Electronically controlled switching circuits for campus

Due to the needs of teaching work, the power supply lines in schools are relatively complicated. The time controller available on the market can solve the problem of ringing the bell during school schedules. If it is equipped with a homemade simple electronic control switch, it can fully control the opening of equipment such as classroom lighting, classroom fans, campus street lights, etc., which can greatly reduce manual management.

Working principle: See the attached figure.

The 220V mains electricity is passed through the step-down circuit composed of R1s and C8, the bridge rectifier circuit of D7-D10, and the voltage stabilizing circuit of R14 and D, to obtain a 12V DC voltage to supply the whole machine power supply. Transistor Ti generates a delay signal. When the time program controller outputs an instruction or the test button S is closed, the base potential of T1 decreases and quickly saturates. At this time, the capacitor Cl is charged and a delay of 1-2 seconds can be obtained. Since C1 has a large value, T1 slowly exits the saturation state after a long delay process and enters the cut-off state, completing the triggering task for the next level for ~ times and waiting for the next trigger. The purpose of C, taking a large value is to ensure that Ti is quickly and reliably saturated and not easily affected by external interference to cause false operation.

T2 and T3 form a symmetrical bistable circuit. When Ti is saturated and outputs a pulse signal, it controls the bistable circuit to flip after passing through the resistor-capacitor coupling circuit, thereby obtaining two signals, "on" and "off".

T4 is the switch output stage and the relay is the output stage load.

When R is saturated and T3 is cut off, T4 enters the saturation state, similar to outputting an "on" signal. At this time, relay Mi is energized and the AC contactor M2 is energized. The M2-K contact is closed to connect the three-phase AC power, and the equipment in the circuit starts working (such as turning on the classroom lights).

When T2 is cut off and T3 is saturated, T4 is quickly cut off, which is similar to outputting an "off" signal. At this time, relay Mi is released, causing the AC contactor M, to also be released, the M2-K contacts to open, the three-phase AC power to be disconnected, and the equipment in the line to stop working (such as turning off the lighting).

In this way, when the time controller outputs the control signal twice, the bistable circuit flips twice, and T4 completes the entire "on" and "off" process, thereby controlling the on and off of the equipment in the circuit. The function of setting the push button switch S is to facilitate the inspection of whether the controller works reliably.

In order to prevent the controller from losing control due to power outage, the battery pack GB and Di are specially set to maintain the circuit to ensure the stability of the bistable circuit. The function of D. is to prevent the mains from charging the battery and damaging the battery. The function of Ds is that when the power is off, the battery only supplies power to T1, T2, and T3 without excessive power consumption. When the mains comes back, the DC voltage rectified by the mains is higher than the battery voltage, so the battery stops supplying power automatically, extending the battery life.

Component Selection

In the attached figure, except for R14, which is a 1W metal film resistor, and Ris, which is a ÷W resistor, all other resistors are 1/8W small resistors. Capacitor C8 is a 400V ferroelectric ceramic capacitor; C2 and C3 are small ceramic capacitors; C4 and C7 can be 25V electrolytic capacitors, and the others can be 16V electrolytic capacitors. D1-D6 are general switching diodes; D2 is a 1A small rectifier diode or bridge rectifier; M1 is a 12V DC small relay; M2 can be a 10A~30A AC contactor (depending on the power).

If there are many controlled circuits, the multi-unit circuit in the figure can be designed into 2-way or multi-way (2-unit or multi-unit) circuits. When designed as a multi-way control switch, the power supply part can be shared, and the other unit circuits are the same; use the SKS type four-way time program controller produced in Shanghai or other types. The program controller and the electronic bell are connected to this electronic control switch, and each way controls one line. The effect is very good and the reliability is high. It is a good choice.