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Design by JOHN CLARKE Words by LEO SIMPSON One-of-nine switch indicator Originally conceived as a track-in-use indicator for model railway layouts, this one-of-nine indicator can used with any selector switch with up to nine positions. It can be used with a bank of reed switches, as might be used on a locomotive turntable or traverser on a model railway layout or with any switch with up to nine positions. Then we realised it had many other uses . . . A ny railway modeller will be familiar with the problem: you have a locomotive turntable or switch-yard and you are never sure which track is actually selected, unless you go and have a close look. Or you could have the same problem with a traverser which selects rolling stock storage tracks. With a locomotive turntable you may well be sure that a track has been correctly “indexed” but you still don’t know which one has been selected. So the solution to that problem is a reed switch associated with each output track and a magnet on the turntable to activate each reed. The bank of nine (or less) switches is wired effectively as a single-pole rotary switch and then can be coupled to this single-digit display. From there, the concept can be applied to any situation where a rotary switch is used, 32  Silicon Chip with one or two provisos which we will come to later. Normally open switches More specifically, this Switch Indicator is designed to operate with normally-open switches, such as reed switches. With no switches closed, the single-digit display will show zero (0). With a switch closed, the display will show the number of the switch. This brings us to another important point – the circuit is designed to operate correctly only if one switch is closed at the one time. The arrangement of the reed switches and magnets should be such that as one switch opens the next switch closes. In other words there should not be a period when two reed switches are closed. If two or more switches are closed, the display will show a blank or an incorrect value which may be quite unrelated to the switches that are closed. For example, a closed 4 and 6 switch will show a 6, a closed 1 and 2 switch will show a 3 while a closed 8 and 3 switch will show a blanked display. Circuit description The circuit for the Switch Indicator comprises the switch inputs, a diode matrix, a CMOS 4511 BCD to 7-segment decoder (IC1) and a single 7-segment common cathode LED display. IC1 has four inputs labelled A, B, C & D. These are normally held low at 0V via the four 10k pull-down resistors. When all four inputs are low, IC1 decodes this condition as zero and it drives the 7-segment display accordingly, to show a 0. This is achieved by pulling its a, b, c, d, e & siliconchip.com.au There’s not much to this versatile project – it simply detects which switch position is high and reads out the appropriate figure on the LED display. An extension board (see overleaf) can show the same digit some distance away. f outputs high to drive the similarly labelled segments of the display via the 1.2kresistors. For the 0 display, the central ‘g’ output remains low and its segment is not lit. For those not familiar with BCD decoders and 7-segment displays, a look at Table 1 will be helpful. The four columns on the left side of the table are labelled D, C, B & A, corresponding to the BCD inputs of the 4511 decoder. What we are talking about is a 4-bit BCD code; BCD stands for Binary-Coded Decimal. So if you look at the top row of the ABCD columns you will see that it shows 0000 and this corresponds to a numeric value of 0, as indicated at the top of the extreme right column. The other columns in Table 1 show BCD INPUTS SEGMENT OUTPUTS DISPLAY D C B A a b c d e f g 0 0 0 0 0 0 0 0 1 0 0 0 0 1 1 1 1 0 0 0 1 1 0 0 1 1 0 0 1 0 1 0 1 0 1 0 1 0 1 1 0 1 0 1 1 1 1 1 1 1 0 0 1 1 1 1 0 1 1 1 1 1 1 1 0 1 1 0 1 1 0 1 1 0 1 0 0 0 1 0 1 1 0 0 0 1 1 1 0 1 0 0 1 1 1 1 1 0 1 1 0 0 1 1 1 1 0 0 1 1 0 1 2 3 4 5 6 7 8 9 8 a f b g c e d Table 1: here’s how the 4511 chip decodes the switch inputs, in BCD (binarycoded-decimal and lights the appropriate segments in the LED readout (1s light, 0s