Mad Teddy's single-solenoid electric engine

Mad Teddy's web-pages


My single-solenoid electric engine
(ca. 2002)

Two of the three books mentioned in the lead-up to this page, "Model Making for Young Physicists" by A.D.Bulman and "The Boy Electrician" by Alfred P. Morgan, each presented a model which could be described as a "solenoid engine". The most obvious difference between them is that one of them (Bulman's) had only one solenoid, while Morgan's had two. The most obvious thing that they had in common is that they both relied on moving contacts.

Having built my two-pole electric motor , and thus knowing the hassles moving contacts can cause, I decided in 2002 to build a solenoid engine built on very different principles.

The fact is, my Dad and I did build a single-solenoid engine in the late 1960's, based somewhat along the lines of Bulman's model, using an old solenoid my Dad had lying around (goodness only knows where he got it from, or what its original function was!). The model did work, although not very well; eventually it was dismantled, and some of the parts found other uses. As you've probably guessed, the moving contacts were the main cause of its ultimate demise.

Reduced to its bare essentials, a solenoid engine of the moving-contact type can be represented as in the following diagram:

At the right is the solenoid - a coil of wire wound on a tube of suitable non-ferrous material with a movable soft-iron core. This is attached to a crankshaft (at left) which bears a slip-ring and a cam, both made from some suitable metal (eg. brass) and electrically connected together.

Here is a view from above:

Two brushes made from some springy metal are attached to the base so that one is in permanent contact with the slip-ring, and the other is in contact with the cam for exactly half the time when the engine is in operation. In the above diagram, the relationship of the cam to the crankshaft is such that the solenoid will pull only when the crank is above the level of the bearings. In this configuration, the pull of the solenoid on the armature - and hence the crank - will cause clockwise rotation of the crankshaft, together with the slip-ring, cam and flywheel. Once the crank has been pulled to the right-hand horizontal position, the cam is no longer in contact with its brush and the solenoid is switched off. The flywheel causes the crankshaft to continue to rotate (or "free-wheel") clockwise until the crank is horizontal again, but this time over to the left. While this is happening, the armature pulls the moving core part-way out of the coil. Then the cam and brush will again come into contact, switching the solenoid on so that once again it pulls - and the cycle continues until the power is switched off.

If the cam were mounted at 180 degrees to the position shown here, the solenoid would pull only when the crank was below the level of the bearings, and the rotation would be anticlockwise instead.

The model my Dad and I built was pretty much along the lines of the diagrams above. There may have been minor differences: the slip-ring and the cam may have been at interchanged positions on the axle; we may have used a second slip-ring with a semi-cylindrical piece of metal instead of an actual cam - small details like that - I really can't remember; but the basic idea was the same. (We probably tried various modifications at different times in an attempt to improve the performance of the thing!)

It is possible to build a model which has a different geometry, but essentially the same basic structure. The model described above looks rather like an old-fashioned horizontal steam-engine. On the other hand, another experimenter (see his own website ) has built an electric engine model which is more reminiscent of a Texan oil pump! The movie of this is well worth a look - you can see the action of the cam very clearly.

(While you're there, check out the movie of his Atkinson engine - nothing to do with the present topic, but fascinating in its own right. Lots of other interesting things there too - spend a bit of time having a poke around.)

In 2002, it occurred to me that it should be possible to build a machine of this basic type without moving contacts, using opto-electronic components to control the switching. Light-Emitting Diodes (LED's) have been around for decades; so also have Light-Dependent Resistors (LDR's). Both are reasonably cheap and can be obtained from electronics shops.

An LDR is a thin slice of cadmium sulphide (CdS), a light-sensitive substance, mounted on a small piece of insulating material and with two interleaved metal grids attached to the top surface. When light falls on the LDR, the resistance between the grids drops dramatically.

This LDR is about 1cm across. In darkness, its resistance is about 45 kilohms (probably more in pitch-dark conditions); with a bright white LED shining on it at close range, the resistance drops to about 80 ohms.

In the 1960's, when the tremolo effect was popular with electric guitarists (remember Hank B. Marvin's style in some tunes by the Shadows, or "Crimson and Clover" by Tommy James and the Shondells?), technically-minded "musos" who owned a "basic" guitar amplifier would sometimes add this effect by building an oscillator circuit capable of flashing a small torch globe (this was before LED's were common or cheap). The globe would be organized to shine on an LDR which was wired into the amplifier's gain control, thus producing the characteristic pulsating effect.

These days, of course, you can buy things like "opto-transistors" which do essentially the same job as an LDR wired into a gain-control circuit, but with everything built into a single integrated-circuit package and made from more recently-developed materials with faster response times.

(By the way: I recently visited a museum which had a display of old-style pigments used for making paints. Powdered cadmium sulphide is - or was - used to make bright yellow paint! So I wonder if my use of a yellow LED to shine on the LDR was an inspired guess - or whether a different colour would have actually been better? Something to look into at some stage...)

A circuit including an LDR can be used to control an electric engine. All that is needed is some method of automatically controlling when a beam of light from a LED falls on the LDR, connected so as to switch the solenoid on and off at the appropriate times. What can be used for such a purpose?

From time to time, for other projects (unrelated to those in these pages), I do a bit of resin casting with clear epoxy-resin. Of course, one always makes up a bit more than is actually needed, to make sure that there is in fact enough for the job in hand. Thus there will be a small amount left over afterwards. Rather than throwing the excess away, I pour it into some small container - a cap from an old spray-can, for example - and leave it to set, on the very sound principle that "you never know when it might come in handy".

Well, a clear plastic disc made in this way did come in handy - just the job for the light-shutter in my single-solenoid engine!

  

In the left-hand picture above you can see the clear resin disc, with about half its area covered with black insulating tape. The right-hand picture shows how a yellow LED is mounted to one side of the disc so that it can shine into a black tube on the other side, provided the black tape is not in the way. An LDR, of the same type as that shown above, is mounted inside the tube, at the far end.

The LED shines all the time while the engine is running. Thus, for about half the time when the rotor is spinning, the LDR's resistance is high; and for the rest of the time, its resistance is low.

The resin disc was attached with four screws to a short piece cut from an old broom-stick. A hole of an appropriate size was drilled through the centre of the assembly so that it would be a snug push-fit on the metal rod chosen for the axle. When it turned out not to be quite as tight a fit as I'd hoped, I included the fibre part of a tap washer in the assembly to improve matters. (This is just barely visible in the right-hand photograph above. You can see it better in the final photo in this page.)

Part of the barrel from an old ballpoint pen was used as the distance-piece between the piece of broom-stick and one of the bearings, as can be seen quite clearly in these photographs (especially the right-hand one).

In the left-hand photograph above, you can see part of a hardboard disc on the far end of the axle. This corresponds to the flywheel in the earlier diagrams. It has its own points of interest; more about it later.

The crankshaft arrangement shown in those earlier diagrams was not used. Instead, another small hardboard disc with a central hole, and another hole drilled near the circumference to accommodate a suitable pivot made from a piece of an old ballpoint pen, was fitted to the near end of the axle as seen in the left-hand photograph above. (The small blue circle in the centre of the disc is also a part from a ballpoint pen, included to improve the disc's grip on the axle.)

The blue object used for the armature is just the handle of an old toothbrush! I simply cut off and discarded the head, and drilled appropriate holes in the ends of the handle.

(You've probably noticed some other bits'n'pieces in the top-left quarter of the right-hand picture above; don't worry about these for the moment - we'll get back to them soon.)

For the solenoid itself, I used wire which had originally been about half of the primary winding of an old soldering-iron transformer which I had dismantled earlier. (Visit my synchronous wheel page to see what happened to the other half.)

Originally, I wound the wire onto part of the plastic barrel of an old ballpoint pen (probably the same one which had provided the distance-piece mentioned earlier). However, this turned out to be a bad idea. In use, the solenoid became rather hot, and the plastic tube melted! This led me to two conclusions: firstly, I was going to need to make the solenoid from more robust components; and secondly, whatever the solenoid was made from, it was going to get hot during use, and I would be well advised to install some sort of automatic switch-off system. (This second point ultimately led to the use of a thermostat switch - more on this weird saga later!)

Eventually, I used an 8.4cm piece of chrome-plated brass tube which had once been part of a telescopic antenna on a small B&W television. (Again, check out my synchronous wheel page to see what happened to other lengths cut from this old antenna.)

I had originally used a couple of little wooden blocks to support the plastic tube, before it melted. Now, I obtained a piece of 1cm-thick aluminium plate, about 11cm x 5.5cm, and cut it into two approximately square halves. Having drilled a hole in the centre of each to accommodate an end of the tube as a snug fit, I assembled the structure and wound the wire on. I used a couple of pieces cut from "L" cross-section aluminium brace to attach the result to the base. Here is a view from above:

The ends of the winding were connected to two green banana sockets attached to the right-hand support below the coil.

Remember the thermostat I mentioned earlier? Here you see it for the first time, clamped to the coil. (Just ignore it for the moment, okay?)

Also, here you get your first look at the iron core which moves backwards and forwards within the solenoid when the engine is running, and which operates the rotor via the amazing blue toothbrush-handle armature!

The core was made from an odd object I found lying around in my "junk" collection: a large iron nail with a circular flange a short distance from the point. I cut the head off and tidied up the cut end with a file. I found a small piece of plastic shaped like a tube closed at one end (probably yet another part of an old ballpoint pen!), and found that the point of the nail fitted tightly in the open end. There was a small hole in the closed end, just big enough to allow a small bolt to be screwed in, thus tapping its own thread. The hole in the solenoid end of the toothbrush armature was made just big enough to allow this bolt to pass through easily.

Now: more detail on how the LED / LDR combination controls the action.

The simplest thing to do would be to put the LDR in series with the solenoid. It might work; however, the resistance of the LDR is still around 80 ohms when it's illuminated - quite a bit higher than that of the solenoid's coil. This means that much of the available power will go into heating the LDR, quite possibly damaging it - and not going to the coil, where it's wanted. So we'd lose twice.

No: there's a much better way. Instead of using the LDR as a series resistor for the solenoid, I used a 2N3055 NPN power transistor, employing the LDR - and a few other components - to control that. As it turns out, this protects the LDR from high currents, as well as getting plenty of power to the coil. The 2N3055 is a fairly sturdy device, quite adequate for controlling the solenoid's power requirements.

This diagram shows the transistor's pinouts. I've used it in its most usual cofiguration in this circuit: the emitter is connected directly to the "negative rail"; the collector is connected to one end of the coil (the other end of which is connected to the "positive rail"); and the base is connected to reflect what's happening in the LDR.

Because the transistor is being called upon to conduct a fairly heavy current, I've mounted it on an L-shaped bracket cut from the end of a long heavy iron brace, to act as a heatsink. This is probably overkill; but it's better to be safe than sorry...

Here is the transistor mounted on its heatsink, both from in front and from behind:

The base is connected to an old-fashioned 200 ohm resistor (and you know where I found it, don't you? ). The other end of this is connected to the wiper of a 5K linear potentiometer; one end of this is connected to the negative rail and the other end to the LDR. The other lead from the LDR is connected to the positive rail. Thus the LDR and the potentiometer together act as a potential divider, which allows the voltage on the transistor's base to be controlled over a fairly wide range, thus providing a speed control for the engine. (The 200 ohm resistor is probably not strictly necessary, but was included as a current-limiting device just in case.)

In this picture, taken from above the model, you can see the transistor on its heatsink; the 200 ohm resistor (red with a black band, connected between two terminal blocks); and a metal box which contains the 5K potentiometer. Note: the blue knob at the right controls this 5K "pot", which has a long shank. The pot itself is mounted in the left-hand end of the box. (The other knob, on top of the box, controls another potentiometer which will be discussed shortly.)

At this point, all the essential items required to make the basic engine have been described. Here, then, is a circuit diagram for the original version of the project:

(The power switch in the positive rail is on the back of the 5K pot.)

I ran it off my old black power supply with my home-built bridge rectifier, set to the maximum voltage (about 27V). It worked reasonably well. As expected, the coil became quite warm, which ultimately led me to think of a way to include an automatic temperature control using a thermostat. However, another issue began to play at the corners of my mind. Although the overall performance was not too bad, there was room for improvement: the engine seemed to run "a bit rough".

The first thing to do was to build a flywheel. This turned into a little project in its own right. It started out simply as a circular piece of hardboard mounted on the far end of the axle, but it acquired a bit more personality later - more on this shortly.

Even with the flywheel, I felt that it should be possible to make the engine work more smoothly. I placed more black tape on the transparent disc, so that light would get through for a bit less than half the time. This was because the solenoid was obviously staying on for a bit too long, and preventing the rotor from free-wheeling as well as it should for the second half of the cycle. This also helped to some extent; but I still believed that it should be possible to do even better.

I started experimenting with extra resistors between the 5K pot's wiper and the negative rail. This had a marked effect, and ultimately led to the inclusion of a 1K linear pot. For different settings of the speed control (5k pot), this gave a "fine-tune" control. Not a perfect solution - a bit clumsy - but interesting, and not too difficult to implement.

I built the 1K pot into a small metal box, which now houses both pots and a small double-pole slider switch. This was included for flexibility: the 1K pot could thus easily be switched in or out of circuit, as required. I used the second pole of the switch to run a green LED in series with a 560 ohm resistor to act as an indicator, showing at a glance when the 1K pot was in circuit - and also because I just like coloured LED's everywhere!

While on the coloured LED "kick", I included two more LED's - one orange and one red - each with its own 390 ohm series resistor. The orange one glows strongly when the transistor is on (and the solenoid is pulling); the red one glows strongly when it's not. I found that when the fine-tune control was used to give maximum smoothness, these would flash to approximately equal brightness, thus giving a good visual indication of correct setting.

I also included one more LED: a white one, coloured purple with a felt pen, in series with a 680 ohm resistor, to act as a pilot light for the entire unit.

Here is the circuit diagram for the unit with all these features built in:

Now: as promised, a bit about the flywheel.

Ever the light-show freak, I decided to embed a white LED into the flywheel. This necessitated an independent power supply to run it. I installed an AA cell holder on the back of the flywheel, and placed a small 12V battery and a 560 ohm resistor inside it. A small slider switch was installed. When this simple circuit is switched on, and the engine set in motion, a ring of white light appears. (This looks really good in the dark, with all the other LED's glowing at the same time.)

Here are photographs of the flywheel, from in front and behind:

Because of the weight of the battery, the flywheel is not perfectly balanced. I've put this fact to good use by arranging that, when the engine is switched off, it comes to rest with the armature in a good position to re-start it when power is re-applied. (I've found that the engine runs better when set up to rotate anticlockwise, as seen from the front.)

Here is a photograph of the entire model, seen from in front and slightly above:

Here's a 10-second mpeg movie (921Kb). I start the camera; my wife switches on the power supply (left, off-screen) and goes around behind me (you'll notice a shadow effect as she moves through the light from the window); and then she turns up the speed-control potentiometer to maximum. (The other potentiometer, used for "fine-tuning" the engine's performance, is set to a position which is an acceptable compromise for the engine's speed between the minimum and maximum settings for this movie.) The rotor spins anticlockwise, as can be clearly seen for the first few seconds; as it picks up speed, the camera's strobe effect makes it seem to rotate clockwise:

sol-eng1.mpg

Next (also as promised) some comments about the thermostat-controlled cooling system.

I went a bit crazy with this. The basic idea was to use a fan from an old hand-held hair-dryer to blow air over the solenoid and thus cool it down.

Here are two pictures of the fan unit, from in front and above:

Here we go - fasten your seat-belt!

You may have noticed a small U-shaped white wire link joining two terminal-block connections together, near to the power terminals (red and black) on the main unit. These connections are marked A and B on the circuit diagram. If this connection is broken, the yellow LED and the LDR are isolated from the positive rail, so that the transistor is switched off and the solenoid draws virtually no power - thus the engine stops.

You'll notice that there are two switches on the fan unit. The left-hand one (with ON and OFF markings) is an SPST (single-pole single-throw) toggle switch which connects between A and B when the fan unit is installed. This gives an immediate manual control over the engine's operation.

To the immediate right of this switch is a DPDT (double-pole double-throw) relay. One pole's "normally-on" contacts are wired to A and B also, so that if the relay switches on (and the SPST switch is in the "OFF" position), the engine will stop. (If the switch is in the "ON" position, the relay's effect is over-ridden, and the engine will continue to operate.)

The "normally off" contacts of this pole operate the fan. When the relay switches on, it may in turn switch the fan on - depending on the position of another switch just to the right of the relay.

This second switch is a DPDT slider switch with a central OFF position. With the slider in that central OFF position, the relay stays off even if the thermostat triggers. However, with the slider pulled toward you, the relay - and fan - will come on if the thermostat triggers.

If the slider is pushed away from you, the relay and fan switch on whether the thermostat has triggered or not.

All this requires only one of the slider switch's poles. The other pole is used to control a little electronic circuit, which takes up the rest of the space on top of the fan unit.

This circuit is an astable multivibrator, based on a 555 timer IC, which flashes a blue LED (this appears clear when off). With the slider in the central OFF position, the circuit is off and the LED is not illuminated. With it toward you, the multivibrator is switched on and the LED's cathode is connected (vai a 2.2K current-limiting resistor) to the 555's output (pin 3) by means of the relay's second pole "normally closed" contacts. The LED then flashes (about once per second).

With the slider pushed away from you, essentially we introduce a bypass for the thermostat, so that the relay switches on and its second pole's "normally off" contacts connect the LED's cathode directly to supply negative, so that the LED glows continuously. (Recall that the relay's first pole is then operating the fan.)

The effect of all this is that, with the slider towards you, the fan is in "standby" mode with the blue LED flashing to indicate this. When the thermostat triggers (or, equivalently, when the slider is pushed away from you), the fan operates with the blue LED shining brightly as it does so. (You are following all this, aren't you?)

To summarize:

The thermostat switch (clamped to the coil) is normally open, and closes at about 60 degrees C (quite hot!). So, when the coil gets hot, the thermostat triggers, thus switching on the relay. If the slider is toward you (so that the blue LED is flashing) when the thermostat triggers, the fan will operate and the blue LED will glow continuously. When the coil has cooled down, so that the thermostat clicks off and the relay contacts return to their "normally off" condition, the fan stops and the blue LED flashes again. If the slider is away from you, the fan runs and the blue LED glows continuously. The SPST toggle switch determines whether the engine runs or pauses while the fan is operating.

A couple of final points:

As mentioned, the engine requires the highest (or at least the second-highest) voltage setting of my old black power supply . For this reason, it was necessary to include appropriate series resistors for all the LED's (including the blue one), the power connection to the 555, and the fan.

The 3.3K resistor (the big one at extreme left of the 555's circuit board) connected to supply positive takes care of the 555's power requirements; and that resistor and the additional 2.2K resistor (at extreme right) make things okay for the blue LED.

The fan is in series with three 5-watt resistors, each 4.7 ohm (for a total of 14.1 ohms). Since these get quite hot when the fan is running, I've arranged for the fan to blow over them as well as over the coil, thus solving both problems at once.

Pretty neat, huh?

Here's the overall circuit diagram for the cooling system:

I'll tell you what: in the dark, with the cooling system on standby - so that its blue LED is flashing - and all the other LED's (including the flywheel's white LED) doing their respective things, this project looks just great.

Now, two pictures of the engine with all its components installed, one from in front...

... and one from behind:

Looks a bit like the old paddle steamer from "Showboat", doesn't it?!

So, what's it all for?

Well, if nothing else, this project shows how an essentially simple idea can grow into something far more complicated than it has any right to be! But it's all OK if it's interesting, fun, and part of a learning process.

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