Canon SPEEDLITE 540EZ Service Manual page 42

Part 2: Technical Information
4.6 Flash Control
(1) Reflected light from the subject is input to the SPD of the camera by the main
flash.
(2) When the camera judges that this reflected light provides a sufficient exposure
intensity, the STSP (flash stop signal) signal state changes from Low to High.
(3) When the No.8 pin of IC7 receives this signal, High is output to pin No.34 of IC7,
and is input to the base of Q17 and Q23.
(4) The High signal to pin No.34 of IC7 is input to pin No.90 of IC8 via RQ17, and High
is output from pin No.19 of IC8.
(5) Operations in steps (3) and (4) turn Q17 and Q23 ON. Turning ON of Q23 turns
Q12 ON and turns Q8 OFF.
(6) This turns IT1 OFF to stop flashing.
* Checking the Results of Light Adjustment
(7) Ifa High input is received by pin No.90 of IC8 within 3.4ms of pin No.88 of IC8
receiving the TRIG signal (IC7 pin No.35 output High signal), the High signal is
output for two seconds from pin No.22 of IC8.
(8) The High signal from pin No.22 of IC8 turns RQ6 ON to light LD1.
Figure 2-11 Circuit 6
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