When the timer resets are high, both timers U4 may be triggered to
(4) When the expose, phase, and clock signal inputs to the first triple-input
NAND gate are all simultaneously high, the output goes low, causing the trigger-input
pin 8 to U4 timer 1 to go low and turn on the output (pin 9), which goes high.
(5) Because the threshold pin 12 of U4 timer 1 is tied to common, the output
will remain on until the timer is reset by removal of the exposure signal from H6T.
(6) The output (pin 9) of U4 timer 1 turns on and turns on the gate of
transistor Q2, which causes a negative-going pulse to the U4 trigger input (pin 6) of
timer 2. This pulse causes timer 2 to turn on.
The output of timer 2 (GPS) provides the following.
(a) The turn-on signal to the SCR contactor.
(b) The gating signal for Q1 which gates Q6 and Q5 on the SCR
contactor to provide the BAS (exposure on) signal.
The input to a triac optocoupler OC4 which illuminates the
(8) Additionally, GPS is connected to the second triple input NAND gate U3,
allowing the clock-pulses clock flip-flop 2 U2 for the duration of the exposure.
c. The exposure terminates when C13, the timing capacitor for the U4 timer 2,
charges to a voltage equivalent to that set by the potentiometer P1 through the
d. In the case of phototimed exposures, a back-up time longer than the
expected time is set on the timer switch and the exposure is terminated by the
automatic exposure control pulling the BSR signal low. This causes the associated
XOR gate to pull the reset pin on timer 2 low, thus terminating the exposure. The
exposure cannot be restarted until 115vac is removed from H6T, which resets
U4 timer 1.
e. The overall-time-adjust potentiometer P1 is used for setting overall timing
accuracy at longer times. The short-time-adjust potentiometer P2 is used for precise
timing at short times (1/60 to 1/30sec).