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Xenon Strobe Transmitter |
Schematic of the power supply and trigger circuit for the xenon strobe transmitter. The AC powered voltage doubler section of the circuit can be replaced by a battery powered inverter from an old camera. Due to the higher impedance of some camera inverters, you could eliminate the 4.7k 10 watt resistor or choke. This resistor or inductor is normally used to prevent the tube from lighting continuously, once triggered, by current from the mains power supply. A small camera inverter may not be capable of charging the flash capacitor quickly enough if it is run at higher pulse rates. When this happens, the voltage on the main flash capacitor will be lower than normal and flash energy will drop. |
For triggering the transmitter, any positive going pulse source can be used. I built a crystal oscillator divider chain for output frequencies in the 1 to 30 Hz range using crystals from 1 to 10 Mhz. I also built a GPS synchronized pulse generator with precise 1 or 3 Hz output for an experiment suggested by Yves, F1AVY. The unit is used with any GPS device that has a 1 second sync pulse output. The oscillator-divider and GPS units both use an internal 5 volt regulator IC for stable operation and can be run off 6 to 12 volts for portable use. |
4024 7 bit ripple counter used in divider chain 
GPS synchronized pulse generator Two IC timers are used in monostable mode to generate the 333 ms and 666 ms delay times. These are OR'd together with the master signal to drive the third IC timer which generates the 3 Hz output pulses. When making 1 Hz pulses, the GPS sync input is switched to an additional OR input on the gate. |
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When the divider chain is used with an external audio input, (from computer sound
card or an mp3 player) the input to the one shot is connected to the divide by 128 output
of the first ripple counter. This provides pulse outputs in the same frequency range as
the crystal oscillator but by using recorded audio tones in the 1000 Hz range.
*Note - The strobe unit and any driver circuits have separate grounds and are isolated from each other since the strobe power supply is connected directly to the AC power mains and is 'hot'. The ground wire from the three wire AC plug, shown in the strobe circuit, is connected to any metal case the device may be installed in. The separate ground of the strobe circuit is isolated from the case. This helps contain any RF emissions from the circuit trigger, which uses a high voltage pulse to trigger the xenon tube. |
| For using this circuit with LaserScatter, the divide by 16 output of the first ripple counter (pin 6) should be used to trigger the one-shot. And since this program uses frequencies above ten hertz, the output energy of the strobe may need to be lowered depending on the tube wattage ratings. |
The xenon strobe tube has a small metal foil reflector wrapped around one side of it. This is so most of the light exits the tube on the "output side" and provides an electrical connection for the trigger wire. With F:1 optics, about 50 percent of the light is collected by the lens?? When you take apart a cheap camera to get the strobe tube and electronics, you will notice they have the tube mounted in a high efficiency parabolic cylinder reflector. This can be used for better transfer of light to the lens. This will also make the output beam larger since the tube is now effectively the size of the square aperture of the reflector. |
Original design of the strobe transmitter box and general layout of internal optics. The unit is aimed by using an internal ccd camera connected to a monitor. The camera images the moon (or other target) and the rectangular area of beam. The camera's field of view is a little wider than the transmitter's beam width. The strobe unit is built in a small plastic case with an output hole cut in the cover for the tube. The strobe tube itself is mounted directly in this output aperture. A longpass IR filter can be placed over the output. This allows the powerful IR lines to pass through while blocking most of the visible light. With the filter in place, only a deceptively dim reddish light remains. Here are spectrums of the output with and without the IR longpass filter. And another spectrum with less exposure to show the 400nm cutoff of the optics better. |
Strobe circuit built into a small plastic case. The oscillator/divider chain circuit (in a similar case) can be partly seen next to the strobe connected to it with a phono style patch cable. |
Short video (WMV) of strobe firing. A nearby photodiode connected to an audio amp is picking up the flash and producing the loud clicks. |
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