extinguish). Any other BCD input results in all 0 s and therefore no segments lit. Note that the “6” shown here is the standard 4511 output – but we’ve modified it so that the “a” segment lights as well (see right . . . ) siliconchip.com.au which of the seven segments of the display are illuminated. Hence, the top row of the table shows that all segments except ‘g’ are illuminated. Going back to the circuit of Fig.1, if switch 1 is closed, the 9V supply is connected to the anode of diode D1 and this pulls the A input of IC1 high. This is equivalent to a BCD value of 0001. IC1 decodes this condition as a 1 and drives the b & c outputs high while all other outputs are kept low. The b & c segments for the display now light to show the 1. This is shown in the second row of Table 2. Similarly, if a different switch is We reckon our 6 (left) looks a lot better than the standard 7-segment display 6 (right). All it costs is two diodes! December 2009  33 REG1 7809 CON1 +9V S1 A D2 A IN A GND 10 F 16V K S2 S3 OUT D1 D18 10 ZD1 18V 1W 100 F 25V K K A K K D3 K A D7 K D6 A K D8 S6 A K D9 A 10 OPTIONAL REMOTE DISPLAY (CON4 CONNECTS TO CON3 VIA IDC CABLE) K A DISP2 LT5543R CON4 D5 S5 9 g f 1 e 2 d 4 c 6 f e b 7 a a b g d c dp k 3,8 K D12 S7 A K D11 A K 4 D10 A S8 0V D4 A A S4 +12V CON2 3 K D13 A 6 K 2 D15 S9 A 1 K 7 D14 A 5 K BI 16 Vdd Og LT Of DD Oe DC DB 7x 1.2k 14 9 DA Ob EL Oa 10k 10k 10k g 11 f 1 e 2 d 4 c 12 6 9 A 13 Vss 10k 10 15 10 IC1 Od 4511B Oc DISP1 LT5543R CON3 A 7 a D17 D16 8 K b f e a g d b c dp k 3,8 K ZD1 7809 GND SC 2009 SWITCH POSITION INDICATOR A IN GND A OUT Special drive for 6 The display for the number 6 requires some explanation. As shown in Table 1, the 4511 decoder creates a 6 by driving the c, d, e, f and g segments. This gives an abbreviated 6 (in our opinion), so we have modified the circuit to also include the top segment (‘a’) in the 6 display, using diode D17. This lights the ‘a’ segment whenever the ‘e’ segment is lit. Diode D16 is included to prevent the low ‘a’ output line from IC1 from being driven high via diode D17. This display modification does not affect any other numbers. This is because for other numbers where the ‘e’ 34  Silicon Chip segment is lit (ie, the numbers 0, 2, and 8), the ‘a’ segment is already lit – and it doesn’t get any brighter if more than one output drives it! Other inputs on the 4511 include pin 3, the Blanking Input (BI), pin 4, Lamp Test (LT) and pin 5, Latch Enable (LE). These functions are not used in our design and so pins 3 & 4 are tied high while pin 5 is tied low. Power for the circuit can come from just about any 12V DC supply (in fact, anything from 11V to 18VDC at 80mA or so will do). Diode D18 protects the input capacitor and regulator from reverse voltage connection while the 10 resistor and 18V zener gives transient protection. A 100F capacitor filters the input to the 3-terminal regulator, REG1. This regulator provides a 9V output for the reed switch common connection and supply for IC1. A 10F capacitor bypasses the regulator output. K D18: 1N4004 A Fig.1: switch positions S1-S9 are decoded by IC1, a BCD-to-7-segment decoder, the result displayed on the 7-segment LED readout. The optional remote display can be used some distance away. pressed, then the diodes associated with that switch pull the respective A, B, C or D lines high to select the required digit to light. Table 1 shows the A, B, C and D input conditions to produce each number. K D1-D17: 1N4148 K Remote readouts So far, we’ve only looked at a single LED display mounted on the main PC board. But we’re sure (in fact we know from experience!) that there will be times when a remote display is also needed. Therefore, we’ve designed the system to be very flexible. You can use the single display on the main PC board, or you can add a second, smaller, display-only PC board via a suitable length of IDC ribbon cable and have an extension readout (obviously this always displays the same digit as on the main board!). Or you can even leave the display off the main PC board and simply have a single display a suitable distance away. How far away? Because the LED segments only draw milliamps there won’t be much