Timer on D-trigger. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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To power many amateur radio designs, autonomous power sources, such as batteries, are used. To avoid useless discharging of the source, it is necessary to turn off the power of the device in time. However, sometimes this is forgotten, so in some cases it makes sense to supplement the design with a timer that will turn off the power of the device if the owner forgot about it.
If the required operating time of the device does not exceed 10 ... 15 minutes, a simple timer is quite suitable, the description of which is presented below. It is controlled by one button, and only one microcircuit is needed to make it. The timer circuit is shown in the figure. It is assembled according to a slightly modified circuit of a pulse shaper on a D-flip-flop, the description of which is given in the article by Alekseev S. Shapers and generators on CMOS microcircuits. - Radio, 1985, No. 8, p. 31-35.
Timer circuit
In the initial state, at the input C of the D-flip-flop (pin 3) and at its direct output (pin 1, Output 1) there is a low logic level. The timing capacitor C2 is discharged. In this mode, the current consumed by the device does not exceed a few microamperes.
When the SB1 button is pressed briefly, the D-flip-flop switches to a single state at the direct output and the charging of the capacitor C2 begins through resistors R2 and R3. Capacitor C1 eliminates the influence of the bounce of the button contacts. When the capacitor C2 is charged to a voltage corresponding to a high logic level, the D-flip-flop will be reset to zero and the device will return to its original state. In this case, the capacitor C2 will quickly discharge through the current-limiting resistor R2 and the diode VD1. With the values \u3b\u2bof the elements R10 and C1 indicated in the diagram, the timer exposure time is about XNUMX minutes. You can turn off the timer before this time by pressing the SBXNUMX button again.
With this timer, it is best to control the FET key. In this case, you can use the signal from both the direct and the inverse output of the trigger. Depending on the current consumed by the load, a high-power (IRF540, IRF640, IRFS34, IRFZ44 and similar) or low-power (KP505, BS170, 2N7000, 2N7002 series) switching field-effect switching transistor with an insulated gate and an n-type channel ( or p-type). An n-channel transistor is connected as follows. Source - to the minus of the power source, gate - to pin 1 of the DD1 microcircuit, drain - to the negative line of the load power supply. The plus of the power supply is connected to the positive power line of the load.
Imported oxide capacitors are used, and it is desirable to use C2 tantalum and, possibly, for a higher rated voltage, in this case the leakage current will be less, and the exposure time will be more stable. Resistors - MLT, C2-23, resistor R3 can be made up of several connected in series. Button - any small-sized with self-return. At a supply voltage of 9 V, a K176TM2 microcircuit can be used, at a voltage of 3 ... 6 V - a 74NS74 microcircuit. In the latter case, a field-effect transistor with a low opening voltage, for example, the IRL2505 series, must be used for switching.
Author: E. Gerasimov
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