Fullscreen HTML5Med Res (??MB)HTML4

Pt.2: By JOHN CLARKE LED Strobe & Tachometer Last month, we published the circuit of our new LED Strobe & Tachometer and showed how to build the main unit and the strobe light. This month, we describe the assembly of the optional Photo-Interruptor and IR Reflector Amplifier boards. We also describe how the unit is used. L ET’S START WITH the assembly of the Photo-Interruptor board – see Fig.11. This board is coded 04108083 and carries just the photo-interruptor itself, a 150W resistor and three PC stakes. The assembly should take only a few minutes. Just be sure to install the photo-interruptor with the correct orientation, ie, with its diode symbol (indicated in blue on Fig.11) on the righthand side. It should be secured to the PC board using two M3 x 6mm screws and nuts before the leads are soldered. The completed assembly is wired 76  Silicon Chip to a 3.5mm jack plug using 2-core shielded cable, with the shield wire used as the 0V (GND) connection (ie, it goes to the sleeve) – see Fig.6 in Pt.1 last month. Make sure that the tip and ring connections are made correctly. The tip connection is right at the end of the plug, while the ring is the separate section just behind the tip. The 0V or ground terminal is the main body connection. Use your multimeter to identify the jack plug terminals if you are unsure. To test the unit, plug it into the main unit, apply power, set the unit to Trigger mode and then return to the main RPM display. If a slotted disc (or some other opaque shape) is now rotated through the photo-interruptor, a reading should appear on the display. In addition, the strobe should flash each time the light path is interrupted. If this doesn’t happen, check your connections. In practice, this unit is intended to be used with a small slotted disk (see photo) that spins within the gap of the photo-interruptor (ie, the disk is driven by the rotating machinery). You will need to manufacture the disk to suit your application. The completed Photo-Interruptor siliconchip.com.au RE G GIRT LA CITP O E B ORTS 3.5mm PLUG A slotted disc attached to a shaft rotates through the slot in the photointerruptor, to interrupt the light path. The resulting pulses are then fed into the tacho unit. 0V TIP SLEEVE E RING 150 04107083 PHOTO INTERRUPTOR BOARD Fig.11 (above): make sure you get the photointerruptor the right way around when building the photointerruptor board (ie, diode symbol to the right). The PC board is mounted on Nylon standoffs, to avoid shorting the tracks. board can be attached to a fixed section the machine. Be sure to mount it using Nylon (not metal) spacers at the output end, to prevent shorts to the soldered joints. IR reflector amplifier This circuit is built on a PC board coded 04108084 and measuring 53 x 32mm. This is housed in a plastic utility box measuring 82 x 53 x 31mm. Fig.12 shows the assembly details. Begin by installing the resistors. Follow these with IC2, making sure it goes in with the correct orientation, then install the three electrolytic capacitors. Be sure to mount these capacitors with the correct polarity. Finally, complete the board assembly by installing the three PC stakes, the infrared LED (IRLED1) and the infrared photodiode (IRSENS1). Note that IRLED1 is mounted at full lead length, so that it can later be bent over horizontally to protrude through the side of the box. Take care with the orientation of both these parts. An accompanying photo shows how the board is mounted in its plastic case. It sits on four M3 x 6mm tapped Nylon spacers and is secured using M3 x 12mm countersink Nylon screws and M3 nuts. Two holes are drilled in one end of the box for the IR LED and photodiode, while another hole is drilled at the other end of the box to accept a cable gland. As before, the PC board is wired to a 3.5mm jack plug using 2-core shielded cable, with the earth shield used as the 0V (GND) connection – see Fig.7 last month. Once again, make sure you get the tip and ring connections correct. Testing the IR reflector board Having completed the assembly, the next step is to test the IR Reflector Amplifier board for correct operation. To do this, first plug it into the trigger input of the LED Strobe & Tachometer unit, then set the Trigger mode and return to the main RPM display. Now wave your hand in front of the sensor end of the IR Reflector box and check that the LCD shows an RPM reading. If this doesn’t happen, check your wiring connections. Note that as well as picking up reflected signals from IRLED1, the circuit will also detect signals from other infrared sources, such as incandescent lights running on the 50Hz mains. This means that RPM measurements are best done in natural light or subdued light. Measuring the RPM of a machine should be done with the sensor about 30-40mm away from the rotating shaft or fan. This means that you have to exercise a great deal of caution, to ensure that neither the sensor or any part of your body touches any moving parts. In complex situations, the best approach may be to mount this sensor unit in a fixed position before switching the machine on. In short, use your common sense. Note that as well as displaying the RPM value, the LCD also indicates rotation by displaying an Up or Down arrow that flashes on and off. Note also