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555 darkroom timer circuit 3

2015-02-03 12:54  
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555 darkroom timer circuit 3
As the figure 17-16 shown, the timing circuit is composed of two branches of 555, IC1 forms a timer, IC2 is a second signal metronome. AN1 is the timer button, clicking the AN1, 555 sets, J pulls in, light exposure lamp H1 is lit, safelight H2 is off. K1 is regular file, when the C1 (or C2) is charged to more than 2/3 threshold level, the 555 resets, J releases. H1 is off, H2 is lit, exposure ended. IC2 and RP2, R4, R5, C4 etc. form astable multivibrator, adjusting the RP2 make the oscillation be 1Hz. The output of drives red LED light by VT1.