Tab Electronics Guide to Understanding Electricity ... - Sciences Club

Special-Purpose Diodes and Optoelectronic Devices
219
Figure 7-6
A triple-LED
flasher circuit.
6 to 12 V
R1 R2 R3 R4 R5 R6
C1
C2
R1 = R3 = R5 =33 k
R2 = R4 = R6 =560
C1 = C2 = C3 =100 F
C3
Q1 LED1
LED2
Q2 Q3
LED3
All capacitors rated at 16 WVdc or higher
base resistor. When the voltage across C1 reaches a high enough potential,
it turns on Q2 and causes LED2 to become dark (the saturation of
Q2 effectively short-circuits the voltage drop across LED2).
Here is where the circuit operation becomes a little tricky. Going back
to the prior condition when Q2 was in cutoff, the voltage on the negative
side of C2 was actually a little more positive than the voltage on the
positive side. This is because the voltage drop across LED2 was a little
higher than the base-to-emitter voltage drop of Q3. Therefore, C2 actually
takes on a slight reverse voltage charge. When Q2 saturates, this has the
effect of forcing Q3 into “hard” cutoff, because a slight negative voltage is
applied to its base, before C2 has the chance to start charging in the positive
direction through Q3’s 33k base resistor. With Q3 in the cutoff
state, C3 begins to take on a small reverse charge, while C2 begins to
charge toward the point where it will drive Q3 back into saturation. The
cycle continues to progress in a sequential manner, with LED1 lighting
next, and so on. Even though C1, C2, and C3 are electrolytic capacitors,
the small reverse charge is not damaging because the charging current
and voltage are very low.
The frequency of operation is a function of the time constant of the
capacitors and base resistors. Increasing the value of either component
will slow down the sequence. Any general purpose NPN transistor
should operate satisfactorily in this circuit. The LED type is not critical,
either, although you would have to adjust the resistor values somewhat to
accommodate the newer “high brightness” LEDs.
Many of the applications that applied to Fig. 7-5 will also apply to this
circuit. One of the advantages of this circuit design is that additional
transistor stages can be added on for a longer sequential flashing string.