Simple Method for Producing Giant Scoreboard-Like Digital Displays
Douglas R. Henry Schoolof Pharmacy Oregon State University Cowaiiis. 97331
Recently there has appeared on the market a pH meter with a eiant LED dieital disolav.' The use of such laree-scale displays is likely i o becode more common. Thereis a simple method that is suitable for constructing a very effective scoreboard-like ac display that can be interfaced easily and a t moderate cost with any instrument that has accessible 7segment LED readouts. The key is the use of an opto-isolator Silicon Controlled Rectifier (SCR). This device is available from General Electric2at a cost of about $3 or .%. The package is a &pin DIP, and one unit is all that is needed for each seement of a 7-segment display, plus one unit for each decim& colon, or dash in the display. The device contains a miniature solid-state Ga-As lamp with a forward current of 60 mA and a reverse voltage of 6 V. This lamp actuates an isolated (2500 V) LighbAdivated SCR (or LASCR), which is an ac device that can he likened to a one-wav- eate with a latch on it. Whenever the latch is lncked. ~ ~ nothing will pass through the gate, forwards or backwards. Once the latch has heen unlocked. or in this care. once lieht from the Ga-As lamp bas struck thk LASCR, forward current will oass (UD to 300 mA a t UD to 200 V). However. since the gate'is one-way, reverse current is hlncked. It so happens that the LASCR will remain unlocked, even after light has stopped striking the SCR, until a high enough reverse voltage has been applied (i.e., untilsomethina has slammed the eateshot). For this particular device, this ;eveme gate voltage is f r o m 6 3 0 ~
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The use of LASCR's in the construction of two segments of a 7-segment display is shown in the figure. The digital readout of the instrument (e.g., pH meter, timer, DMM) is disconnected, and the input to a given segment of the readout of the instrument is connected directly to a LASCR. Connection is made a t whichever pin, 1or 2, is appropriate, deoendine on whether the LED readout of the instrument in ~~~~~~~-~~~~~ k m m w cathode, or common anode. This can be checked with an ordinan, LED. if necessarv. The comolementarv nin of the LASCR is-then connected tb the comlhon of the"&ticular digit being interfaced. AC current is fed into pin 5 of the LASCR, and the output, a t pin 4, is connected to the lights which make up the corresponding segment of the scoreboard display. These lights should he wired in parallel. We have found that four clear 7-W "night-light" bulbs make a convenient segment, which consumes about 150 mA of current a t 110 V ac. If lower voltage is desired, a transformer can serve as the source of ac current, but i t should he kept in mind that the voltage must exceed the reverse gate voltage requirements of the LASCR, in order t o switch off the current. The operation of the circuit is straightforward. When current from the digital instrument passes through the Ga-As lamp, the SCR is turned on for the duration of each positivegoing cycle of ac current, and the corresponding segment of the scoreboard display is illuminated. Most LED readouts are multiplexed. This means that only one digit of a multi-digit readout is ever actually turned on a t any given instant; this reduces the number of connections reouired. and a t ranid multiplexing rates, the effect is that of continuous display of all digits. Unlike the digital readout however, the LASCR, ~
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242 / Journal of Chemical Education
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IIOvolts AC
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Scoreboard Display
000 Clear 7W
a b c cathode Cmstruction of two segments of a giant smreboa~llkedigital display. inputs e seoments Mreadwt at tha len have been intemted and ~10 tha dand ~ ~ - of tha ~ fed directly to tha LASCR's whlch arks the c~nsspandmgssgmemr 01 the scoreboard display. Note ms common cathode config.rstlan of me LED. l h ~ s scheme will work wlm any insnumen1 with sccessoo e LED resoouts, as well as with directdrive lc's
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once actuated, remains on until the ac voltage switches, regardless of the multiplexing rate of the digital instrument. This is advantageous, since it allows the lights t o remain on longer, and gives a brighter display. I t should also be noted that the LASCR can he turned on externally by the application of a 5-V signal a t pin 6. We have successfully used this technique to interface with a number of circuits. Two particularly useful single-chip circuits are Intersil's ICM7205 and ICL7107 IC's.3 These are both direct-drive IC's, capable of multiplexing and supplying current to LED readouts without the need for transistors. The first circuit is a complete stopwatch-on-a-chip, and it can be used to make a versatile 6-digit timer (0.01 s to 1hr). The second is a complete auto-polarity DVM, whichgives a 3-112 digit readout. When interfaced to a scoreboard display, this voltmeter can be used with pH meters, spectrophotometers, or any analog-output devices, and is impressive in even the largest lecture hall setting. Of course, ac current must be used when working with LASCR's, hut GE also manufactures ohoto-transistor isola~ HllB2) and photo-ljarlingtons (HII A I and tors ( H I I B and HllA2). These can be used iust as simply and effectively fur dc displays, using LED'S oriow-voltage lamps. 'Chemtrix, Inc., 163SW Freeman Ave., Hillsborn, OR97123. Type
80 pH Meter.
=GeneralElectric Opto-Electronics part HllCl or HllC2. The units differin their surge voltage characteristics, ,andeither is suitable. The HllC2 unit is less expensive. They are available through lmal GE solid state component distributors, or write GE Semiconductor Products, Electronics Park, Syracuse, NY 13201. 3Available from Intersil Franchised distributors, or write Intersil, Inc., 10900 Tantau Ave., Cupertino, CA 95014.
