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Build this low-voltage LED stroboscope If you need to measure the speed of rotating machinery in revs per minute, try this new lowvoltage LED stroboscope. It uses pulsed highintensity LEDs as the light source to stop motion & gives a readout of the RPM on a 3-digit LED display. By DARREN YATES Have you ever had to measure the speed of rotating machin­ery? Unless your head is mounted on a 360 degree swivel axis and has an inbuilt rev counter, it’s quite a difficult job – without a stroboscope, that is! There are all sorts of situations where a stroboscope is a useful tool. Typical applications include calibrating and check­ing motor speed controllers in industry, measuring engine idle speed in model aircraft, and checking the speed of lathes and other rotating machinery. 22  Silicon Chip A stroboscope is also useful as a diagnostic tool because it can effectively slow down motion. Many machines operate at a pace that’s faster than the eye can see, so when a malfunction occurs it can be difficult to locate the source of the problem. However, by using a stroboscope that runs slightly out of sync with the machine being monitored, it’s possible to slow the motion down so that the eye can actually follow what is happen­ing. For example, newspapers coming off a printing press are automatically folded by a machine that works at high speed. Because of this, it can be very difficult to locate the exact cause of any problems, such as paper tearing. By using a strobo­scope, it’s possible for the operator to visually “slow” the machine down, locate the problem and make the necessary adjust­ments to correct the problem. The concept behind a stroboscope is easy enough to under­stand. It’s basically a device that emits a high-intensity flash of light at a set interval. The frequency of these flashes is usually adjustable by means of a potent­iometer. In this unit, the flash frequency can be set anywhere from 1Hz to 317Hz – a range that effectively covers from 6019,000 RPM. In order to measure RPM, the strobe light is pointed at a white dot or line painted on the axis of the machine. The flash frequency is then adjusted until the white dot appears to be stationary (equivalent to one flash per rev) and the speed of the machine read off the digital display directly in RPM. There’s just one point to watch out for here – the dot (or line) will also appear to be stationary if the strobe flash­es at some exact multiple or fraction of the rev rate (eg, twice per rev or once every two revs). For this reason, it’s always necessary to use the flash setting at which the line is brightest when it ap­pears stationary. Slow motion effects are made possible by adjusting the flash frequency so that it is slightly out of sync with the machine being monitored. This has the effect of making the ma­chine appear to be in slightly adjacent positions for each suc­cessive flash, even though it may have gone through several cycles between flashes. As a result, the machine appears to run in slow motion. Adjusting the stroboscope to give slow motion effects is not as difficult as it sounds. You simply aim it at the machine and rotate the pot for the desired effect. In most stroboscopes, the active flashing element is an xenon tube. But although this is capable of producing a bright light, it does require a high voltage to drive it – typically around 350V or so. This high voltage is usually derived by charg­ing up a capacitor which is then discharged via the xenon tube when it is triggered by a pulse transformer. The main drawback of this technique is that the high vol­tage required to fire the tube is dangerous. Certainly, the voltage that appears across the main discharge capacitor is potentially fatal, so due care must be exercised in the design and construction of such devices. High-brightness LEDs By contrast, this design is completely safe because there are no high voltages involved. This has been achieved by elimi­ nating the xenon tube and substituting an array of high-bright­ ness LEDs instead. The whole circuit runs off a 12V DC plugpack supply, so