Electronic timer with long exposure time. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Electronic timer with long exposure time. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Consumer electronics

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An electronic timer with a long exposure time allows you to fix the range of time intervals (exposures) from 2 to 70 minutes in increments of 1 minute and an accuracy of about 2%.

Electronic timer with long holding time

The device contains a time relay on two transistors and signaling device A1. The relay can be powered by a 9 V battery or a stabilized power supply based on transistors V4 and V5.

The timer works as follows. Set the shutter speed with a calibrated resistor R2 and turn on the power with switch S3. At the same time, transistors V1 and V2 open, relay K2 is activated and contacts K2.1 prepares the signaling device A1 for switching on, and contacts K.2.2 turns off the signal lamp H1.

When the S1.1 "Start" button is pressed, its contacts turn on the relay K1 and lock the transistor V2, putting the time relay into countdown mode. In this case, relay K2 is de-energized, and K.1 is self-locking by contacts K1.1, and contacts K1.2 prepare the circuit for switching on the signaling device. As soon as the transistor V2 closes, the capacitor C2, previously charged through the open transistor V4, begins to discharge, and when the voltage across it drops to the cutoff voltage of the field-effect transistor V1, the latter will open itself and open the transistor V2, which will cause an even greater increase in drain current and relay K. 2, whose contacts K2.1 close and turn on A1, signaling the end of the time delay.

You can remove the sound signal by briefly pressing the S2.1 "Reset" button, while the blocking of the relay K1 is removed, and it turns off, opening the circuit of the signaling device A1.2 with the contacts K1, the contacts S2.2 short-circuit the resistors Rl, R2, and the capacitor C4 is charged. The device returns to its original state.

Details. Resistors type MLT-0,125 or VS-0,125 (R7 type MLT-1), R2 type SP-1A; electrolytic capacitors of the K50-12 or K50-6 type; C4 type IT-2, C7 type MBGO; switch S3 type MG3; relay K.1, K2 type RES-47; signal lamp H1 type ТН-02; signaling device A1 - any sound device operating at a voltage of 8,5 V and consuming a current of not more than 170 mA.

The timer is adjusted by selecting the maximum exposure time with resistor R2, and the minimum exposure time with resistor R1, then calibrate the scale of potentiometer R2 using an exemplary stopwatch.

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