Traditional Chinese Practical Memory timer: Circuit Design, Parts List & Workshop Prototype Guide

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memory timerThe advantages of memory timers are as follows:(1) Simple structure, low cost, low power consumption, no heating elements, stable performance, and the delay range can be adjusted arbitrarily from a few minutes to more than ten hours.(2) No contact, reliable action, long life, large control power, and has two functions of delayed power on and delayed power off.(3) Use the meter head to display the time, and use the add and subtract buttons to set the delay time. The remaining time can be monitored midway, and the original set time can be quantitatively added or shortened as needed.(4) It has memory function and is not afraid of power outage. During the delayed start-up process, the device can be disconnected from the power supply without consuming any power, and the delay time will not be affected at all. Once the power is turned on again, it will still be powered on at the original set time.Basic principles:The circuit schematic diagram is shown in Figure 132, which consists of a time circuit, a trigger circuit, a control circuit and a power supply circuit.Before point A, the time circuit was formed with the field effect transistor VT1 as the core. When the power is first turned on, VT1 has no bias, so the drain-source saturation current IDSS is large, and the potential of point A reaches the maximum value of about 12V. If you need to delay, press SB1, and the 13V power supply charges C7 through the drain-source resistor of VT1, C7 and R1. The charging process lasts for several seconds. After releasing SB1, since the charged C7 is connected in parallel between the gate and source of VT1, the gate of VT1 is reverse biased at this time, so the drain-source current IDS is very small, and the potential of point A drops to a few tenths of a volt.Since the gate-source insulation resistance of the field effect transistor is extremely high, C7 can only be discharged through R24~R26. Select different values by S1 to get different discharge times (ie, delay time); of course, if SB2 is pressed, C7 will be discharged through R2, and the discharge time is only a few seconds. As time goes by, C7 slowly discharges, the gate-source reverse bias voltage of VT1 gradually decreases, the drain-source current gradually increases, and the potential of point A gradually increases with time. Galvanometer μA directly reflects the change in potential at point A, which also reflects the change in time.This part of the circuit has several notable features: First, by selecting C7 and R24~R26 that are large enough, it is easy to achieve a long delay of more than ten hours. Secondly, if you briefly press the time-added button SB1 during the discharge of C7, some charge will be added to C7 and the delay time will be increased; while pressing the time-subtracted button SB2 will cause C7 to quickly discharge some electricity and shorten the delay time. The control quantity can be displayed by the meter header. Third, C7 is not affected by the power supply during the discharge process. …

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