high-voltage mains wiring is also eliminated. The LEDs specified are 5mm red high-intensity types which are available from Altronics (Cat. Z 0149) for 50 cents each in quantities of 10 or more. They have a brightness of about 1000mCd and are arranged in a circular pattern inside a torch body. By the way, all stroboscopes work best in subdued light conditions. They can’t work in bright light because you cannot see the flashes. How it works Fig.1 shows the circuit details of the LED Stroboscope. It’s virtually identical to the Digital Voltmeter for PARTS LIST 1 PC board, code 04112931, 100 x 55mm 1 PC board, code 04112932, 100 x 55mm 1 PC board, code 04112933, 53mm diameter 1 plastic zippy case, 130 x 67 x 42mm 1 front panel label 1 torch case (see text) 1 12VDC 500mA plugpack 1 2.1mm DC socket 1 5-pin DIN plug 1 5-pin DIN socket 1 1-metre length of 3-pair telephone cable 4 10mm x 3mm tapped spacers 4 5mm untapped spacers 1 100mm length of 0.1-inch spaced ribbon cable 1 10kΩ log potentiometer (VR1) 1 1MΩ 5mm horiz. trimpot (VR2) Semiconductors 1 LM358 dual op amp (IC1) 1 4049B hex inverter (IC2) 1 MC14553 3-digit counter (IC3) 1 4511 7-segment display driver (IC4) 1 BC548 NPN transistor (Q1) 3 BC557 PNP transistors (Q2,Q4,Q6) 3 BC337 NPN transistors (Q3,Q5,Q7) 1 BD679 NPN Darlington transistor (Q8) 1 1N4004 diode (D1) 1 7809 3-terminal regulator 3 HDSP-5303 7-segment common-cathode displays 31 high-brightness LEDs (LEDs1-31) (Altronics Cat. Z-0149 or equivalent) Capacitors 1 2200µF 16VW electrolytic 2 0.1µF 63VW MKT polyester 1 .033µF 63VW MKT polyester 2 .01µF 63VW MKT polyester 2 .0033µF 63VW MKT polyester Resistors (0.25W, 1%) 1 2.7MΩ 1 3.3kΩ 2 470kΩ 3 1kΩ 4 100kΩ 1 390Ω 2 47kΩ 7 270Ω 7 10kΩ 8 47Ω 1 4.7kΩ This view shows the control module of the LED Stroboscope. It consists of two PC boards which are stacked back-to-back on 5mm spacers & secured to the lid of the case. The three LED displays are viewed through a Perspex window. Miscellaneous Solder, screws, nuts & washers December 1993  23 24  Silicon Chip 47  VR1 10k 3 IC2a 4049B +9V 2 C E VIEWED FROM BELOW B 2.7M 0.1 VR2 1M 47k 47k 100k 0V +12V 500mA PLUG-PACK 4 10k 470k .0033 IC1a LM358 I GO B Q1 BC548 IC2b E C 3 2 7809 GND .033 IN 7 10k 1 6 10k 10k E CB IC2c 0.1 OUT LED STROBOSCOPE 5 4.7k 2200 16VW D1 1N4004 A 470k .0033 100k 100k +9V +V K 8 +9V 8 10 4 IC1b IC2d 9 1 5 6 7 +9V IC2e 11 12 10 4 3 A LE 9 7 7 1 B 6 2 C IC4 4511 4 3.3k 15 D2 D1 D0 5 6 D 15 E Q8 BD679 C B 2 1 10k K A 47  15 E    47         47  10k 1k 3 47       31xRED LED 10k E c b 47  C Q4 BC557 E B d DISP1 HDSP-5303   C a g Q3 BC337 C B e f  B 10 9 1 2 10 9 6 4 7 11 Q2 BC557 7x270  12 13 f g 14 e d c b a 16 MR 13 8 IC3 MC14553 IC2f CLK LE 14 11 12 16 +9V 100k .01 .01 +9V 8 5 3      1k Q5 BC337 47  B 5 DP      E C 3 47  B Q6 BC557 DISP2 HDSP-5303 390     C E +9V 1k +V B Q7 BC337 +9V E C 3 DISP3 HDSP-5303 The VCO board (left) & the counter board (right) are joined together via a 10mm-length of 5-way rainbow cable. This allows the two boards to be “folded” together so that they can be stacked on 5mm spacers. Note how the 2200µF capacitor on the VCO board is mounted (bottom left). Cars pub­lished in the June 1993 issue, despite the fact that the two projects perform totally different functions. VCO operation ▲ Op amps IC1a and IC1b (LM358) are connected to form a vol­tage controlled oscillator (VCO). IC1a is wired as an integrator while IC1b acts as an inverting Schmitt trigger. In operation, IC1a’s output (pin 1) ramps up and down due to the presence of Schmitt trigger IC1b and transistor Q1 in its negative feedback loop. When power is first applied, IC1a’s output ramps down line­arly until it reaches the lower threshold of IC1b (about 3V). At this point, pin 7 of IC1b goes high and turns on Q1. This pulls pin 2 of IC1a low via a 4.7kΩ resistor and so the voltage on pin 1 rises as the .033µF capacitor charges in the opposite direc­tion. When it reaches the upper threshold of the Schmitt trigger (about 6V), pin 7 of IC1b switches low again and Q1 turns off. Pin 1 of IC1a Fig.1 (left): the complete circuit of the LED Stroboscope. IC1a & IC1b form a