voltage drop over a ribbon cable, even several metres long. siliconchip.com.au D1 10k 1 4148 D2 10k 2 2 4148 D3 3 3 4148 D4 4148 D5 4 4 4148 D6 5 5 4148 D7 6 6 4148 D8 4148 D9 7 4148 D10 8 8 4148 D11 9 V+ 9 4148 D12 4148 D13 10 F 100 F 4148 D14 10k 4148 D15 10k 7 +V 4148 D17 4148 1.2k 1.2k 1.2k 1.2k LOCATING LUG UNDER DISP1 1.2k 18V ZD1 1.2k 1.2k IC1 4511B 1 D16 H1 8x2 IDC HEADER (CON3) 10 D18 V+12V 21+ REG1 7809 V0V 0 CON2 19021190 OPTIONAL REMOTE LED INDICATOR BOARD DISP2 YALPSID NIART 4148 R OTA CID NI G NIDIS K CART NIART CON1 RIGHT ANGLE IDC HEADER 29021190 (CON4) LOCATING LUG 16-WAY IDC CABLE IDC LINE CONNECTOR (DISPLAY END) IDC LINE CONNECTOR (MAIN BOARD END) Fig.2 (top) shows the component layout for both the main PC board and the (optional) remote or extender board. The extender draws its power from the main board and is connected via the 16-way IDC cable, shown immediately above in Fig.3. We’re not stating a maximum distance – it’s probably tens of metres or more – but if the remote display is noticeably dimmer than the main display, you’ve reached the limit! Construction The Model Railway Storage Track Indicator is constructed on a PC board coded 09112091 and measuring 104 x 62mm. This can clip into the integral mounting clips within a UB3 plastic case if required. Alternatively, four corner mounting points are provided for mounting in a different box or mounted under a track layout. Fig.2 shows the component layout on the board. The remote LED display PC board measures 35 x 43mm. This board is coded 09112092. Its layout is also shown in Fig.2 Begin construction by checking the PC board for breaks in tracks or shorts between tracks and pads. Check that siliconchip.com.au Parts list – Switch Position Indicator 1 PC board coded 09112091, 104 x 62mm 1 Display PC board coded 09112092, 35 x 43mm * 1 plastic UB3 box, 130 x 68 x 44mm 1 TO-220 mini heatsink 19 x 19 x 9.5mm 6 PC mount 2-way screw terminals with 5.08mm pin spacing 1 1m length 16-way IDC cable * 1 16-way PC mount IDC header * 1 16-way PC mount right angle IDC header * 2 IDC line sockets * 1 20-way IC socket strip Items marked with 1 DIP16 IC socket an asterisk (*) are for 1 M3 x 6mm screw optional remote display 1 M3 nut Semiconductors 1 4511 BCD to 7-segment decoder (IC1) 1 LTS543R common cathode LED display (DISP1) (or 2*) 1 7809 9V regulator (REG1) 1 1N4746 18V zener diode (ZD1) 17 1N4148 switching diodes (D1-D17) 1 1N4004 1A diode (D18) Capacitors 1 100F 25V PC electrolytic 1 10F 16V PC electrolytic Resistors (0.25W 1%) 4 10k (brown black orange brown or brown black black red brown) 7 1.2k (brown red red brown or brown red black brown brown) 1 10 (brown black black brown or brown black black gold brown) December 2009  35 Here’s the display showing switch position 6 along with the extender board which obviously has to show the same thing! You can elect to have the main display only, the extender display only (by leaving out the main board LED readout) or indeed both displays, as we have shown here. the hole sizes are correct for each components to fit neatly. The screw terminal holes are 1.25mm in diameter compared to the 0.9mm holes for the IC, resistors and diodes. REG1 should have a 3mm mounting hole for the metal tab and the corner mounting holes should also be 3mm in diameter. The first components to insert are the diodes and resistors. The diodes must be mounted with the orientation as shown. Diode D18 and ZD1 have a larger body size compared to the other diodes (D1-D17). When inserting the resistors, use the resistor colour codes shown alongside the resistors in the parts list to check the resistor values (both 4-band and 5-band types are shown). A digital multimeter can also be used to measure each value as it is inserted. REG1 mounts on a small heatsink with its leads bent at right angles to insert into the PC board holes. Make sure the leads are bent at the correct length so the regulator tab can be secured to 36  Silicon Chip the PC board using a screw through the mounting hole in the PC board. Do this before soldering its leads. The screw terminals can be mounted next, noting that the 10-way section is made from five 2-way sections locked