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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Electronic stopwatch on two transistors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches This simple device, assembled on just two transistors, is an electronic stopwatch, since it can be used to count time intervals set on a scale. As the capacitor C1 charges, the voltage across it increases, and the charge current gradually decreases. The drain current (and the source current, which is almost equal to it) drops, and the voltage drop across the resistor R4 decreases. This leads to an increase in the negative voltage at the base of the transistor VT2. When this voltage reaches a certain threshold, the transistor VT2 opens and the audio frequency generator, which is assembled on it, starts working. Generation is provided by the presence of positive feedback, which is supplied from the winding 2-3 of the transformer T1 through the capacitor C2 to the base. Loudspeaker BA1 starts to reproduce a sound signal, the frequency of which is determined by the inductance of the winding and the capacitance of capacitor C2. Now with the toggle switch you can turn off Schematic diagram of the stopwatch is shown in the figure.
In the initial state shown in the diagram, the device is not supplied with power from the battery GB1, and the capacitor C1 is discharged. To start the stopwatch, turn on the SA1 toggle switch. In this case, the SA1.2 contacts supply power to the circuit, and the SA1.1 contacts open and the resistor R1 is disconnected from the capacitor C1, which starts charging. While the capacitor is discharged, the voltage at the gate of the field-effect transistor VT1 relative to its source is equal to the power supply voltage of +4,5 V and the transistor is fully open. Almost all the voltage of the power supply drops, and the potential of the base of the transistor VT2 relative to the emitter is close to zero. As a result, he is locked up. As the capacitor C1 charges, the voltage across it increases, and the charge current gradually decreases. The drain current (and the source current, which is almost equal to it) drops, and the voltage drop across the resistor R4 decreases. This leads to an increase in the negative voltage at the base of the transistor VT2. When this voltage reaches a certain threshold, the transistor VT2 opens and the audio frequency generator, which is assembled on it, starts working. Generation is provided by the presence of positive feedback, which is supplied from the winding 2-3 of the transformer T1 through the capacitor C2 to the base. Loudspeaker BA1 starts to reproduce a sound signal, the frequency of which is determined by the inductance of the winding and the capacitance of capacitor C2. Now, using the toggle switch, you can turn off the power while simultaneously discharging the capacitor C1 through the resistor R1, preparing the circuit for the next turn on. The time elapsed from the moment the power is turned on by the SA1 toggle switch until the sound signal (exposure) appears is determined by the charge rate of the capacitor C1, which is why it is called the time setting. The rate of its charge depends on the time constant of the charge, equal to the product of the capacitance G1 and the resistance of resistors R2 and R3 connected in series. The larger the time constant, the slower the capacitor charges and the longer the exposure. To change it, a variable resistor R3 is used. With the parameters of the elements indicated in the diagram, the minimum exposure corresponding to the minimum resistance of the resistor R3 is about one minute, and the maximum is about 15 minutes. If shorter shutter speeds are required, the capacitance of capacitor C1 should be reduced. When it is reduced by 10 times, the shutter speed decreases by about 4 times. To set a particular shutter speed, the variable resistor R1 must be equipped with a scale and programmed using a conventional stopwatch. The circuit can be mounted on the board shown in the figure, without printed conductors, using the leads of the parts as reference points.
As a transformer T1, any output transformer from a small-sized transistor radio is used. Capacitor C1 should have minimal leakage. Therefore, it is advisable to use a tantalum capacitor of the ETO or K52 type with an operating voltage of 6 V.
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