VCO, with VR1 setting the output frequency. The pulse output appears at pin 7 of IC1b & drives an array of high-brightness LEDs via Darlington transistor Q8. It also clocks pin 11 of IC3, a 3-digit counter. IC4 decodes the counter outputs &, together with IC3, drives three 7-segment LED displays to show the speed of the rotating object in RPM. now ramps down again and so the cycle continues indefinitely. As a result, a sawtooth waveform appears on pin 1 of IC1a, while a corresponding pulse waveform appears at pin 7 of IC1b. This pulse waveform has a duty cycle of about 5%, as set by the ratio of the 4.7kΩ and 100kΩ resistors on pin 2 of IC1a. Its repetition rate is directly proportional to the input voltage set by VR1 – the higher the voltage on VR1’s wiper, the higher the output frequen­cy. The output from the VCO is used to switch Dar­ lington transistor Q8 (BD-679) and this in turn drives the LED array. Thus, each time pin 7 of IC1b goes high, Q8 turns on and lights the LEDs. The LED array consists of 31 high-brightness LEDs, arranged in five lines of five series LEDs plus two lines of three series LEDs. A 47Ω current limiting resistor is fitted in series with each line of LEDs to limit the pulse current through them to a safe value. This current is quite high but is still within the LED ratings due to the short duty cycle. Counter circuit As well as driving the LED array, the VCO also directly drives a counter circuit with a 3-digit LED readout. This counter measures the VCO frequency and is calibrated to read directly in RPM. In greater detail, the pulse waveform at pin 7 of IC1b clocks pin 11 of IC3, a CMOS 4553 3-digit counter. This IC con­tains three separate dec- ade counters, as well as the necessary output latches and multiplexing circuitry for three 7-segment LED displays. The .01µF capacitor between pins 3 & 4 sets the frequen­cy of an internal oscillator and this in turn sets the speed at which the outputs are multiplexed. The BCD outputs appear at pins 5, 6, 7 & 9 and are decoded using IC4, a CMOS 4511 7-segment display driver. This IC converts the 4-bit BCD code from IC3 into 7-segment outputs which then directly drive the LED displays via 270Ω current limiting resis­ tors. Each display is switched on at the correct time by the D0-D2 digit driver outputs from IC3. These outputs switch the dis­plays via PNP/ NPN transistor pairs Q2-Q7. IC2 provides the required latch enable (LE) and memory reset (MR) timing signals for IC3. IC2a and IC2b are used to form a standard squarewave oscillator. Its output fre­quency can be adjusted using VR2, which provides calibration. Each time pin 4 of IC2b switches high, pin 6 of monostable stage IC2c switches low and this provides the LE pulse for IC3. Each time a pulse is received, the current count in IC3 is latched into the output registers and the display is updated. After latching, the counters inside IC3 must be reset so that a new count can begin. This task is performed by the MR pulse and this is obtained by feeding the output from IC2c through a delay circuit consisting of stages IC2d-IC2f. Normally, pin 12 of IC2e is low but when pin 6 goes high at the end of the LE pulse, pin 12 also goes high for a brief peri­od. When pin 12 goes December 1993  25 This close-up view shows the completed VCO board, after it has been stacked with the counter board. The trimpot (VR2) allows the counter circuit to be calibrated, so that it shows the correct speed of the rotating object in RPM. low again, pin 15 of IC2f goes high and resets IC3 to 000. IC3 then begins counting the pulses applied to its clock input from the VCO as soon as the MR signal goes low again. Power for the circuit is derived from a 12V DC plugpack supply. This drives a 7809 3-terminal regulator to derive a 9V supply rail, while a 2200µF capacitor provides supply line decou­pling. Diode D1 protects the circuit against damage if the supply is connected with reverse polarity. Because the high-brightness LEDs and the LED displays draw a fair amount of current, the plugpack should be rated at 500mA. A 300mA plugpack will work but LED brightness will be