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eck out engine RPM on yo~r . odel airplane wiJh .tltis-easy7 .. uiJd:opticaJ tachometer. . ti can also use it to measure··.· -spQed of fans and'. rotating·-· 0 . 4 ' fts> · By JOHN CLARKE & GREG SWAIN -; PARTS LIST 1 PCB, code SC4-1-688, 85 x 56mm 1 plastic utility case, 130 x 68 x 41mm 1 Scotchcal front panel, 126 x 63mm 1 meter scale, 52 x 43mm 1 MU45 50µA meter 3 SPOT toggle switches 1 LED bezel 1 9V battery clip Semiconductors 1 4093 quad Schmitt NANO gate 1 4013 dual-D flipflop 1 555 timer 1 7805 3-terminal regulator 1 2N5485 N-channel FET 2 BC549 NPN transistors 1 BC327 PNP transistor 3 1N4148, 1N914 diodes 1 LD271, CQY89A IR diode 1 BPW50, BP104 IR photodiode 1 5mm red LED Capacitors 2 1 OOµF 16VW PC electrolytic 1 4 7 µ,F 16VW PC electrolytic 1 22µF 16VW PC electrolytic 1 1Oµ,F 16VW PC electrolytic 18 SILICON CHIP ·. ! 2 0 .1µ,F metallised polyester 1 0 .022µ,F metallised polyester 1 0.001 µ,F metallised polyester 1 680pF ceramic Resistors 2 x 470kn, 1 x 100kn, 2 x 68kn, 1 x 4 ?kn, 1 x 22k0, 1 x 1 Okn, 1 x 6.8kn, 1 x 3 .3k0, 1 x 1kO, 1 X 3300, 2 X 1000, 1 X 330, 1 x 200kn miniature vertical trimpot, 1 x 20kn miniature vertical trimpot Miscellaneous Rainbow cable, twin shielded cable, standoffs for meter terminals (only if required , see text). It's easy to measure the speed of rotating objects with this project. There are no wires to connect, since the circuit counts pulses of reflected infrared light. You just point the tacho at the propeller or whatever and check the reading in RPM directly on the meter scale. Actually, the idea for this project started when a colleague became interested in flying model aircraft and sought our help after several in-flight engine failures. Model aircraft engines require careful adjustment, particularly when new, if they are to run reliably. An optical 100k +5.6V 10 + 16VWI 100 + 16VWJ ""er~\o, ~ 3 BP104 CK IC3b 68DpF +0.51V .,- .,. 470k .,. .,. .,. .,. 1k +1 DETECT LE02 DIVIDE S2 '/>. +5.6V +2 A .,. POWER ~-------+5.6V 22 16VWI -: D1 1N4148 10k 47 + 68 16VW+ k 03 1N4148 0-20000RPM 0·2000RPM VR1 200k RANGE S3 3.3k 0.1 ,,, 100 16VW~ .001.:r .,. OPTICAL TACHOMETER SC4-1-688 ~ IN OUT GNO SENSITIVE ARE~ .qK Al I ll: K A BPW50 BP~04 + G<at>D B EOc VIEWED FROM BELOW Fig.1: the circuit uses 555 timer IC1 and Ql to provide a pulsed (20kHz) infrared signal. This signal is reflected by the rotating object, picked up by photodiode ID1, and processed to drive the meter movement. tachometer was required to monitor engine speed as carburettor and idling speed adjustments were made. Engine speed measurements are even more important for multiengined models. Here, the engines must be carefully adjusted so that they have the same speed regardless of throttle setting. Differences in engine speed of more than 100 RPM or so will make the model uncontrollable. Of course, our optical tachometer can do more than just measure the speed of model aircraft engines. You can use it to measure the speed of virtually any rotating machine, including multibladed fans and rotating shafts. Pulsed infrared When the instrument is turned on, an infrared LED (light emitting diode) at one end of the case emits a continuous stream of infrared pulses at 20kHz. The blades of the rotating propeller then reflect pulses of this infrared light back to a detector mounted adjacent to the LED. The pulses are then processed by the circuit and used to drive the meter movement. Why have we chosen to pulse the infrared beam at a 20kHz rate rather than simply use a continuous source? There are two reasons. First, it allows the circuit to function reliably under various lighting conditions, such as sunlight and fluorescent light. Second, the pulsing technique allows the infrared LED to be driven much harder to increase the light output. This, in turn, increases the useful operating range between the tachometer sensor and the rotating machine. The RPM readout is displayed on a meter with two ranges: 0-2500 RPM and 0-25,000 RPM. These ranges are selected using a toggle switch. A second toggle