Flash-meter

  
Insulated-gate, field-effect transistor (IGFET), Q2 and silicon photo cell Zl form the heart of this circuit. Transformer T1 is an audio-output type, but it`s reversed in the circuit. A sudden flash from a photoflash unit detected by Zl sends a voltage pulse through the low-impedance winding of Tl via Rl. That voltage pulse is stepped-up in Tl`s 500-!l, primary winding before being rectified by Ql. Transistor Ql is used as a diode; its emitter lead was snipped off close to the case. Ql then charges Cl to a value proportional to the amplitude of the electrical pulse generated by the light from a flash unit.
Flash-meter - schematic

Capacitor Cl controls the current flowing through Q2, which has a very high-input impedance. The current through Q2 is read by meter Ml, a 0-50 p.A de unit, which has been calibrated in f-stops. The extremely high internal resistances of Ql and Q2 will allow Cl to retain its charge for several minutes; this is more than enough time for you to take your reading of Ml. The charge on Cl is shorted to ground and returned to 0 V by depressing reset button Sl. The flashmate is ready to read the next photoflash. Trim potentiometers, R7 through Rl4, are.adjusted to values which will yield correct readings for corresponding film sensitivities, or exposure indexes.




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