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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING A simple telegraph key. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications Although modern automatic telegraph keys are usually assembled by radio amateurs on digital microcircuits, on PIC controllers or on specialized microcircuits, a good key can also be made on analog elements. One of the options for such a device was proposed by G3PVH in his article ("En enkel noklingskrets", published in "Amator radio", 1998, Juni, p. 9. 10). Like most of these switches, it consists of a relaxation oscillator and a comparator. The generator (see figure) is assembled on an analogue of a unijunction transistor, which is made on devices of different structures (VT1, VT2), and the comparator is on an operational amplifier DA1.
The operation of the device is determined by three voltage levels: the opening level of the unijunction transistor analogue (about 7 V), the release level of relay K1 to the op-amp input (about 5 V) and the voltage level on the capacitor C1 (about 3 V). In the initial state, the capacitor C2 is charged to the power supply voltage (9 V). If the movable contact (M) of the manipulator (connected to the emitter terminal of the transistor VT1) is connected to the "-" ("dash") contact, then the capacitor C2 will almost instantly discharge to zero. In this case, relay K1 will operate and at the same time the charge cycle of this capacitor will begin through resistors R3, R4 according to a law close to linear. As soon as the voltage on the capacitor reaches the level of 5 V, the contacts of relay K1 (not shown in the figure) will open. If by this time the movable contact of the manipulator is in the neutral position, then the formation of a single dash will be completed, and the capacitor will charge up to the voltage of the power source after a while. In the case when the movable contact of the manipulator is still connected to the "-" contact, the capacitor charge will continue only up to a voltage of 7 V. When this voltage is reached, an analog of the unijunction transistor will open and the formation of the second dash will begin. While the voltage on the capacitor C2 rises from 5 V to 7 V, a pause is formed between dashes. If the movable contact of the manipulator is connected to the contact "." ("point"), then the process described above will be repeated, with only one difference - the capacitor C2 will start to discharge not to zero, but only to the voltage level on the capacitor C1. For this reason, when the capacitor C2 is charged, the voltage on it will quickly reach the release level of the relay, which will lead to the formation of a parcel shorter than a dash - a dot. The transfer rate is regulated by a variable resistor R4. When adjusting the key, the movable contact of the manipulator is transferred to the "dash" position and the trimming resistor R5 achieves the optimal "dash-pause" ratio (about three). Then, the tuning resistor R2 sets such a transmission rate of points at which their duration is close to the duration of the pause. Both of these adjustments are somewhat interdependent, so the optimal position of the trimmer resistors R1 and R5 is determined by the method of successive approximations. In an analogue of a unijunction transistor, you can use, for example, a pair of KT315 - KT361 with any letter indices and a current transfer coefficient of at least 50. Operational amplifier - K140UD7 or any other close to it in terms of parameters. Relay K1 - for a response voltage of 6 ... 7 V. The current consumed by it should not exceed the maximum output current of the operational amplifier.
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