switch provides selectable divide-by-1 or divide-by-2 readings. The divide-by-2 switch setting is used when there are two light reflections per revolution; eg, when measuring a two-bladed propeller. If there are more than two reflections per rev, you simply divide the reading by the appropriate figure: eg, divide by 5 for a five-bladed fan. How it works Fig.1 shows the circuit details of our new optical tachometer. We'll start with the transmitter section which is based on ICl. Ql and LED 1 provide the 20kHz pulsed infrared signal. ICl is a 555 timer wired in astable or free running mode. Its output at pin 3 is high while the O.OOlµF capacitor on pins 6 and 2 is charging via the 68k0 and 3.3kn resistors, and low when the O.OOlµF capacitor is discharging via the 3.3k0 resistor. These timing components set the frequency of operation to about 20kHz, with the output (pin 3) being low for 2.3µs and high for 49.4µs. The output of ICl drives PNP transistor Ql via a 1000 base resistor. Each time the output of ICl switches low, transistor Ql switches on and drives the LED. Since the LED is driven for only about 4.6% of the time, it can be safely pulsed with currents of more than lOOmA. The infrared pulses reflected from the rotating object being MAY1988 19 K :~ 10 METER 11<at> 7~ ~LE02 A Fig.2: here's how to mount the parts on the printed circuit board. Twin core shielded cable must be used for the connections to the photodiode (ID1) but all other wiring connections can be run using rainbow cable. measured are picked up by photodiode IDl. This produces a 20kHz pulse train which has been interrupted by the rotating object. The voltage pulses produced across the 68k0 resistor are buffered by the FET source-follower QZ and then fed to the base of Q3 via a 680pF capacitor. Q3 and Q4 are a DC feedback + pair with 100% DC feedback from the emitter of Q4 to the base of Q3. Q3 is biased from the emitter of Q4 and the values of the resistors in the circuit are selected to give approximately 1/2Vcc [ie, half supply) at Q4's collector. AC current feedback is also applied from the emitter of Q4 to the base of Q3 and the gain is set by the ~ ,.f o::f" I~ ml ~'1 2'- Fig.3: this is the actual-size etching pattern for the PCB. 20 SILICON CHIP ratio of the 470k0 resistor to the output impedance of the source follower [QZ). So Q3 and Q4 together provide a gain of several hundred times. The amplified '20kHz pulse train on Q4's collector is now squared up by Schmitt trigger IC2a. Thus, whenever 2.3µs pulses are received by IDl, the output of IC2a goes low and discharges the 0.022µF capacitor at the input of IC2b via diode DZ. This, in turn, causes the output of Schmitt trigger IC2b to switch high and clock D-type flipflop IC3a. At the same time, the output of IC2b is inverted by IC2c to light the Detect LED. When no pulses are being received by IDl, the output of IC2a remains high and the 0.022µF capacitor charges to the positive rail via a 22k0 resistor. Because the RC time constant is about 0.5ms, the 20kHz signal is filtered out by this network. IC3a is part of a 4013 dual-D flipflop and divides the signal on its CK (pin 11) input by two. Its job is to provide a square wave with a duty cycle of exactly 50%, which is necessary for the following stage. The output frequency appears at the Q output [pin 13) and depends on the number of times the rotating object reflects the infrared light. The other half of the 4013, IC3b, is clocked by the Q-bar output of IC3a. It also divides by two and provides an output on pin 1 which is half the frequency on pin 13 of IC3a. Switch S2 selects between the output of IC3a and IC3b to give the divide-by-1 and divide-by-2 functions. From there, the signal is fed to a O. lµF capacitor which differentiates the square wave signal to give a series of negative-going voltage spikes. Diode D3 prevents the input to IC2d from going more than 0.6V above the positive supply rail. VRl , the 10kD resistor, and range switch S3 set the differentiator time constant. When the 0-2500 RPM range is selected, VRl sets the time constant so that broad negative-going pulses are produced at the input of IC2d. When the 0-25,000 RPM range is selected, the c. ..l... MU-45 CLASS-2.5 • • Fig.4: this artwork is used to replace the existing meter scale. As shown in this view, the bodies of the 0.1µ,F and 0.022µ,F capacitors lie flat against the PCB. Make sure that all polarised components are installed correctly. This view shows how the PCB mounts on the back of the meter. The pen points to the 0.1µ,F capacitor which is soldered to the back of the PCB for calibration of the high range (see text). 