Typical Strobe Light Trigger Circuit

The Basic Circuit
More Advanced Stuff

The Basic Circuit

This circuit typically uses a voltage around 300 volts for the main high voltage supply and needs a low voltage supply of typically 6 to 15 volts.
                               )     trigger lead to flashtube
                               )  trigger
                               )  coil
                               )  secondary
To positive end of             )
main storage cap               )
____________    _______________)
            VVVV    |          )  trigger
     47K 1/2W typ.  |          )  coil
                    |          )  primary
 Trigger          -----        |
 Capacitor        _____  anode--- SCR such as Radio
 (.047 uF typ.)     |          V  Shack 276-1020
                    |  cathode---        ___________________
____________________|__________|\        |  -------------  |
To negative end of  |            \       |  |    555    |  |
main storage cap    |             >      |  |1         8|  |
                    |    470 ohms >      |  |           |  |
                   gnd            >      +--|2         7|  |
                                  +-VVVV-|  |           |  |
                                  |_________|3         6|--+--
                                            |           |    |
                               timing       |4         5|    |
                               resistor     |           |   --- timing
                                            -------------   --- capacitor
555 pin connections not shown above:

1   goes to ground / negative of both low and high voltage supplies

4,8 both go to positive low voltage supply, of 6 to 15 volts.

7   does not get connected to anything.

The 555 should not be a TLC555, but should be an LM or NE 555.

The timing capacitor should be at least 100 picofarads. The timing resistor should be 1K to 5 meg.

The pulse (flash) frequency (in Hz or flashes per second) is approx .72 divided by the product of timing resistance (in ohms) and timing capacitance (in farads). This oscillator does not work well at frequencies above something like a megahertz. Since most strobes don't work well above 10 Hz or something like that, the timing capacitor would be a minimum of .01 uF in nearly all cases, usually at least .22 uF.

For a typical variable speed strobe light with a flash rate adjustable from about .35 to 7 Hz, I recommend a timing resistor of 10K in series with a 200K variable one, and the timing capacitor would be 10 microfarads.

For a 200K variable resistor, you can use the two sections of Radio Shack's dual 100K pot (271-1732) in series with each other. I recommend using the center terminal and the first one clockwise from it, of each section, in order to take advantage of the nonlinear resistance distribution ("taper") of this device.

You are encouraged (but at your own risk) to try different timing resistors and timing capacitors to get the results you want.

In the event you are trying to get a rapid flash rate and the trigger capacitor is not charging quickly enough, you can try a lower value for the resistor that charges it from the main high voltage.

More Advanced Stuff

For now, I advise people to get the Radio Shack "Engineer's mini-notebooks" on digital circuits and on 555 timer IC circuits.

One IC that is good for some strobe stuff, especially multiple and unevenly spaced flashes, is the 4017. This is a decade counter with ten outputs 0 to 9. Connect a square or rectangular wave input to the clock input (Pin 14). For normal use, ground Pin 13 ("NOT clock enable"). Ground Pin 15 ("reset") for normal counting 0 to 9 and starting again with zero. For a shorter count, connect the reset pin to the appropriate output pin instead of to ground. For example, to have the count have a cycle of only six possible outputs, connect reset to the "6" output.
B+ is Pin 16 and B- (ground) is Pin 8. "Carryout" is Pin 12. All the other pins are the ten outputs:

"0" is Pin 3
"1" is Pin 2
"2" is Pin 4
"3" is Pin 7
"4" is Pin 10
"5" is Pin 1
"6" is Pin 5
"7" is Pin 8
"8" is Pin 9
"9" is Pin 11

The clock input (14) would typically get output from Pin 3 of a 555 oscillator like that from the circuit above.

For use triggering SCR's, I recommend the Teccor SCR TCR22-6, Digi-Key catalog number TCR22-6-ND since this one only needs .2 mA to trigger. Feed the gate from a 4017 output via a 10K resistor, maybe less if the supply voltage is 6 volts or less and the 4017 output in question is not feeding anything else. It is OK to use as low as 3.3K at any supply voltage as long as the 4017 output in question is not feeding anything else.

If you want more than one 4017 output to feed the same SCR, each output should go through a diode (traditionally of type 1N4148 or 1N914) to keep it from interfering with other 4017 outputs. These "steering diodes" would converge onto the 10K or 3.3K resistor feeding the SCR gate.
Do not attempt to fire the flashtube from adjacent number 4017 outputs. This is generally not recommended. If necessary for some reason to attempt this, put the combination output (after the steering diodes) to one input of a 2-input AND gate. This input needs a 100K resistor to ground. The other input goes to the clock. The output of the AND gate goes to the 10K or 3.3K resistor feeding the SCR gate. This AND process makes the input to the SCR go low for half a count tick - otherwise it would stay "high". If you use a NAND gate, you have to "invert" the output. You can use an additional 2-input AND stage (a 2-input AND or NAND IC usually has four of these) - connect the inputs together and to the output that needs to be inverted, and connect the inverted output through the 10K or 3.3K resistor going to the SCR gate.

One 4017 can even control two or more SCRs - such as a dual strobe with the two flashtubes flashing at different times as part of one overall pattern.

Written by Don Klipstein.

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