Speed Controller

by Chuck Davenport

You can make a speed controller for your hobby drill or use one in conjunction with a variable speed hobby drill that will slow the drill to one revolution per second.

The idea is to obtain a variable speed drill at a sufficiently low cost to justify the project. Drills sold for home use frequently suffer from armature burn out or gear damage due to improper or extended use. If you have one of these drills, you've a perfect candidate for the project. You might also acquire one at a flea market or yard sale. Ensure the variable speed trigger works, however.

You will also need the following items: commercial, 3-wire female plug, 6" length of 16 gauge wire (extension cord works fine), common hand tools, and 0.040 plastic card or 1/8" plexiglass.

1. (Fig. 1) Remove the gear and armature housing. Many are secured to the pistol grip via 3 screws at the front of the gear housing. The armature may come away with the housing or remain attached to the pistol grip. An all metal drill may have the armature housing and the pistol grip cast as one unit. If that is the case, you will need to cut the armature housing away with a hacksaw. DO NOT DO ANY SAWING UNTIL YOU HAVE DISCONNECTED THE WIRING IN STEP 2.

2. (Fig. 2) This drawing shows an exploded view of the principal connections that concern you. The drill housing and trigger have been omitted for clarity. The rectangular box depicts the GENERAL type of trigger you will likely find. There are many variations but the important features are standard. Two wires, one white, one black, come from the cord and enter the trigger assembly. These are the power leads. DO NOT DISTURB THEM. A green wire also comes from the power cord. This is the ground wire and is either attachd to the (metal) case or to some metal part of the drill. If attached to the case, leave it. Otherwise, remove the wire from from its attachment point. If possible, leave whatever connector that is crimped/soldered to the wire attached to the wire. If you cannot, cut the wire with nippers.

There will be a number of wires coming from the trigger assembly. Two, black or white (or possibly other colors) will be attachd in some fashion to the carbon brush contacts at the commutator. Remove these. They will be your power leads. Remove any wires that lead to the coil by removing the wire from the trigger assembly. Do not cut the wires as they could short on the case and ruin your day. Insert a paper clip into the small hole adjacent to the wire you wish to remove from the trigger assembly. You must force the spring contact to release the wire. You may have to use some force here. Once you have the wires removed, you can do any cutting to the case that may be required. Just remember to diagram how the trigger assembly and wires fit back into the housing.

3. (Fig. 3) Trace the outline of the pistol grip cross-section onto plastic card (two sections of 0.040 glued together as in Fig. 4) or plexi and cut to size. This is the mounting plate for the female plug. Also, transfer mounting hole locations to the card and trill these out. You may have to create some mounting lugs if your drill lacks a sufficient number (2 to 3 should do quite well). If all else fails, you can gape the mounting plate to the pistol grip with duct tape. I do not recommend gluing it should you need to make adjustments/repairs at a later date.

4. (Fig. 4) Disassemble the female plug and trace its outline onto the card or plexi and cut out the hole. Check the fit of the plug to the mounting plate.

5. (Fig. 4) Attach the two power leads (from the trigger) to the female plug by threading the wires through the plug base. The plug has 3 screw connections. The two vertical posts are for power. Polarity is not critical. You may need to remove 3/4" of insulation by scoring the circumference of the wire. The insulation will pull away. Attach a section of 16 gauge wire to the ground terminal of the plug and make a wire loop at the other end by removing 1 1/2" of insulation. This can be attached to the case at the locations shown or wired directly to the green wire from the cord. Cover bare wire with electrical tape.

6. Once wiring is complete, test the tool to ensure it works. Check for loose or disconnected wires. You may have also removed one too many wires from the trigger assembly. Some assemblies feature a wire that electrically connects the lower part of the trigger assembly (input from the cord) to the upper section (output to contacts and coil). If you removed this, reattach it.

7. Assemble the controller by sandwiching the plastic plug mounting plate between the two ends of the female plug. Secure the plug with screws provided. Attach the plug mount to the pistol grip with screws or duct tape. Your speed controller should resemble the one on the overview.


  • Overview
  • Profile view
  • Wiring diagram
  • Outlining plate
  • Exploded view

  • Ed. Note: I've collected the full sized scans into an Adobe PDF file. The most current version of the Adobe Acrobat software may be obtained free of charge from the Adobe web site:

    Charles T. Davenport
    IPMS #25544