitted. When you position the fuse clips, be sure to place them so that the retaining lugs are at the outside, or else the fuse will not clip in. On our board, the pad for the ground spring (which makes contact with the aluminium screen inside the case back) had been screened with green solder resist and this had to be scraped off before the spring could be soldered. Now turn the PC board over and fit the input jack sockets and the transistor test socket. One end of the input socket sleeve is slightly larger and this should be inserted into the PC board. You may find it convenient to put the board into the front of the A long spring is soldered to the PC pattern. This makes contact with the adhesive aluminium shield plate inside the back of the meter case. 64  Silicon Chip case to align the input sockets when doing this – just tack solder each socket in place then remove the board and run solder right around each socket. The transistor test socket can be flush with the front of the case but must not protrude or the front label will not fit properly. Final assembly The next stage is the mechanical assembly and then your multimeter will be ready to test. The first step is to assemble the liquid crystal display The rotary switch is incorporated into the PC pat­tern, while six phosphor bronze spring contact wipers must be inserted into the back of the switch knob, as shown here. (LCD) in its rectangular bezel. Remove the clear protective cover from the front of the LCD and place it face down (mirror side up) in the bezel frame. Drop the rectangu­lar insert in to hold the LCD in place and slide the elastomeric connectors (incorrectly referred to as “rubber sponges” in the Asiansourced instruction manual) into the top and bottom chan­nels. These connectors consist of a sandwich of two pieces of pink non-conductive rubber with a centre section of black rubber which has alternating (invisible) non-conductive and conductive strips. These unseen strips carry the signal from the LCD metallised terminals to the printed circuit board connector strips. The 16 metallised edge terminals on the glass of the LCD are virtually transparent but can just be seen if the panel is angled to the light to make them stand out. This front assembly must now be mounted on the PC board. Take the assembly and hold it so that it will not come apart. Now look at the front in a good light and angle it so that you can see the digits which should read “-1888” and then carefully clip it into the front of the PC board (non- component side). Note that the elast­omer strips should make contact with the 16 connector pads on the PC board. Next, clip the six spring contacts onto the switch as shown in one of the accompanying photos. The first two seem difficult but once you have done them the rest are easy. Sit the knob on the PC board with the spring contacts touching the board. Insert the two springs (also shown in one the photos) and sit the steel balls on top of each spring. Now comes the tricky part. Lift the board and gently place the case front over the PC board making sure the knob comes cleanly through the hole. Hold the board against the case with one hand and screw the three small screws into the front (one at each end of the 7106 and the third under the centre of the fuse). Check that the switch operates smoothly and shake the case to ensure that both steel balls are located. If everything is OK, set the switch to the OFF position. Next, remove the backing paper from the adhesive aluminium screen and stick it centrally inside the back of the case. This done, check to make sure that the ground spring on the PC board contacts it when the case is assembled. This screen is used to shield the sensitive inputs of the 7106 IC from interference. Clip the back in at the top and use the two 10mm long self-tapping screws to hold it in place. Stick the serial number label into the recess on the battery cover, plug in the battery and slide the cover into position. Remove the backing from the front panel decal and carefully place it in position. Now for the big moment. Turn the selector knob one click either way The surround for the rotary selector incorporates an indexing plate to provide positive switch location. This is achieved with spring-loaded ball bearings. Fig.3: this diagram shows the positions of the components on the PC board. Most of the resistors are mounted “end-on”. from the OFF position and if you are greeted with 000 everything is probably OK. In our case the a, b and f segments were missing from the first digit and the f segment from the second digit. We disman­tled the unit, moved the top elastomer connector to the right from the rear, reassembled the unit and it worked fine. It may take one or two attempts to get all display segments The two spring loaded ball bearings are inserted into the rotary switch plate (one on either side) as shown in this photo. June 1995  65 RESISTOR COLOUR CODES ❏ No. ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 ❏  2 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 Value 1MΩ 820kΩ 547kΩ 470kΩ 330kΩ 352kΩ 220kΩ 200kΩ 100kΩ 90kΩ 11.5kΩ 9kΩ 2kΩ 1kΩ 900Ω 380Ω 100Ω 10Ω 9Ω 0.99Ω 0.01Ω but eventu­ally you will get it right. Testing & calibration Six 1% resistors, 1MΩ, 100kΩ, 10kΩ, 1kΩ, 100Ω and 24Ω, were supplied with the kit (from Altronics) to check the “Ohms” rang­es. Our readings were 995kΩ, 99.1kΩ, 9.92kΩ, 992Ω, 99.2Ω and 24.2Ω which are all well within the specification of ±0.8% of reading ±2 digits. There are no calibration adjustments for the Ohms ranges, but if you have mixed up resistor values on the PC board it may show up here. The voltage ranges have to be cali- 4-Band Code (5%) brown black green gold grey red yellow gold Not applicable yellow violet yellow gold orange orange yellow gold Not applicable red red yellow gold red black yellow gold brown black yellow gold Not applicable Not applicable Not applicable red black red gold brown black red gold Not applicable Not applicable brown black brown gold brown black black gold Not applicable Indicated on resistor No code; this is a metal bar brated and this is prob­ably the most difficult task for the hobbyist. If you have access to another multimeter, get a battery or a regulated power supply set to about 1.5 volts. Connect both meters and carefully adjust RV1 until the readings are the same. You will have to remove the back to gain access to this adjustment. On the other hand, if you do not have access to another multimeter, there is a good alternative. Just go out and buy the cheapest silver oxide 1.5V button cell (as used in cameras, watches and calculators) you can find. You should be able to buy one The completed multimeter, prior to the back being clipped into place. Note how the vertical resistors have been bent inwards to provide clearance for the back panel. 5-Band Code (0.1%, 0.5% or 1%) Not applicable Not applicable green yellow violet orange green Not applicable Not applicable orange green red orange green Not applicable Not applicable Not applicable white black black red green brown brown green red brown white black black brown green Not applicable Not applicable white black black black green orange grey black black brown Not applicable Not applicable white black black silver green Indicated on resistor No code; this is a metal bar for around $3.00. It will have an open circuit voltage of 1.55-1.56V which makes a good reference. Switch your new multimeter to the 2V DC range and check the voltage of the cell. If the reading is not between 1.55V and 1.56V, adjust RV1 until the meter reading is 1.555. Troubleshooting If the display is completely blank when you first turn on your finished multimeter, do not panic. It’s probably because the backplane signal to the LCD is not getting through the elastomer connector. This signal comes from pin 21 on the 7106, so try repositioning the top elastomer connector. If, after a couple of attempts the display is still blank, check that the polarised components are in the correct way on the PC board. Finally, check your soldered joints and check the values of all the comSC ponents above the 7106 chip. Where To Buy The Kit The kit for this digital multimeter is available from Al­tronics and their dealers and from all Dick Smith Electronics stores. 66  Silicon Chip