circuit design - How to control a motor with only relays and push button?

Hello I´ve been working in this project for some days, I need to make a circuit with only relays that can make a dc motor start, stop and a reverse. The thing is that when you push the start or the reverse button the motor need to keep running, I can´t keep it pushed.

This is as far as I can get. SW1 turns the engine on, SW5 turns it off and SW7 reverse the motor but only if it is already running. What i need is that the reverse button also turns the engine on. Please help.

notes: -i can only use three push buttons and as many relays as needed. Yes is a school assignment.

THANKS!

Answer

Your specification is a little unclear, but I'm going to assume that

• one button starts the motor forward


• one button starts the motor in reverse


• one button stops the motor

mainly because the alternative

• one button toggles the direction of the motor


• one button starts the motor in the selected direction


• one button stops the motor

can't be done with only three relays.

Does this give you enough of a hint to get going?

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Here's one possible way to do it. Note that this version requires you to hit "Stop" before reversing direction. Can you see why?

^{simulate this circuit – Schematic created using CircuitLab}

With a little more work, you can eliminate that requirement.

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In case anyone is curious, here's the simplest circuit that I can find that implements a "toggle" function:

^{simulate this circuit}

This assumes that K2 or K3 won't drop out in the time that it takes K1 to switch. Adding catch diodes to their coils would help make sure this works, by increasing their "hold" time.

This allows us to implement the "alternative" controller described above, using three buttons and five relays:

^{simulate this circuit}

Note that the motor wiring (in blue) uses its own separate contacts, so the motor power could be different from the power used for the logic (relays).

Note also that K2 switches when you press the "Reverse Direction" button, and K3 switches when you release it. This means that while you're holding the button, the motor is unpowered, which is probably better than instantly slamming it into reverse.

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Multiplexer diagram:

^{simulate this circuit}

Ladder diagram:

^{simulate this circuit}