Time Delay Photo Flash Trigger

Q1 is a phototransistor that is normally illuminated by a beam light. When the beam is intercoupled, pin 1 of U1A goes high, and forces pin 4 U2B low. Then, C2 discharges through R2 and R3. After a certain time delay, pin 10 U1C goes high, triggers SCR1, and sets off the flash. R4/C1 charges, causes U1D output to go low after about 1/2 second, and resets U1A and U1B to the initial state. This delay prevents accidental double flash exposure.

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Time Delay Flash Trigger Circuit

The circuit is built around a single 4093 quad 2-input NAND Schmitt trigger. Two gates from that quad package (U1-a and U1-b) are configured as a set-reset flip-flop.

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Sound Triggered Flash

Audio input from a microphone drives amplifier Q1/Q2/Q3 to produce an ac voltage across R5. C4 couples this to TR1, causing it to conduct, triggering photoflash or other device that is connected to J1.

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Photo Event Timer

S2 is used to initiate timing. A light-to-dark or dark-to-light transition stops this timer, depending on the setting of S5. S3 offers a direct operating mode, rather than through the latch. IC3 and IC4 supply 0.1- or 1-second timing pulses. IC7 drives a time display counter, a 12-Vdc unit that draws less than 200mA.

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Flash Meter

Insulated-gate, field-effect transistor(IGFET), Q2 and silicon photo cell Z1 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 Z1 sends a voltage pulse through the low-impedance winding of T1 via R1. That voltage pulse is stepped-up in T1's 500-ohm, primary winding before being rectified by Q1. Transistor Q1 is used s a diode; its emitter lead was snipped off close to the case. Q1 then charges C1 to a value proportional to the amplitude of the electrical pulse generated by the light from a flash unit. Capacitor C1 controls the current flowing through Q2, which has a very high-input impedance. The current through Q2 is read by meter M1, a 0-50 uA dc unit, which has been calibrated in f-stops. The extremely high internal resistances of Q1 and Q2 will allow C1 to retain its charge for several minutes; this is more than enough time for you to take your reading of M1. The charge on C1 is shorted to ground and returned to 0 V by depressing reset button S1. The flashmate is ready to read the next photoflash. Trim potentiometers, R7 through R14, are adjusted to values which will yield correct readings for corresponding film
sensitivities, or exposure indexes.

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Darkroom Enlarger Timer

The NE555 circuit is a basic one-shot timer with a relay connected between the output and ground. It is triggered with the normally open momentary contact switch, which when operated, grounds the trigger input at pin 2. This causes a high output to energize K1 which closes the normally open contacts in the lamp circuit. They remain closed during the timing interval, then open at time out. Timing is controlled b a 5-MW potentiometer, Rt. All timer-driven relay circuits should use a reverse clamping diode, such as D1, across the coil. The purpose of diode D2 is to prevent a timer output latch-up condition in the presence of reverse spikes across the relay.
With the RC time constant shown, the full-scale time is about 1 minute. A scale for the 5-MW potentiometer shaft position can be made and calibrated in seconds. Longer or shorter full-scale times can be achieved by changing the values of the RC timing components.

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