reduced. Construction The LED Stroboscope is built on three PC boards: a VCO board (code We mounted the LED array & the speed control pot (VR1) inside an old torch case but a suitable piece of conduit could also be used. The various connections to the control module are run via a 5-way cable fitted with a DIN plug. 04112931), a counter board (code 04112932) and a LED array board (code 04112933). The first two boards measure 100 x 55mm and are mounted back-to-back on 5mm spacers inside a plastic case. The LED array board is circular in shape and is mounted separately, along with the speed control pot, inside the torch body or in some other suitable tube (eg, plastic conduit). It is connect­ ed back to the control Brief Specifications Range .................0-19,000 RPM Light Source .......High-brightness LEDs Power Supply .....12V DC <at> 500mA Readout...............3-digit LED display Resolution...........100 RPM Accuracy..............1% ± 100 RPM circuitry via a 1-metre cable fitted a 5-pin DIN plug. Fig.2 shows how the parts are installed on the boards. The parts can be mounted in any order, although it’s always best to mount the smaller parts first. Don’t forget the small wire link immediately beneath DISP 3 and make sure that all polarised parts are correctly oriented. These include the tran­sistors, diodes, ICs and electrolytic capacitors. The six transistors on the counter board all face in the same direction but be sure to use the correct type at each loca­tion. Q2, Q4 & Q6 are all BC557 PNP types, while Q3, Q5 & Q7 are BC337 NPN types. It’s easy to get these transistors mixed up so take care when installing them on the board. Note that each transistor should be pushed down onto the board as far as it will comfortably go before RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 2 4 2 7 1 1 3 1 7 8 Value 2.7MΩ 470kΩ 100kΩ 47kΩ 10kΩ 4.7kΩ 3.3kΩ 1kΩ 390Ω 270Ω 47Ω 4-Band Code (1%) red violet green brown yellow violet yellow brown brown black yellow brown yellow violet orange brown brown black orange brown yellow violet red brown orange orange red brown brown black red brown orange white brown brown red violet brown brown yellow violet black brown 5-Band Code (1%) red violet black yellow brown yellow violet black orange brown brown black black orange brown yellow violet black red brown brown black black red brown yellow violet black brown brown orange orange black brown brown brown black black brown brown orange white black black brown red violet black black brown yellow violet black gold brown December 1993  29 Q2 Q4 Q3 10k 1k DISP3 Q5 270  270  270  Fig.2: install the parts on the three PC boards exactly as shown in this wiring diagram. Take care with the orientation of the three LED displays & note that the 2200µF capacitor is mounted with its body flat against the VCO board, as shown in one of the photographs. 10k 1k 1 2 3 4 5 Q6 Q7 DISP1 10k 1k DISP2 270  270  IC4 4511 IC3 4553 270  270  1 390  .01 1 8 2 9 100k 100k 2200uF 0.1 0.1 47k 47k 100k D1 IN GND OUT 4.7k 1 .033 Q1 IC2 4049B 1 0.1 2.7M VR2 7809 3 5-PIN DIN SOCKET .0033 .01 IC1 LM358 5 7 470k 47W 10k Q8 6 100k 470k 10k 7 10 9 6 8 0.1 10k 10k SUPPLY SOCKET 3.3k E C B 1 4 10 1 2 3 4 5 .0033 7x 47  soldering, so that it doesn’t later foul the front panel. Don’t force them down too far though, otherwise you could fracture the leads inside the transistor bodies. Take care also when installing the 7-segment LED displays. They must be oriented with the decimal point of each display at bottom right. The 7809 regulator is mounted flat against the VCO board by bending its leads at right angles so that they mate with the mounting holes. It is then secured to the board using a screw and nut. The LED array board is easy to assemble. Just make sure that the LEDs 1 4 5-PIN DIN PLUG 2 3 31xLED are all correctly oriented. If any LED does go in the wrong way, then all the LEDs in that row will fail to light because that LED will be reversed biased. Once all the parts are in, the VCO STROBOSCOPE X 1000 RPM + 12VDC 500mA 30  Silicon Chip VR1 5 and counter boards can be placed end-to-end and their 1-5 terminals connected