Relatively new to circuits, but I have found this good motor control circuit for a DC motor (I have 2 microswitches, each one reversing the direction of the motor), the only thing is I am having some trouble figuring out where I should place my speed control unit. This is the speed control unit I am using. It has a +/- battery input and a +/- motor output.

This is the circuit diagram I am using, the terminal block I am using is a DPDT relay with a socket base. I would've drawn a circuit for it, but to be honest I don't know what it would look like, but I feel like it would make it easier for me to understand where to put the speed control, so if someone wouldn't mind drawing the circuit for me it would be greatly appreciated, but not necessary.


simulate this circuit – Schematic created using CircuitLab

circuit diagram

To see a full overview of what I am doing (minus the speed control) take a look at this link : http://cheesycam.com/cheesycam-diy-auto-reverse-polarity-motorized-video-slider-update/

I know that I cannot put the speed control in between the motor and the terminal base, because the speed control will not work when the polarity changes to reverse the direction. I also don't believe I can simply put the speed control in between the battery and the terminal base.

Anyone have any ideas? As usual any help is greatly appreciated!

  • \$\begingroup\$ Can you add the pinpout for the relay? Or better, create a schematic for whole thing (minus speed controller). \$\endgroup\$ – RJR Apr 30 '14 at 1:21
  • \$\begingroup\$ @RJR yeah, working on the schematic. what would be the pinpoint for the relay? I can add that as I work on the schematic \$\endgroup\$ – Adjit Apr 30 '14 at 1:22
  • \$\begingroup\$ Pinout is here: ebay.com/itm/121034195696 - halfway down the page. \$\endgroup\$ – RJR Apr 30 '14 at 1:25
  • \$\begingroup\$ @RJR same exact pinpoint as that relay, except switch 7 & 8. Working on the schematic now, so will let you know when its done \$\endgroup\$ – Adjit Apr 30 '14 at 1:26
  • \$\begingroup\$ Look at this a bit more I think the issue is that the relay coil is activated through the same relay contacts as the motor - the relay coil doesn't care about polarity. The only way I can see this work is to separate the circuit that switches/powers the relay from the motor - that means you need a relay with 3 switches. An other option would be to use a latching relay. A third (an maybe simplest) way would be to get rid of the relay completely and use a DPDT switch (with enough current to drive to motor) to change direction. Do you need to use a relay? \$\endgroup\$ – RJR Apr 30 '14 at 1:47

Option 1: do away with the relay completely:


simulate this circuit – Schematic created using CircuitLab

The relay isn't really required unless you want to be able to change direction using some electrical signal from a microcontroller for example. Since you're using a speed controller with a manual knob, I assume direction will be manual also.

Option 2, now using a 3PDT relay. Imagine SW3 being part of the relay:


simulate this circuit

Basically SW2 and SW3 are in parallel. SW3 'latches' the relay until it's released by breaking SW1. This way the relay is powered directly of the power source while the motor source comes from the speed controller.

  • \$\begingroup\$ Direction won't be manual. That is the idea of this. So I can leave it on and the motor will go back and forth infinitely \$\endgroup\$ – Adjit Apr 30 '14 at 2:03
  • \$\begingroup\$ That's a bummer. I guess the only way then is to separate out the motor and relay coil switches using a relay with 3 DP switches. You'd then 'feed' the first 2 with the motor controller, and use the 3rd to 'latch' the relay. \$\endgroup\$ – RJR Apr 30 '14 at 2:44
  • \$\begingroup\$ think you could draw a circuit up for that? \$\endgroup\$ – Adjit Apr 30 '14 at 2:57
  • \$\begingroup\$ Just have one question about this... it is probably a stupid one... but can a 12V battery charge a 24V relay coil? \$\endgroup\$ – Adjit Apr 30 '14 at 18:46
  • \$\begingroup\$ @Adjit - probably not I'm afraid. However, since you'll have to buy another relay anyway (assuming you don't already have a triple-pole one), getting a 12v one shouldn't be an issue. \$\endgroup\$ – RJR May 1 '14 at 1:48

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