Canon SPEEDLITE 540EZ Service Manual page 44

Part 2: Technical Information
4.8 AF Auxiliary Light
* When AF cannot be detected because the brightness of the subject is below a fixed
value, the red AF LED lights according to the following procedure.
(1) When the camera judges that additional light for focusing is required, a High signal
of about 5V is generated on the TV line. (This is a pulsed signals. Generation of this
signal is repeated up to 10 times until detection of AF.)
(2) When pin No.6 of IC7 receives this signal, High is output to pin No.44 of IC7.
(3) This turns RQ8, Q13 and Q15 ON, and AF-LED (LD3) lights.
(Q13 is for stabilizing the current flowing to the LED.)
Note: The High signal to pin No.44 of IC7 is also input to RQ10 to inhibit the main
capacitor being charged during lighting of the AF-LED. (For details, see 4.9
Charge Inhibit.)
(4) When the camera communicates the instruction to light LED3, High is output from
pin No.11 of IC8.
(5) This turns RQ15, Q14 and Q16 ON via D45. If RQ8 is ON, AF-LED (LD2) lights.
(Q14 is for stabilizing the current flowing to the LED.)
e Flow of Operation: LD3 lights
(1) > (2) > af
(4) > (5) > LD2 lights
BATT1
TVv(CCC)
Figure 2-13 Circuit 8
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