Fabrication of high performance microwave controlled switch

Fabrication of high performance microwave controlled switch

Microwave control switches are widely used in automatic control, alarm and other fields. The high-performance microwave probes introduced in many publications require special control chips. They are expensive and difficult to buy. Simple probes made with op amp LM324 have been introduced in many electronic publications, but they all have a major disadvantage: poor anti-interference ability, false operation, and little practical value. The author combined various materials to design and produce the microwave control switch introduced in this article. Its biggest feature is that it is made of universal devices and cleverly uses the counting and automatic reset functions of CD4017 to greatly improve the anti-interference ability of this probe. Its performance is comparable to commercially available products. If it is made with disassembled parts, the cost is only a few yuan, which is good value for money and very suitable for electronics enthusiasts to make. As shown in the attached figure.

1. Circuit Principle

Q1, L1, and C1 in the attached figure form a microwave oscillator circuit, and L1 is both a transmitting coil and a receiving coil. When someone moves in the microwave field, a small voltage change occurs at both ends of R4, and then coupled to the frequency-selective amplifier composed of U1A and U2B through C2. The gain is determined by the ratio of R9 to R5, and R10 to R6. This circuit is designed to be 80dB. The output signal of U2B is sent to the voltage comparator composed of U3C. Proper adjustment of R11 can make U3C output a low level when there is no signal, and output a high level pulse signal when there is a signal. The pulse signal is sent to U5 for counting. When a human body moves in the microwave field, U3C will continuously output more than 4 pulses within 2 seconds. At this time, the Q4 pin of U5 outputs a high level, causing the bistable circuit composed of U4 to flip, Q3 to turn on, and the relay to close to control the load to work. LED1 serves as both a power indicator and a receiving indicator. It will go out when a signal is received and light up again after 2 seconds.

If the pulses sent in 2 seconds are less than 4, and the interference pulses rarely reach 4 within 2 seconds, U5 will automatically reset after 2 seconds of delay through R15 and C3, and re-enter the standby state. The introduction of the automatic reset circuit greatly improves the anti-interference ability of this circuit.

2. Component selection

Q1 can use high-frequency tubes such as 9018 and C3355, L1 can be wound with Φ1mm enameled wire into a coil with a diameter of 10cm, the op amp can use LM324, U5 can use CD4017, U4 can use CD4013, Q2 and Q3 can use low-power NPN tubes such as 9013 and 9014, and other resistors and capacitors can be selected according to the diagram. The power supply can be made with a three-terminal voltage regulator 7812.

3. Installation and debugging

The circuit can be soldered by yourself after arranging the components on the universal board. After soldering, do not connect C2. Use a multimeter to measure the output voltage of U3C. At the same time, adjust R11 so that U3C just changes from high level to low level. At this time, the trigger sensitivity is the highest. To reduce the sensitivity, you can appropriately increase the voltage of U3C's ⑩ pin. Connect C2. Shake it in front of L1 with your hand. After seeing LED1 go out, you can hear the sound of the relay closing.

The debugging is now complete. Install the circuit board into a suitable housing and it can be put into use. The microwave detection part in the circuit is replaced by an infrared receiver. Send the output signal to the (14) pin of U5 to make a universal infrared remote control switch. At this time, just press the remote control button 4 times in 2 seconds to turn the appliance on and off. Under normal circumstances, the remote control is rarely pressed 4 times in 2 seconds. The probability of malfunction is very low. Other probes can also be used to replace the microwave part. Realize the wide application of this signal detection part.