that it may be necessary to average the readings to account for slight TIP SLEEVE E 1k 3.5mm PLUG ebortS 470k 150 10k 480701140 100 F 1k IC2 LM358 100k RING 10 F 100 F IR SENS1 K A K A 100k IR LED1 150 IR REFLECTOR AMPLIFIER BOARD The IR reflector amplifier board must also be mounted on Nylon spacers, to avoid shorting tracks. siliconchip.com.au Fig.12: follow this diagram to build the IR reflector amplifier. Take care to ensure that IR SENS1 & IR LED1 are installed with the correct polarity. September 2008  77 Using White LEDs As Strobes: Busting A Myth B EFORE ATTEMPTING TO use a white LED as a strobe we had to be sure of its suitability. Initially, we had our doubts because we had read somewhere that white LEDs cannot be strobed at a fast rate. The reason given was that unlike coloured LEDs, white LEDs contain a phosphor and the persistence of this phosphor prevents them from switching on and off at a fast rate. One of the reasons behind this story is that most of the phosphors we are familiar with do have long persistence. These include those used in toys that glow for hours after being exposed to light and in fluorescent lights that continue to glow for a short time after being switched off. Similarly, some white LEDs do glow for a short period after the power is switched off. In this case, we wanted to use a Luxeon white LED as a strobe for this project so we set out to test its suitability. First, we checked the manufacturer’s data sheet and this specified less than 100ns for both the turn-on and turn-off periods. From this, it is clear that white Luxeon LEDs do indeed switch on and off very quickly and so would be quite suitable for our proposed strobe. How they’re made Further research on the web revealed that there are several ways in which white LEDs can be made. One way is to use red, green and blue LED chips and mix their outputs together to produce white light. These have a fast response because no phosphor is involved in converting the colour. Another way to achieve white light is to use a phosphor that converts the emission from a single colour LED into a white spectrum. There are two types, one based on a blue LED and the other on a near-ultraviolet LED. The blue-LED-based white LEDs use a speed variations while the machine is running. Using the strobe/tacho unit Each time you switch it on, the unit shows the main readout on the LCD. This will either be in Generator mode or Triggered mode, depending on the last selection. 78  Silicon Chip FTP100 phosphor that adds in colours to+5V ward the yellow end of the spec1k trum so that the combination of the blue light and the phosphor C E emission produces a white light. B C This construction is the most TO B FTP100  OSCILLOSCOPE common form used for white E LEDs. However, the phosphor 100 used does not phosphoresce but emits light by a process Fig.13: this simple phototransistor circuit called “scintillation”, an effect was used measure the response time CIRCUIT FORto MEASURING RESPONSE TIME OF of WHITE LED that has no light persistence. the white LED used in the strobe. The alternative white LED construction is not so comand a Cree XR-C white LED from 10% to mon and is based on a near UV LED 90% full brightness to be just 290ns which and a mixture of a red and blue emitting is really quite fast. The “fall-time” response phosphors plus a green emitting copper from 90% to 10% brightness was 360ns. and aluminium doped zinc sulphide. The Next, we wanted to make sure that emission works in a similar manner to were measuring the response time of the fluorescent lights. We do not have any phosphor rather than the light from the information about the response time for blue LED itself. To do this, we placed a these LED types but presumably these Polaroid red circular polarising filter over do have a long persistence. the white LED to block the blue spectrum For our strobe, we use the more comfrom the phototransistor. When we did this, mon blue-LED-based white LED. This the response times remained the same, type is manufactured by Luxeon, Cree although the amount of light available for and several others. the measurement diminished markedly. Measuring the response time This all means that the white LED response is very likely to be better than To further assess its suitability, we 100ns, just as the manufacturers claim. decided to measure the response time of The slower response times we measured a 1W Luxeon white LED. This was done are actually the phototransistor response using a phototransistor to detect the white times – ie, the phototransistor is slower light as shown in Fig.13. than the white LED. This circuit uses a low-value (1kW) From this, it is clear that the 1W white collector resistor to ensure that the photoLEDs specified are more than fast enough transistor switches on quickly. In addition, for strobe applications. However, one the 100W resistor from base to ground question remains: if white LEDs do have ensures that the phototransistor quickly a fast response, why do some continue switches off in the absence of light. to glow for a short time after the power is By pulsing the LED and monitoring this switched off? on one channel of a 200MHz oscilloscope, The main reason is because they are we could measure the response at the coloften driven by a supply with a filter