together, before they are inserted into the PC board. IC1 can either be soldered directly into the board or you can solder in a 16-pin DIP IC socket – either must be oriented with the notch as shown. Two 5-way socket strips are used for the LED display. If you intend using the separate display board, you will need to mount a 16-way IDC PC-mount header for the interconnecting cable. This header has its notch closest to the display. Install the two capacitors next, ensuring they are oriented correctly. If the display is to be mounted on the main PC board then this can be inserted now. The decimal point is oriented to the lower right as shown. That completes the main PC board assembly but if the remote display is required, the display PC board will also require assembly. It too should also be checked for breaks in tracks or shorted tracks and that hole sizes are correct. The right angle mount 16-way IDC header mounts as shown and the display can be mounted on two 5-way socket strips. The 16-way IDC cable is made as shown in Fig.3, using a length of 16way IDC cable and the two IDC sockets at each end. They are attached to the ribbon cable by clamping the socket halves around the cable in a vise. Make sure the cable is oriented correctly, with the red stripe side located at the pin 1 edge of the IDC sockets. Pin 1 is indicated with a triangle shaped arrow embossed on the location lug side of the socket. Testing Apply power and check that the display shows a 0. If it does not, check that there is 9V between pin 16 and 8 of IC1. If there is no voltage here, siliconchip.com.au 2 REED SWITCH 3 3 REED SWITCH 4 REED SWITCH 5 REED SWITCH 6 REED SWITCH 8 REED SWITCH 9 4148 4148 4148 5 4148 4148 4148 7 4148 8 4148 9 V+ USING SEPARATE REED SWITCHES 4148 4148 4 6 REED SWITCH 7 4148 18V V21+ 4148 V0 4148 4148 1 3 2 3 4 +9V (R) 5 9 8 7 0V 4148 4148 4148 4148 4148 4148 4148 7 4148 8 4148 USING A SINGLE ROTARY (OR SLIDER) SWITCH 4148 4148 5 9 V+ 4148 4148 4 6 6 +12V 19021190 CON1 2 (POWER) CON2 4148 1 Fig.4 (left) shows how you would wire a set of reed switches, such as would be used on a model railway turntable or traveller with a magnet strategically placed on the moving section. 18V V21+ 4148 4148 4148 4148 V0 R OTA CID NI G NIDIS K CART NIART 1 REED SWITCH 2 4148 4148 R OTA CID NI G NIDIS K CART NIART CON1 REED SWITCH 1 (POWER) +12V 0V CON2 19021190 Fig.5: the wiring for a conventional 9-position switch. This could be part of a dual or multi-pole switch, as long as the poles remained isolated. This arrangement can be used for any number of applications requiring “in use” identification. check for approximately 9V at the output of REG1. If the regulator does not deliver the right voltage it may be faulty (or the wrong type!) or installed incorrectly (not easy to do!), diode D18 or Zener ZD1 may be faulty or installed backto-front (much easier to do!) or there may be a short circuit between the 9V and common ground on the PC board. Otherwise there is not much else that can be wrong. When the display is working, a connection between the 9V terminal on CON1 and the 1 input should change the display to show a 1. siliconchip.com.au Similarly a connection from the 9V to the 2 input should have the display showing a 2 and so on. A transparent red acrylic or Perspex filter can be used over the display to improve the contrast (and therefore visibility) of the number. In use If the circuit is used with reed switches, Fig.4 shows how these are wired. One side of each switch is common and connects to the 9V terminal. The free end of each reed switch connects to the terminals on CON1. Not all nine reed switches need to be used - only the number of reed switches associated with the storage tracks need to be connected. Unused inputs are left disconnected. Fig.5 shows the equivalent connection for a single-pole rotary switch. We imagine that most applications requiring switch position indicators will in fact use a double-pole (or even multi-pole) switch. Just be certain to keep the original application and the Switch Indicator wiring isolated from each other! Any other uses for the Switch Indicator should follow this basic approach. SC December 2009  37