10k0 resistor is switched into circuit to give much narrower pulses. IC2d inverts these pulses which are then averaged by VR2 and a 100µ,F capacitor to drive the meter movement. Calibration adjustments are made by means of VRl and VR2. VRl provides calibration for the low (0-2500 RPM) range, while VR2 provides adjustment on the high (0-25,000 RPM) range. A 9V battery powers the circuit. This feeds a 7805 3-terminal regulator which has its GND ter- minal connected to earth via series diode D1. This "jacks up" the output of the regulator to give a nominal + 5.6V regulated supply for the circuit. Building it Most of the parts are accommodated on a small printed circuit board (PCB) coded SC4-1-688 and measuring 85 x 56mm. The board is mounted on the back of the meter and the whole assembly is housed in a plastic box measuring 130 x 68 x 41mm. We have produced a front panel artwork to suit the case, along with a suitable meter scale. Fig.2 shows the parts layout for the PCB. No particular procedure need be followed when installing the parts but take care with the orientation of polarised components. These include the electrolytic capacitors, diodes, regulator, transistors and ICs. The 0.1µ,F and 0.022µ,F capacitors must be mounted flat against the PCB as shown in the diagram, to provide sufficient clearance for the meter. Once assembly of the PCB has been completed, holes can be drilled in the front of the box for the infrared LED and photodiode. The hole for the photodiode should be filed to shape so that it is a tight fit. Secure the parts using an epoxy adhesive but be careful not to get any adhesive on the face (active area) of the photodiode, otherwise its sensitivity will be degraded. The front panel artwork can now be attached to the lid of the case and the holes drilled to accept the meter, switches and Detect LED. Mount the various items in position, then complete the wiring as shown in Fig.2. Rainbow cable can be used for the switch and LED wiring, but you must use twin-core shielded ea ble between the photodiode and the PCB. Once the wiring has been completed, the PCB can be mounted on the meter terminals (see photo) and secured with the meter screws. Be sure to install the two spring washers supplied with the meter between the screw heads and the PCB. These will bite into the copper pads to provide a good connection to the meter terminals. MAY 1988 21 We made up our strobe disc using a paper cutout attached to the back of a conventional turntable strobe. is because the cheap meters available these days will have a different zero setting depending on whether they're in vertical or horizontal orientation. The 0-25,000 RPM range is very easily calibrated using the light from an ordinary fluorescent lamp fitting. We simply take advantage of two facts: (1) a fluorescent lamp is extinguished at 100 times a second, and (2) it contains some infrared energy and therefore can be used with the infrared detector diode. To calibrate the unit, we first need to modify the circuit slightly to make the unit sensitive to the frequency of fluorescent lights. This involves shunting the 680pF capacitor at the source of Q2 with a 0. lJLF capacitor (ie, connect the two in parallel). You can do this by The low-range is calibrated by using a turntable set to 45 RPM and a strobe disc (see Fig.6). Adjust VR1 for a reading of 900 RPM (see text). If you want to be doubly sure, the washers can be soldered to the PCB pads. Depending on the meter supplied, it may also be necessary to add a couple of 6mm standoffs to