capacilector of the phototransistor on the second tor and it takes time for the filter capacitor channel of the oscilloscope. We measured to discharge after switch off. the rise-time for both a 1W white Luxeon In Triggered mode, the LCD shows the RPM on the top line, then the word “Trig” and either an up or down arrow if there are incoming trigger signals from an external sensor. This arrow will flash on and off, with an up arrow displayed when rising edge triggering is selected and a down arrow when falling edge triggering is selected. The second line shows the frequen­ cy in Hz and following that the division ratio (ie, 0.5 and 1-8). An asterisk (*) on the far righthand side is displayed whenever the strobe is flashing correctly but is not displayed when the strobe LED is continuously lit (as happens when the flash period is longer than the time between flashes). siliconchip.com.au In the Generator mode, the display shows the RPM in the top line followed by the word “Gen”. The second line shows the frequency in Hz. As before, an asterisk (*) is shown on the righthand side when the strobe LED is flashing. In this mode, RPM adjustments are made using the Up and Down switches and the fine adjust potentiometer. The Up and Down switches adjust RPM in 100 RPM steps, while the potenti­ o­meter adjusts in 1 RPM steps over a 100 RPM range. Selection of either mode is made using the Mode switch. When pressed, the display shows ‘Trig/Gen’ on the top line and the selection (either ‘Gen or Trig’) on the second line. The selection is then made using the Up or Down switch. The Infrared Reflector Amplifier board is mounted in a small plastic case. Note how the IR sensor and IR LED are arranged. Options When the Generator mode is selected, a further press of the Mode switch brings up the “Flash Mode” option. This can be set to either “Automatic” or “Fixed” using the Up and Down switches. Pressing the Mode switch again brings up the “Flash Period” setting. If the “Fixed” mode is selected, the period can be adjusted from 32ms to 6.5ms in 25.4ms steps. The display shows the value in “ms” for readings less than 1000ms (1ms) and in ms for readings above 1ms. Note that because the flash period is fixed, it is possible for the frequency of the RPM signal to be high enough for the LED to stay fully lit (as indicated earlier), ie, when the unit is flashed at a faster rate than the update period. Correct operation is indicated by an asterisk (*) at the lower righthand side of the LCD. When the asterisk appears, the strobe is flashing. Conversely, if the strobe is lit continuously, the asterisk is off. If the Automatic mode is selected, then the display will show the automatic percentage value from 1-10% (ie, this is the strobe’s duty cycle). These value is altered using the Up and Down buttons. Press the Mode switch again returns the unit to the main tachometer display mode (showing RPM and frequency). Flash Mode, Flash Period and Averaging (of the reading). As before, these are selected using the Mode switch. First, the trigger edge can be set to either rising to falling. In this case, the LCD shows “Edge” on the top line, while the second line shows either “Rising” followed by an up arrow or “Falling” followed by a down arrow (depending on the selection). The Up and Down switches allow the setting to be changed. The Division selection allows the number of incoming trigger pulses to be divided by a set value, to give the correct reading on the LCD. When this is selected, the top line shows the word “Division”, while the second line shows the divide-by value. Division values of 0.5 and from 1-8 are available and are again selected using the Up and Down switches. For example, if you wanted to use the IR reflector sensor to measure the rotational speed of a 3-bladed fan, the division value would be set to three. The Flash Mode and Flash Period settings are adjusted in the same way as for the Generator mode. The Averaging mode is included to smooth out irregular measurements on a machine that is not running smoothly. You can average over 1-10 measurements and this is set using the Up and Down buttons. Higher averaging may be useful when the measured machine rotation varies markedly. Finally, when the main RPM and frequency reading is displayed, the strobe firing position can be altered using the Up or Down switches. Note that this feature is available only when the division is set to two or more. Using a Hall Effect sensor If you wish, you can use a Hall Effect trigger instead of the photointerruptor. As with the latter, this can be wired directly to the tachometer unit using 2-core shielded cable and a stereo 3.5mm jack plug. Note that the supply for the Hall effect sensor connects between the ring (+5V) and the ground 0V. The tip connection is for the Hall effect senSC sor’s output signal. Issues Getting Dog-Eared? Keep your copies safe with these handy binders Trigger mode Available Aust. only. Price: $A13.95 plus $7 p&p per order (includes GST). Just fill in and mail the handy order form in this issue; or fax (02) 9939 2648; or call (02) 9939 3295 and quote your credit card number. The Trigger mode allows even more selections. These are Edge, Division, Buy five and get them postage free! siliconchip.com.au REAL VALUE AT $13.95 PLUS P&P September 2008  79