In this article I will show ten simple circuits or devices that are assembled on a mechanical relay. The relay consists of a winding with a resistance of several hundred ohms and contacts, of which there can be one or more. The relay body indicates what voltage and current the contacts and winding are designed for.
1.The first device is called a flasher on a relay. No transistors, but the lamp will flash, and the relay contacts will click. The frequency of flashes of the lamp depends on the capacitor.
2.The second device is a two-button remote control. When the SW2 button is pressed, the relay is activated, the contacts of which close, blocking the SW2 button. Now it can be released, and the load will be turned on. When the SW1 button is pressed, the circuit opens, and the load is de-energized.
By pressing the SW2 button the load is switched on
When SW1 is pressed, the load is disconnected
3.The third device is protection against supplying the wrong polarity of voltage to the device. Usually this is done on one diode, but it should be taken into account that there will be a voltage drop on it, especially if the load current is large. Here there will be no voltage drop with the correct connection
We supply the correct polarity and the voltage goes to the load, the relay will not work because the diode will not allow it to do so
And now the polarity is wrong. The current will go through the diode and through the relay coil winding. The relay contacts will de-energize the load and the flashing LED will turn on, which will notify about the emergency situation
4.The fourth device is an automatic load switch depending on the illumination. When it is light, the light falls on the solar cell and this leads to the closing of the transistor. When it gets dark, the transistor opens and the relay operates, including a powerful load
The resistor must be used to adjust the sensitivity of the device
5.The fifth device is a load switch for a certain time. This time can be adjusted using a capacitor installed in the gate. The larger the capacitor capacity, the longer the duration of the load switch-on. While the button is not pressed, the load will be disconnected. We press the button briefly and the capacitor will charge, the transistor will be open and the load will be connected. When the capacitor is discharged to a certain value, the transistor will close and the load will be de-energized
6.The sixth device works as follows. When power is applied, the load will be disconnected, but at this time the capacitor is charged. After some time, the capacitor will be charged, the transistor will open, and the load will be turned on. The charging time is regulated by resistor R2
7.The seventh device is a soft switch for a lamp. It is known that incandescent lamps cannot be turned on abruptly at negative temperatures. Because of this, the filament may burn out, because its resistance will change. So that incandescent lamps, especially halogen ones, do not burn out, the current must be supplied to them gradually when turned on, then the filament will not burn out. This device is designed for soft switching on of halogen lamps or lamps that consume sufficient current. How it works. When power is supplied, the current goes to the lamp through a thermistor with a resistance of 10 ohms. The lamp does shine by about a third. The relay does not work, and the thermistor heats up and its resistance drops. After 3 seconds, the relay will work and block the thermistor with its contacts, and the current will go to the lamp completely and the lamp will shine at full intensity.
8.The eighth device is switching on and off a powerful load with one low-power button. When power is supplied, the lamp or load does not light. We press the button and the lamp lights. After about 3 seconds, we press the button and the lamp is de-energized. You cannot press the button without waiting a few seconds, otherwise there will be no switching.Relay with closed and open contact, winding resistance 300 Ohm, with a different resistance it is necessary to select resistors. When the relay is off, the capacitor is charged, by pressing the button, the relay is triggered, the capacitor is discharged to the resistor. By pressing the button again, the capacitor will bypass the relay and the relay will again return to its initial state
9.The ninth device is short-circuit protection. When power is applied, the load is de-energized and the LED lights up. We press the button, the relay is triggered, and current flows to the load. The LED goes out. We make a short circuit in the load, the relay is de-energized, current does not flow to the load, and the LED lights up again. The button must be selected in accordance with the current that will flow in the circuit
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