Electronics workshop: bistable relay

Electronics workshop: bistable relay

We share a new practice electronics in our workshop  to better understand the operation of logic circuits . We explain the assembly step:

With this simple mounting electronic hobbyists and students learn practical and fast way the operation of the logic circuits . Normally, electronic appliances on and off is done by simply pressing a key or button.

This function is called bistable , as there are two (on / off) clearly differentiated states that remain constant until the key is pressed. You can see the installation diagram below:

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Fig.1: Assembly diagram of the bistable circuit.

The core or heart of this design is the flip – flop of the JK type . This scale is implemented in an integrated CMOS circuit 4000. The output series of the scale (Q) is the pin 1 of integrated . This pin is only possible existence of a logic high level (12V in this case), or low logic level (zero volts).

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Components for the bistable relay circuit.

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List of components required for the circuit.

The passage of a high to low level, occurs on each clock pulse (pin 3) as long as the two pins JK are high (12v).

The pins Set (set to 1) and Clear (are reset to a low logic level, to not interfere in the operation of the circuit). The pulses are applied through the switch, thus to each pulse the logic level output pin 1 changes value.

The R1, R2 and capacitor C1 resistors form a ‘debounce circuit’. Each time a switch or button is actuated, closing its contacts results in a series of ‘rebound’, which can be interpreted by the circuit as several clock pulses which would result in the output is unstable and will vary from level very quickly . To avoid this, it is essential to use these components which absorb these rebounds. The current delivered by the integrated output is of little value to load or activate a relay, so it must be amplified by a small transistor. Resistor R3 is responsible for limiting the base current T1.

T1 is a small power bipolar transistor that activates the saturated relay. LED and limiting resistor R4, inform us in a visible way of relay activation status. The diode D1 is responsible for absorbing reverse voltage pulses generated in the relay coil when it is off.

In the electronic scheme we include the pinout of the transistor and integrated with its numbered pin.

We remind you that the numbering of the pins are made in an integrated positioning notch it up and numbering them on the left side from top to bottom, and right from the bottom up.

Given the relative complexity of the circuit, we recommend doing the first tests on a type ‘breadboard’ board then mounted on a plate definitely the type of lines or dots. The circuit is powered 12v , but the relay can be activated either loads or alternating current of any value, provided they do not exceed the isolation voltage and maximum current relay contacts.

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Image circuit mounted bistable relay.


You can ride around the circuit in a sealed plastic box and use it for a lot of applications . For example, it’s easy to make a practical car burglar or a starter circuit thereof, by means of a simple push open or close contacts starter. We can also use the circuit to turn on or off light bulb with one touch.

If you found it interesting this assembly, in magazines Todoelectronica and in our electronics course you will find many more proposal to enjoy electronics.

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