the meter terminals to provide sufficient clearance for the PCB. We used a couple of LED bezels for this job and substituted longer meter screws. regulator (7805) is at about 5.6V. The voltages around Q3 and Q4 should also be checked to confirm that they correspond with those marked on the circuit diagram. Now check that the range switch is set to the 0-2500 RPM range. You should now be able to get a reading on the meter by moving your hand rapidly back and forth in front of the infrared LED. Testing High range calibration Now for the smoke test. Connect .up a 9V battery, switch on, and check that the output of the Before calibration, you must decide whether you want to use the unit vertically or horizontally. This lo 0 0 w ~ w ,- + 0 :aE 0 () 0 <t N + I- J: 0 ~ (.) z w w 0 <t () 0 0 ~ a. 0 0 IC ...J L.: :i cc 0 0 ,X 0 0 0 0 ,X ..:J Fig.5: here is an actual size reproduction of the front panel artwork. 22 SILICON CHIP The prototype meter was calibrated 0-20,000 RPM but was later modified for readings to 25,000 RPM. soldering the 0. lµF capacitor to the pads for the 680pF capacitor on the copper side of the board (see photo). Next, set switch S2 to the 7 1 position, S3 to xl000, and VRl to mid-position. VR2 can now be adjusted so that the meter reads exactly 6000 RPM in the presence of fluorescent light. (Remember to zero the meter first, in say the horizontal position, and then calibrate it in the horizontal position). You will find that this method of calibration works extremely well. You don't even have to be up close to the fluorescent light; as long as the photodiode is pointing towards the light, it can be a couple of metres away. Low range calibration The low range is calibrated using an ordinary phono turntable and a strobe disc (see Fig.3). First, remove the 0. lµF capacitor on the back of the PCB, set S3 to xlO0, and set the turntable speed to 45 RPM. The infrared LED and photodiode can now be positioned a few millimetres above the strobe disc, near the edge, and VRl adjusted for a meter reading of 900 RPM. Why 900 RPM? Because the turntable speed is 45 RPM and there Fig.6: this strobe pattern makes low-range calibration a cinch. Cut the pattern out carefully, place it on a turntable set to 45 RPM, and adjust trimpot VRl for a meter reading of 900 RPM. are 20 lines across the strobe disc (ie, 20 X 45 = 900). Note that the meter circuitry will have to be positioned outside the case during this adjustment procedure, so that the meter is oriented either vertically or horizontally. Do not lry lo calibrate the unit with the meter upside down as this greatly upsets the meter zero setting. Because the two calibration trimpots interact, you should now go back and repeat the calibration procedure for the high range. Having done that, check the low range again and repeat the calibration procedure once more if necessary. Using the optical tacho To check model aircraft engines, hold the unit close to the propeller blades and observe the Detect LED to confirm correct operation. If the LED is fully lit (and there is no reading on the meter), the sensor is continually receiving reflected light and so cannot respond to the rotating blades. When this happens, it's simply a matter of moving the unit away from the blades until the LED dims (indicating that the LED is flashing) and a steady reading is obtained. The range switch should be set to the correct RPM range and the 7 2 position selected for two-bladed propellers. For rotating shafts, the situation is a bit different since there are no blades to reflect the light. This problem is easily solved by attaching a reflective (or non-reflective) strip to the shaft so that there is some difference in reflectivity. This means that a non-reflective strip should be attached to a shiny shaft, while a reflective strip (eg, white paint) should be attached to a dull shaft. As before, the detect LED can be used to determine the correct position for the optical tacho. Just adjust the distance so that the LED switches on and off as the shaft rotates, depending on the position of the strip. ~ MAY1988 23