Vectronics VEC-1500K - Manual (Page 66)

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Theory of Operation
IC U1, a 555 timer, along with resistors R1 and R2, and capacitor C3 form a
simple astable multivibrator circuit. The output of U1 on pin 3 is a square wave
pulse train with a repetition rate that is variable from about 100 mS to about 10
mS.
The pulse train drives IC U2, a CMOS 4017 decade decoder IC. Each positive
going transition of the square wave generated by U1 causes the count of the
4017 to advance by one. The active output of U2 is a high signal level. At any
one time, only one of the ten decade outputs will be in a high signal state. A
clock cycle begins when the square wave goes high, advancing the count of the
4017 by one.
Each of the decade outputs drives a 2N3904 switching transistor. When the
decade output of the 4017 goes high, the associated 2N3904 is driven into
conduction. Bias current to the base of the 2N3904 is limited by a 10K resistor.
When the transistor base is forward biased by the decade output, the transistor is
driven into saturation, or full conduction. This provides a return path to ground
for the Light Emitting Diode (LED), through a 1K current limiting resistor, at the
transistor collector.
Specifications
Power Requirements ....................... Nine volt transistor battery at about 15 mA.
Circuit will operate over a 6 to 15 VDC
supply range.
Flash Rate ....................................... About 100mS to 10mS sequence.