circuit-designdc motordpdtreversereverse-polarity
I request some knowledge regarding a circuit I am building.
Basic I have a circuit connected to a dc 12v motor and a dpdt switch to reverse the polarity of the motor and a variable potentiometer , I will need the motor to be stopped temporary via a microswitch, only in the reverse direction however the microswitch must have no affect in the forward. Is this possible using only 1 battery pack? And how would I have to wire the switch into the circuit for this to work?
Thankyou in advance
C.
You will need a memory element such as a relay to hold the current direction of the motor.
simulate this circuit – Schematic created using CircuitLab
Here a DPDT relay and two microswitches are used. The motor M1 drives to the "left" when the top terminal is positive and to the right when the top terminal is negative.
Suppose the relay is dropped out. The top terminal is positive, SW2 and SW3 are in the positions shown. When the motor reaches the left limit, SW2 is actuated, energizing the relay. The motor immediately reverses, but the top contact has pulled in, keeping the relay energized through D2. The motor operates until SW3 is actuated, which interrupts current to the relay (D3 catches the flyback voltage). The relay drops out, releasing the hold contact and it remains dropped out, and we're back where we started.
Interrupt V1 to switch everything off (and it will always drive to the left when power is applied). Interrupt only the current to M1 and the memory of the previous direction will be maintained as long as V1 is applied (which means continuous draw if the relay is energized).
A solid-state solution could use a single CMOS gate package (eg. 1/2 a CD4011B) and a MOSFET H-bridge to drive the motor:
To be able to drive a motor both ways from a single supply, use something called a "H bridge". The motor is put between two drivers that each can either output high or low. Set them to high-low to run the motor one way, and low-high to run it the other way.
Since your motors can apparently run from only 3.6 V, here is a really simple H bridge that can be safely driven from two 5 V logic outputs:
This is a pretty idiot-proof H bridge that will drive the motor with about 3.6 V given the 5 V supply as shown, up to a few 100 mA. A and B are 5 V logical level inputs. The motor is off when both are the same. It is driven one way when A is high and B is low, and the other way when B is high and A is low.
I would use a small microcontroller running from 5 V to drive these lines. The micro would be monitoring the light sense line thru a A/D input, and the rest is logic in firmware. If you want to sound a alarm and light a light, these can be driven by additional outputs from the micro. The signal from the limit switches would also be inputs to the micro, which it uses to know when to stop driving the motor.
Using a micro significantly simplifies the electronics, and puts the logic in firmware where it is easier to tweak and change as you decide you want something different after getting some experience with the device.
Best Answer
What you want to do is quite easy.
However, the question of NC/NO needs to be addressed first. I assume that your microswitch is used as a mechanical limit switch. In this case, the "regular" or "normal" operation is with the switch closed, and it opens when the limit is reached. With this in mind this
simulate this circuit – Schematic created using CircuitLab
shows the way to wire the system without a limit switch. Note that the indicated position of the switch corresponds to forward travel. Activating the lower position produces reverse by reversing the polarity of the motor drive. This
simulate this circuit
uses the limit switch to interrupt current to the motor, but only in the reverse position. SW2, the limit switch, is shown in the NC position. When it opens, reverse drive will be interrupted.
There is one more condition, though. This assumes that the motor is a brushed DC motor. The circuit will not work on stepper motors or BLDCs (brushless DC motor) or AC motors.