Two Transistor LED Flip-Flop (Astable)

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Two 2N3904 transistors with cross-coupled timing capacitors and two alternating LEDs. 9 V supply.

Parts

Picture

Simulation

This circuit is set up for a transient (time domain) analysis: .tran 0 4 0 1m uic. Run it in your browser, change values and see the waveforms update.

SPICE netlist

* Two Transistor LED Flip-Flop (Astable) - LED Flip-Flop
.param Rb=13340
.param Ct=47u
V1 vcc 0 9
R3 vcc n001 {Rb}
R1 vcc col1 4.7k
R2 vcc col2 4.7k
R4 vcc n002 {Rb}
Q1 col1 n001 0 2N3904
Q2 col2 n002 0 2N3904
R5 vcc n003 1k
D1 n003 col1 LED_RED
R6 vcc n004 1k
D2 n004 col2 LED_GREEN
C1 col1 n002 {Ct}
C2 col2 n001 {Ct}
.model 2N3904 NPN(Is=6.734f Xti=3 Eg=1.11 Vaf=74.03 Bf=416.4 Ne=1.259 Ise=6.734f Ikf=66.78m Xtb=1.5 Br=.7371 Nc=2 Isc=0 Ikr=0 Rc=1 Cjc=3.638p Mjc=.3085 Vjc=.75 Fc=.5 Cje=4.493p Mje=.2593 Vje=.75 Tr=239.5n Tf=301.2p Rb=10)
.model LED_RED D(Is=1e-18 N=1.8 Rs=10 Cjo=10p)
.model LED_GREEN D(Is=1e-20 N=1.8 Rs=10 Cjo=10p)
.ic V(BASE1)=0 V(BASE2)=0.7
.meas tran q1max MAX V(COL1) FROM=1 TO=4
.meas tran q1min MIN V(COL1) FROM=1 TO=4
.tran 0 4 0 1m uic
.end