Mini stroboscope

Mini pocket stroboscope
Designed by Tomi Engdahl

DANGER HIGH VOLTAGE

This circuit is a mini storoscope which you can make so small that it can fit into your pocket. The circuit is not very powerful, but is works out of two small 1.5V batteries for an hour constantly and maximum flash rate. The flash rate is variable from zero to about 10 Hz. Quite nice small device to have with you in parties to gain attention.

How does a strobe light work?

A strobe light passes a brief, intense pulse of electric current through a gas, which then emits a brilliant burst of light. The gas is usually one of two inert gases, xenon or krypton, that emit relatively white light when they're struck by the fast moving electrons in the electric current. Because krypton and xenon atoms have a great many electrons and their electronic structures are very complicated, they emit light over a broad range of wavelengths. Thus the strobe emits a rich, white light during the moments while current is passing through the gas.

Supplying the enormous current needed to maintain the brief arc in the strobe's gas is done with the help of a capacitor. A high voltage power supply pumps charge to the cpacitor (usually to 200-600V range). You can often hear a whistling sound as this power supply does its work.

The capacitor plates are connected to one another through the gas-filled flashlamp that will eventually produce the light. However, current can't pass through the gas in the flashlamp until some electric charges are injected into the gas. These initial charges are usually produced by a high voltage pulse applied to a wire that wraps around the middle of the flashlamp or to the metal reflector near the flashlamp. A cascade of collisions quickly leads to a violent arc of charged particles flowing through the flashlamp and colliding with the gas atoms. The flashlamp emits its brilliant burst of light that terminates only when the capacitor's separated electric charges and stored energy are exhausted.

The circuit

I build this mini strobosocope using compotents mostly taken from an old pocket camera flash unit and some cheap components I already had. On the picture below you can see the picture of the flash unit I used in my circuit (quite similar camera flash unit can be bought from American Science & Surplus):

The transformers (T1 and T2), flash tube (X1), neon bulb (N1) and circuit board were from original flash unit. Nearly all other parts have been changed.

This picture is available in GIF and Postscript versions.

How the circuit works

Principle of the operation: Q1, R1, T1 and D1 form a DC-DC converter to convert +3V voltage from batteries to +200..+500V voltage which is charges the main flash capacitor C1. Resistor R4 and potentiometer P1 form a voltage divider and C2 is changed from that voltage though R3. When C2 reaches 70V voltage, the neon bulb N1 in the circuit starts to conduct and trigger the triac Q2. Thyristor causes C2 discharge through trigger transformer T2, which generates a short high voltage (2..4 kV) pulse which triggers the flash tube X1. Then main flash capacitor C1 discharges through flash tube and the tube generates a bright flash. The the charging of C1 starts again.

Where I got the components

Components T1, T2, D1, X1 and N1 were taken from old camera flash unit. There is nothing special in other components and they should be widely available. You can substitute Q2 with any suitable triac or thyristor which can withstand 400V and few amperes. You can also use any suitable power transistor (>2A and >40V voltage rating) as Q1 if you change the value of R1 to more suitable value fot that transistor. Anyway you can try other values for (100 to 2000 ohms) to tune the circuit to work best with the transformer you are using as T1 and the transistor you are using as Q1. The xenon flash tube X1 should work at 200-400V voltage range and trigger at 4 kV triggering voltage generated by T2.

Construction notes

If you are constructing this circuit remember that the voltage in the curcuit are at dangerous levels. Do not touch any parts at the high voltage side of the circuit when it is operating.

The circuit should be placed inside an insulating case like one above. There should be transparent plastic "window" in front of the flash tube. All parts should be well securated into the circuit board and the board should be well secured to the case. If you are using this at parties, some level of resistance to liquids and mechanical shocks is recommended.

Parts list

D1  1N4007
Q1  TIP 41A
Q2  MAC 216-4
T1  Switcher transformer taken from pocket camera flash unit
T2  Xenon flash tube trigger transformer
R1  500 ohm
R2  500 ohm
R3  4.7 Mohm
R4  220 kohm
P1  1 Mohm potentiometer (lin)
C1  470 nF 400V
C2  22 nF 200V
X1  xenon flash tube taken from pocket camera
N1  Small neon bulb (60V)


Tomi Engdahl <then@snakemail.hut.fi>