This mains powered stroboscope, designed by ELV, offers a wide range of settings as well as an external trigger feature so that it can be used as a slave flash unit.
A stroboscope is an instrument that produces light flashes with an intensity far greater than may be obtained with common light bulbs. The flash is a brilliant burst of light produced as a result of firing a gas (usually xenon) in a glass envelope, by means of a high-voltage pulse. Since the rate of the light flashes can be control- led accurately, moving objects illuminated by the stroboscope appear to stand still. This effect is obtained when the flash rate of the stroboscope corresponds to the period of the movement of the illuminated object. Useful applications of a stroboscope include the visual examination of rotating or relatively fast moving objects or parts such as flywheels and camshafts. Among the less useful, but certainly interesting, applications are lighting effects on theatre stages, on dance floors, in disco-theques and window sills. The stroboscope presented here has two basic modes of operation: as a continuously operating standalone light effects unit with an adjustable flash rate of 0.5 to 5 flashes per second (= 30 to 300 per minute); · as a slave flash unit with an adjustable trigger delay of up to one second. ln this mode, the stroboscope is triggered by a light flash from `another unit. After the set delay, the slave stroboscope produces its own flash. Exciting lighting effects may be obtained by using a single (mother) stroboscope and an array of slave units, each with its own trigger delay.
Operation and controls
The stroboscope is simple to use since the complete circuit is contained in a single ABS enclosure that can be plugged straight into a mains outlet. Operating the TRIGGER push-button in the lower right-hand corner of the front panel switches the unit from stand—alone (continuous) operation to slave operation, or vice versa (toggle function). A green and a red LED indicate the respective modes of operation.
This sensitivity, and that of 1 the associated circuitry, is such that even 2 relatively weak flashes, or flashes from a s distance of 10 rn or more, are reliably detected to enable the stroboscope to be triggered. Although the unit is largely f insensitive to light from normal bulbs or r sound-to-light units, two points should be noted in relation to the external triggering mode·
• the sensor must not be illuminated direct
by a constant light source;
• flickering luminescent tubes may cause
erroneous triggering owing to the light
pulses they emit.
The shape of the reflector behind the xenon tube ensures a light distribution that is particularly suitable for effects applications. Since a straight xenon tube is used, the reflector is U-shaped rather than spherical as in, for instance, a torch.
Circuit description
Power supply and flash tube circuit The power supply of the circuit consists of mains transformer Tr1, diodes D1—D6 and capacitors C1~C3. Note that although a mains transformer is used, the circuit is not isolated from the mains: a path exists via Ri, R2, D1, D2 and C2. This means that the circuit must never be used when it is not enclosed in the ABS case supplied with the kit. After removing the stroboscope from the mains outlet, always wait at least 30 s before opening the enclosure so as to allow the flash capacitors to get rid of their lethal high voltage. Diodes D3—D6 and capacitor CS provide voltage regulator IC with its direct input voltage. The output voltage of lCi is 15 V.
The mains voltage is applied to a two- phase voltage doubler, D1-C1-D;-C2, via ° power series resistors R1 and R;. The Hash 1 voltage of about 600 V exists between the +terminal of Cl and the -terminal of C2. The xenon tube, H1, is fired by a high-frequency, high—voltage burst at its trigger electrode. This burst is provided by the discharging of C4 across the primary winding of the firing transformer, Tr2. Voltages in excess of 10,000 V occur at this point.
The firing capacitor, C4, is charged via Rsa, Rab and the primary winding of Tr2. When thyristor Thyi is fired via R4, it con- ducts and enables C4 t0 be discharged via the primary winding of Tr2. The voltage induced in the secondary winding fires the xenon tube. Since the xenon gas in the tube conducts during the flash, Ci and C2 are rapidly discharged. The energy stored in these capacitors is thus converted to light. When the high voltage has fallen to about 100 V, the xenon tube turns into a high impedance again, so that the buffer capacitors can be charged again via R1 and R2. The firing capacitor, C4, is also charged again via R3a and Rab. The values of the components used in the firing and supply circuit around the xenon tube are such that up to five flashes per second can be produced.
Continuous operation and mode selection
When the stroboscope is used in the stand- alone mode (continuous operation), the firing pulse for thyristor Thyl is provided by an oscillator formed by lC3;—lC1. This is a fairly conventional two-gate stable multivibrator with potentiometer Rw acting as an output frequency control. Resistor R17 may have to be adapted to ensure the highest flash rate of 5 per second with R19 turned fully counter-clockwise. When this highest flash rate is exceeded, increase R17 to l20 kQ. When it is too low, , change RI7 to 82 K. When R19 is turned fully clockwise, the flash rate should be 0.5 per second, i.e., one flash is produced every two seconds.
The oscillator output signal is applied to input pin 5 of NAND gate lC4d. An- other NAND gate, IC4a, is provided with external trigger pulses. The bistable com- posed of lC5c-lC4c and push—button Tai determines whether the oscillator output signal or the external trigger output volt- age is passed to lC4d. Each time the push- button is pressed, the selection changes between lC4b (continuous trigger) and IC4a (external trigger).
A differentiating network, C14-R25, changes each level transition at the output of lC4d into a positive going needle pulse, which is fed to inverter IC5d. The two parallel-connected inverters that follow IC5d, IC5e and IC5, make this pulse positive again for firing Thyi via R4.
External trigger .
When photodiode Dll detects externally generated light flashes, amplifier lC2c supplies a positive output pulse, which is converted into a negative-going rectangular signal by comparator lC2b. This signal sets bistable lC3a-IC3b via pin 1. The out- put, pin 4, changes from high to low so that buffer pair lC5a-IC5b supplies a positive pulse. This results in C11 being charged via potentiometer R16. When the delay has lapsed, comparator IC2d toggles and provides IC4a with a negative pulse. Provided the stroboscope is in the continuous trigger mode (selected by Ta1), the pulse obtained from the external trigger circuit causes the xenon tube to fire as described above. It also causes the rapid discharge of C10 via R15 so that bistable lC3a-lC3b is reset via its second input, pin 6. The result is that C11 is rapidly disc charged via IC5a-IC5b and D7 to prepare this circuit for a new trigger pulse. The short delay introduced by R15-C10 is required to prevent the stroboscope being triggered by its own light flash.
Construction
The circuit is constructed on two printed- circuit boards. Construction is mostly straightforward on the electronic side; the following descriptions therefore detail mainly certain points in the mechanical work. Start the population of the flash tube board with the nine wire links. Fit the two potentiometers at the track side of the board, and secure them with the nuts provided. Push-button Tai is mounted on two solder pins to enable it to protrude from the from panel. The reflector is fitted with the aid of three screws as shown in Fig. 3. The cathode (marked by a black ring) and the anode of the flash tube are connected to solder eyes fitted on M3 screws. Nuts are used to provide the correct mounting height of the reflector. The high-voltage transformer, Tr2, is mounted on to the board as indicated by the component overlay. The firing voltage is carried by the flexible, insulated wire at the top of the transformer. Carefully remove the insulation material over a dis- tance of about 12 mm at the end of this wire. Wind this wire end around the xenon tube, roughly at the position indicated in Fig. 3, and join the turns of the winding by soldering rapidly and carefully. This completes the construction of the flash tubeboard.
