Darkroom exposure timing circuit (1)

It is a darkroom exposure timer light circuit suitable for developing and developing photos in white-light photography. The NE55a circuit
, the potentiometer RP, and the capacitor form a monostable flip-flop. Normally, the circuit is in reset state.
The ③ pin of NF555 outputs low level, the relay K does not operate, and the exposure light E does not light up. The light-emitting diode [．FD2 is Working
means j, burn, you can illuminate the dial of the potentiometer RP. At this time, the internal discharge tube of NTF555 is turned on and the capacitor (1. is short-
circuited and cannot be charged.
    If exposure is needed, just
adjust the potentiometer RP to the required time ,
press the button SB1. The bulb E
will be exposed according to the prescribed call time.
Its working principle is:
after pressing SB1, the NE55 trigger terminal ② pin gets
a negative pulse, and the monostable
trigger flips into temporary love. The pair of
output pins ③ outputs the boat level,
the relay K absorbs the stage, and its moving contact
point kl closes the stage, the exposure lamp E is powered on and emits light, and at the same time, I and EDl also light up and emit light indicators. At this time, the inside of the base circuit is called
When the discharge tube is cut off, C is charged through RP, so that the level of pin ⑥, the threshold terminal of NE5r)5, continues to rise. When it rises to
2/3 of the power supply voltage, the circuit flips and resets, and pin ③ returns to low level. The circuit state returns to its original state, K is released
, the light F goes out, and the exposure ends. Since the transient time of NFj55 (that is, the exposure time fhI) is related to
the values ​​of and is related to the power stack has nothing to do, so the exposure time accuracy is very high and the repeatability is good. S is the Dingzhou gear
switch . When it is turned to position ② or ③, the resistors R, and R. will be connected in series respectively, and the two gears K can be opened d SB2 is a manual button switch
. Press SH2, the light will turn on, and the exposure time can be controlled manually. .
    Rp must use a linear (X-type) potentiometer , and R and R should use RI-1 with higher precision. For 8w type metal film
resistors, it is best to use ferret electrolytic capacitors with a withstand voltage of 25V or above L to ensure the accuracy of the setting. K can use
JZC 22E, DC_12V small medium-power electromagnetic relays. SBL and Sm use small light-touch wireless Lock key switch.