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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING LED car strobe. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Automobile. Ignition It is known how important it is to set the optimal ignition timing of the combustible mixture in the cylinders of a gasoline engine to ensure its maximum power, economy and correct temperature conditions. Performing this work without instruments requires some experience, takes a lot of time, and the accuracy of the installation may be low. A simple stroboscope will allow you to set the ignition timing quickly, accurately and with a minimum of hassle. The light emitter in prefabricated stroboscopic instruments is an inertialess flash lamp, which provides flashes of light so bright that it is possible to set the ignition advance even in conditions of high ambient light. Unfortunately, the service life of flash lamps is short, and it is not easy to get a new one of the right type. With the appearance on the market of domestic LEDs with a luminous intensity of more than 2000 mcd (for comparison, for LEDs of the AL307-M series at the same current, the value of this parameter is 10 ... 16 mcd), it became possible to use them in amateur stroboscopic devices. The design described below uses a group of nine KIPD21P-K red LEDs. The prototype of the device was a device published in the Bulgarian magazine "Radio, Television, Electronics", 1988, No. 8, p. 37. The operation of a stroboscope is based on the so-called stroboscopic effect. Its essence is as follows: if you illuminate an object moving in the dark with a very short bright flash, it will visually appear as if motionless "frozen" in the position in which the flash caught it. By illuminating, for example, a spinning wheel with flashes following at a frequency equal to the frequency of its rotation, one can visually "stop" the wheel, which is easily seen by the position of any mark on it. To set the ignition timing, the engine is started at idle and special installation marks are illuminated with a strobe light. One of them - movable - is located on the crankshaft (either on the flywheel or on the generator drive pulley), and the other is on the engine housing. The flashes are synchronized with the moments of neoplasm in the ignition plug of the first cylinder, for which the capacitive strobe sensor is attached to its high-voltage wire. In the light of the flashes, both marks will be visible, and if they are exactly one against the other, the ignition timing is optimal, but if the movable mark is displaced, the position of the breaker-distributor is corrected until the marks match. If an electronic octane corrector is installed on the car, the labels are matched with the appropriate adjustment knob. How to prepare the engine for this operation can be found in the book "Electrical equipment of cars" (Handbook), ed. Chizhkova Yu. P. - M .: Transport. 1993. The device is powered from the on-board network of the car. Diode VD1 (see diagram in Fig. 1) protects the stroboscope from erroneous reversal of the supply voltage polarity.
The capacitive sensor of the device is a conventional crocodile clip, which is attached to the high-voltage wire of the first glow plug of the engine. Voltage pulse from the sensor, passing through the circuit C1R1R2. enters the clock input of the trigger DD1.1, enabled by a single vibrator. Prior to the arrival of the pulse, the one-shot is in its original state, the direct output of the trigger is low, and the inverse is high. Capacitor C3 is charged (plus on the side of the inverted output), it is charged through resistor R3. A high-level pulse starts the one-shot, while the trigger switches and the capacitor begins to recharge through the same resistor R3 from the direct output of the trigger. After about 15 ms, the capacitor will be charged so much that the flip-flop will again be switched to the zero state on input R. Thus, the single vibrator responds to the pulse sequence of the capacitive sensor by generating a synchronous sequence of high-level rectangular pulses with a constant duration of about 15 ms. The duration of the pulses is determined by the ratings of the R3C3 circuit. Positive drops of this sequence start the second one-shot, assembled according to the same scheme on the trigger DD1.2. The pulse duration of the second single vibrator is up to 1,5 ms. At this time, the transistors VT1 - VT3, which make up the electronic switch, open, and powerful current pulses - 1 ... 9 A flow through the group of LEDs HL0,7 - HL0,8. This current is much higher than the rating of the maximum allowable pulse direct drive (100 mA) set for LEDs. However, since the duration of the pulses is short, and their duty cycle in the normal mode is at least 15. no overheating and failure of the LEDs were noted. The brightness of the flashes, which is provided by a group of nine LEDs, is quite sufficient to work with a stroboscope even during the day. In order to verify the reliability of the device, a control electrical run of the light emitter was carried out at a current in a pulse of 1 A for an hour. All LEDs passed the test, and no overheating was detected. Note that usually the time of using the device does not exceed five minutes. It has been experimentally established that the duration of flashes should be within 0.5...0.8 ms. With a shorter duration, the feeling of a lack of brightness of the illumination of the marks increases, and with a longer duration, their "blurring" increases. The required duration can be easily selected visually while working with a stroboscope with a trimming resistor R4. entering the time-setting circuit R4C4 of the second one-shot. The purpose of the first one-shot is to protect the LEDs from failure if the engine speed is accidentally increased while using the strobe. Typically, the ignition timing is set at engine speeds close to idle. If the frequency of sparking increases, the duty cycle of the flashes will begin to decrease (since their duration is fixed). With a high sparking frequency, the heat generation in the LEDs can become excessively large, which will lead to their failure. The duration of the pulses of the first single vibrator is chosen such that when the crankshaft speed reaches about 2000 min-1, the duty cycle of the output pulses of this single vibrator approaches 1. With a further increase in the input frequency, the operation of the DD1 trigger goes out of synchronism with it and the single vibrator begins to generate pulses of random duration and frequency . The average response frequency of the second single vibrator in this mode is significantly less than the dangerous limit. Resistor R9 contributes to a more complete closing of the powerful transistor VT3 in the pauses between flashes. This transistor must be selected with a minimum collector-emitter saturation voltage, then it will be much easier to provide the required flash brightness. If the brightness is still insufficient, you can try to assemble the output transistor switch according to the circuit shown in Fig. 2. In this case, by the way, the collector current of transistors VT1 and VT2 will be limited at a safe level.
Resistors R6-R8 limit the current through the LEDs. Capacitor C2 suppresses voltage pulses in the power supply circuit of the device, which can cause trigger malfunctions. Resistor R5 limits the base current of transistor VT1. Chip K561TM2 can be replaced by K176TM2. as well as on 564TM2, taking into account the features of its body. Instead of a KD209A diode, KD208A is suitable. but the best result will be given by diodes KD226A, KD213A-KD213G, KD2997V, KD2999V, since they have less direct voltage drop. The tuned resistor is SPZ-196 or SP5-1. Capacitors - KM-5, K73-9 or others; C1 must withstand voltage up to 200 V. KT315B transistors can be replaced by any of the KT3102 series. KT342, and KT815A - any of the series KT815, KT817. The conductor from the sensor to the device should not be too long and must be shielded, since the sensitivity of the device is very high. Switch SA1 - any car or toggle switch TV2-1. It is most convenient to assemble a stroboscope in a plastic case from a flashlight. The LEDs are mounted on a 1 mm thick disk of foil fiberglass, close to each other, and the disk is fixed in place of the lamp lamp. The handle of the resistor R4 can be brought to one of the walls of the case near the SA1 power switch. A properly assembled device does not require adjustment. It is only necessary to set the optimal brightness of the illumination and the clarity of the observed marks with the resistor R4. Author: P.Belyatsky, Berdsk, Novosibirsk region.
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