Automatic control switch circuit _ light control switch circuit and temperature control switch circuit - Database & Sql Blog Articles

Probe domestic switch needle KG-300K, needle head diameter is 3.0mm, normally open switch needle

By making a few small modifications to the delay switch circuit, you can create an automatic control switch that responds to specific conditions.

Light Control Switch Circuit

The light control switch circuit is illustrated in Figure 4. It uses a 2CU type photodiode as the photosensitive element. The resistance of the photodiode decreases when the light intensity increases. By adjusting the position of the movable contact on resistor R, the transistor V1 and V2 can be turned on at a certain light level, which activates the relay. An electrolytic capacitor C is used to provide a delay, preventing unstable operation of the relay under critical conditions. The value of R determines the threshold of light intensity; the higher the movable contact is adjusted, the lower the light intensity required to trigger the switch, thus increasing its sensitivity (note that the movable contact should not be placed at the lowest point).

This light control switch can be used to manage electric lights. If the normally closed contacts 1 and 2 are used, it can implement the "turn off when daylight is present" function. If the normally open contacts 1 and 3 are used, it can activate the "turn on when it's dark" function.

Figure 5: Temperature Control Switch Circuit

The temperature sensing component in this circuit is a KF-12 thermistor, which has a negative temperature coefficient. Its resistance decreases as the temperature rises. By adjusting the resistance of R, the transistor V1 and V2 can be triggered when the thermistor reaches a certain temperature, causing the relay to engage. This allows the temperature control switch to operate as intended. The resistor R can be set to a specific critical temperature value for optimal performance.

This temperature control switch can be used to control an electric fan via its normally open contacts. When the temperature rises to a certain level, the fan will automatically turn on. Similarly, a heater can be controlled using normally closed contacts, turning on when the temperature is low and turning off when it becomes high.

In applications where high precision is not required, both the photodiode and thermistor can be replaced with a 3A×81 or 3A×31 transistor after removing the top cover of the case. For instance, the 3A×81 transistor can be used by scraping off the black paint on the glass bulb, and then replacing the photodiode with the collector and emitter terminals. Similarly, the thermistor can also be replaced using the same method. Before soldering, it’s recommended to use a multimeter to test the resistance between the terminals of the transistor. You can adjust the light intensity (by placing it near a lamp) or the temperature (by placing it near a soldering iron) to select the appropriate resistance that changes with light and temperature levels.