Solenoid Engine Prototyping

I’ve started thinking about how I might build a solenoid engine. I bought a couple solenoids on Amazon, Uxcell brand, and they seem to have quite a bit of zip to them, so I’ve been looking into how to mount them to a block in a way that would allow them to rotate around an axis, eliminating the need for the connecting rod to rotate with the piston head, which in this case is just the armature of the solenoid.

Pictured below is the first three attempts at printing a little box to hold a solenoid. The little wings on the side have 1/8″ holes in them that will fit over a rod, allowing the solenoid assemblies (I plan on having two of those) to lie directly over the crankshaft and rotate back and forth with the cranks.

You can see the design process as it unfolded, first with no venting on the sides, then with vertical venting and finally diagonal venting to allow the solenoids’ windings to cool off better during operation. I don’t know how much of a problem heat will be, though, since I’ll be running somehow around half the rated service voltage of the solenoids, at 50% duty cycle to boot.

I did melt the diagonal vents a little with a heat gun while trying to de-string the object. Oops.

The car moves

I did it: the car moves. It drives right across the kitchen floor, picking up speed. That said, it does not handle carpet very well (no surprise).

I can’t get a very good video of it by myself, so I’ll work on that soon. In the meantime, I’ll post a still.

I made a couple changes near the end:

  • Changed the gearing. The motor just doesn’t have enough torque to move the car with a 1:1 ratio, so I pressed some old Erector crown and pinion gears into service.
  • Added a toggle switch to turn the power on and off.

Future improvements would include a higher-voltage battery and a proper connector for it (so I don’t have to gator-clip the motor to the battery).

Updated motor circuit

I have updated the motor circuit with the changes I’ve made. Notably:

  • Introduced a 5v linear regulator (LM78L05A) to provide supply voltage to the S-R latch, the inverter, the Hall-effect sensors, and the Vcc for the level shifter.
  • Replaced the CD4010B with a CD4504B and connected its select pin (pin 13) to ground.

These were rookie mistakes. The latch and inverter chips need 5v in, not the 15-20v I plan to supply, and the buffer/level-shifter was entirely the wrong kind of chip (it was a high-to-low instead of a low-to-high; we want to go from the 5v logic level to the 15-20v level). Live and learn, I guess. The new schematic is pictured below.

Mounted motor video

I made some interesting discoveries in the days leading up to this video. First, I completely forgot not all of the chips in the circuit can handle more than 5 volts. I actually burned out a chip as a result (it held on bravely for a long time), so I ordered some 5v linear regulators to provide a constant 5v to these devices. I’ll update the EasyEDA schematic with them at some point.

Second, the cycle time of the logic circuit was dramatically reduced by a diagnostic LED I had connected to the output from the S-R latch. Removing this LED significantly increased the speed at which the motor turned.

Third, one of the coils is evidently shorting at voltages higher than 10. Above 10v, the current through the coils increases sharply and the motor stops spinning robustly. At first I thought this was a problem with the control circuit, but all of that stuff appears to be working correctly. Instead, I think on of the coils suddenly begins shorting when enough voltage is present. I have it narrowed down to two of them, so I just need to test them individually and see if this really is the problem. If it is, I’ll need to wind a new coil to replace that one.

Even so, the motor works really well at 10v. The following video demonstrates the current state of the project.

Motor mounting

I mounted the motor to an official pinewood derby block over the weekend. It actually worked pretty well with the Erector wheel/axle/transmission setup I put together.

Kind of a busy photo, but you can see the gearing and the wheels. This is the rear end of the car.

I’m currently awaiting the arrival of some voltage regulator chips from Mouser. I had been getting lucky by overrating the SR latch and inverter chips with voltage, but they finally gave out, so I’m going to incorporate the regulator to hold them at the 5 volts they need. When that part arrives, I should be able to do some real testing.

Motor upgrade

Hey, I’ve been working on the motor for some time now. I got some 10mm Mn-Zn ferrite bars to use as cores and some 26 AWG magnet wire and wound new coils that each have around 3 ohms of resistance, thus eliminating the need for the big power resistors. I got it all hooked up tonight and had some really impressive torque/speed out of it.

Here are a couple of pictures. The first one shows the new motor itself, and the second shows a comparison between the old nail coils and the new ferrite ones.

I finally glued that one sensor in place.

I tested the two coils with a paperclip (highly empirical) and they didn’t seem much different, but on the actual motor, they are much different. The thing is almost violently torquey now, which is good. I’ll upload a video as soon as I have someone available to help film it.

New coils?

I’m going to wind some new coils and see how they compare to the old ones. These will be superior in a couple of ways: better cores, and better winding technique.

The cores will be these ferrite rods (replacing the hot-galvanized nails I used the first time. These are the rods:

For the winding technique, I’m working on building a winding jig out of plywood. The idea is to make a gear system out of wooden gears using the approach detailed here:

A crank handle will turn the gears, which then turn a cheap drill chuck that will hold the workpiece (in this case one of the ferrite rods mentioned above). Wire feeds onto the rotating workpiece from a spool after a tensioning device applies tension to the wire. I’m not sure what I’ll use for a tensioning device yet, but it could be as simple as a series of holes or pegs for the wire to pass through or a clamp that squeezes the spool of wire. Having a set of gears allows me to get several turns of the workpiece for each turn of the crank.

I used a different template generator the create the gears than the one provided on It creates involute gears, which aren’t as easy to cut out, but transfer power more efficiently:

I drilled and cut out the gears using a drill press and scroll saw. They’re not perfect, but they may answer the purpose. If these don’t work well, I may try again with the template generator, which creates less complicated geometries for the teeth.

Quick update on new engine block

The new block works nicely. At 2 amps, the motor has some of the most torque I’ve seen yet. It was able to continue turning as I attempted to stop the rotor by dragging my finger along its top.

You may notice the wooden shaft collar above the Erector set rotor parts. This is meant to be a cheap and disposable way to permanently fix the 2mm shaft to the Erector set parts, which I don’t want to commit permanently to the project. Two Erector machine screws go¬† up from the bottom of the rotor assembly, through the yellow plate and one of the silver crosspieces, and up into the wood. I used a 1/8″ drill bit to make the holes for these screws, and the holes took the threads from the screws really well.

Here’s the photo from tonight: