» Electronics DIY Relay Board

DIY relay board


The relay is used for switching AC circuits ..., AC loads such as lighting lamps, various fans for working in automatic mode to reduce light, increase temperature, etc.

We also encounter situations when we need to control equipment remotely using a smartphone or we have a sensor that detects the presence of a person and turns on the light, turns the fan on and off. To control these devices, we use a relay board. Let's make a relay board that can be used along with logic circuits or micro-controllers to handle an AC load or high voltage DC load.

Required Details
1. Relay 5/6 in
2.2 resistors 1K
3. 1 1N4007 diode
4. 1 BC548 transistor or similar
5.1.3 pin screw connector
6.1 MCT2E / 817 / 4N35 Optocoupler

Theory and Test Layout

A relay is an electromagnetic switch. Initially, when there is no input signal, com (common) and NC (normally closed) are connected. When the voltage is applied to the input coil, a magnetic field is created and becomes an electromagnet. This magnetic field attracts to com-connects and a contact forms between com and But (normally open).

Relay circuit board

The optocoupler circuit is just an optical isolator ... it has an IR LED at one end and a phototransistor at the other end. When the IR LED lights up and the light hits the base of the phototransistor, the transistor turns on.
The signal from the microcontroller or logic circuit is fed to the IR LED .. and turns it on.
The emitter of the Phototransistor feeds the NPN transistor to the T1 base of BC548 through a 1K resistor, therefore the Darlington configuration is obtained, now B1 * B2 + B1 + B2 (B1 is the current gain of the phototransistor and B2 is the current gain BC548) .... Now that the signal line is high, the IR is on, the phototransistor and BC548 and the current flows through the relay winding and feeds it .. then contact com goes to the contact and, therefore, com and But are closed, .. when the signal line is reduced com and H3 are closed ..
D1 is used as a reverse diode. The circuit works for a while and then turns off, the accumulated induction energy is reset, the voltage can reach 40-60 V, during a very short interval and can damage other components, the diode used provides a circular path for the accumulated energy and dissipates in the diode, keeping the components safe ..
We assemble on the layout and look, with the right connection everything should work ...
DIY relay board

Now, after testing the board, we proceed to soldering, look at the circuit and begin to carefully solder. Be careful, because we are dealing with high voltage, so one mistake can ruin everything ... carefully observe the chains with a magnifying glass and light. Test yourself with a tester to find No and NZ, Common.
Now test it under DC load. After successful tests, you can switch to AC loads.

I wish you successful experiments!
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8 comments
Here the meaning is galvanic isolation.
And yes, a little stupid. The output of Arduina is the output of the ATmega328 controller. Output current <40mA.
A simple (small-sized) relay for 5V has a current of ~ 50 ... 60 mA. And the inductance of an electromagnet relay is decent.
The total power supply current of the controller is ~ 200mA. Therefore, it is not possible to connect a lot to the outputs (low-resistance Rн). The crystal will burn.
I don’t know, it may be stupid of me, but any arduino output can provide normal relay operation. The current at the Arduino output is enough even for 2. And both of them can be kept in the on position at the same time. If more than 2, then here you can resort to the help of a transistor. 1 transistor 1 transistor, and so on, you can continue until the pins on the arduino end. Thus, I managed to turn on 7 relays. According to this scheme, you can control the relay and 12, 24, 42 volts.
And plus to the above through this resistor, the leakage of the phototransistor and external interference are shunted. You can’t throw a base hanging in the air at a transistor!
Quote: Nruter
Why is he there?
For normal T1 closure: without a resistor between the base and the emitter, there is nowhere to flow back to the base-collector junction current, so it will flow into the base-emitter junction and open the transistor.
Why is he there? Everything is drawn correctly. Denomination 1K will be a little small, but it will.
NO is a normally open contact, NC is a normally closed contact. The diagram is drawn incorrectly.
There is not enough resistor between the T1 base and ground.

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Hand it for the smartphone ...