together via a short length of 5-way rainbow cable. The 5-pin DIN socket and the power supply socket are now mounted at either end of the case Fig.3: this full-size artwork can be used as a drilling template for the front panel. The cutout for the LED displays can be made by drilling a series of small holes around the inside perimeter, then knocking out the centre piece & filing the job to size. comes from the 9V regulator, while the LED array is supplied from the 12V plugpack via D1. Test & calibration Fig.4: check your PC boards against these full-size etching patterns before installing any of the parts. In particular, check that there are no broken tracks or shorts between tracks due to incorrect etching. and the remaining wiring installed – see Fig.2. Note that these two sockets must be positioned towards the bottom of the case, to provide sufficient clearance for the PC boards. At this stage, it is a good idea to go back over the board assemblies and check for wiring errors. When you are satisfied that everything is correct, the two boards can be stacked togeth­er using 5mm spacers and 10mm-long screws inserted from the VCO board side. The assembly is then secured by fitting a 10mm tapped spacer to each mounting screw – see photos. Connecting the LED array A 1-metre length of 5-way cable is used to connect the LED array board and pot VR1 to the control circuit (we actually used 3-pair telephone cable, with one wire left unused). This cable is fitted with a 5-pin DIN plug at one end, while the other end passes through a hole drilled in one end of the torch before connecting to VR1 and the LED array. The pot is mounted by drilling a hole through the side of the torch case, while the LED array board can be secured using silicone sealant. Be sure to remove the switch contacts from inside the torch to prevent it from shorting against any of the circuitry. The 10kΩ pot we used was a 16mm type which was easily fitted in the torch case. If you have a bigger housing than the one we used, you could use a standard size pot. Note that differ­ent supply voltages are used for the pot and the LED array. The pot supply To test the unit, apply power and check that the three 7-segment displays light up. At this stage, the readout won’t be calibrated but you should see recognisable numbers appear and the readout should vary as you vary the control pot (VR1). The high-brightness LEDs should also begin flashing as soon as power is applied, depending on the setting of VR1. Check that the flash rate can be varied with VR1 (note: for higher settings of VR1, the flash rate is so fast that the LEDs appear to be continuously lit). A word of warning here. If you suffer from migraine head­ aches or epilepsy, then stay well away from this project. The bright flashes of light produced by the strobe can quickly trig­ger an attack. Assuming everything works correctly, the unit can now be calibrated. You will need a digital frequency meter for this job. The first step is to set VR1 so that the VCO frequency at pin 7 of IC1b is 200Hz (as measured on the DFM). This done, VR2 is adjusted until the stroboscope display reads 12.0 (corresponding to 12,000 RPM). Alternatively, you can calibrate the unit against a machine that rotates at a known speed. To do this, set VR1 to the lowest setting at which the reference line appears stationary and adjust trimpot VR2 for the correct reading on the display. Final assembly All that remains now is to install the control module inside the plastic case. The first step is to attach the front-panel label to the lid and use it as a drilling template for the board mount­ing screws. This done, drill a series of small holes around the inside perimeter of the display cutout area. The centre piece can then be knocked out and the job filed to a smooth finish so that the red Perspex® window is a tight fit. It’s now simply a matter of securing the control module to the lid using four 5mm-long machine screws. If necessary, the Perspex window can be glued into position using epoxy resin but don’t use too much as this would spoil the appearance of the unit. SC December 1993  31