PWM solenoid use

[hevans1944]

Hop, I have looked for the definition for the VDS, the acronym has ??? different explanations. could you please give me the electronic definition for Vds?

Vds is the Voltage applied between the drain and source terminals of the MOSFET. It is specified on the device datasheet as the maximum voltage that can be applied when the MOSFET is off. I have attached an application note from a semiconductor manufacturer that will help you read a MOSFET datasheet.

Vds is always applied to the MOSFET through a current-limiting impedance, a solenoid coil for example, because Rds(on) is quite small (a few milli-ohms). Without a series impedance to limit the on current, the MOSFET would self-destruct when turned on. To avoid device failure, Vds applied to the MOSFET when in the off state must always be less than the maximum Vds rating from the device datasheet. That is why a "flyback" diode across an inductive load is used. Without the diode, a sudden change in the MOSFET conduction from fully on to off (which occurs many times per second in a PWM driver) will result is a large "back EMF" or "flyback" voltage across the coil, which voltage adds to the initial Vds provided by the power supply. This "spike" in Vds can damage the MOSFET if it isn't suppressed or reduced to a level the MOSFET can tolerate. I generally allow a "safety margin" of at least a factor of two when selecting the maximum Vds. So, for a 12 to 14 V DC automotive power source, select a MOSFET with a maximum Vds rating of at least 28 V. A higher Vds rating generally does no harm, other than increase the price perhaps, and it does provide an extra margin of safety. The diode, or a diode in series with a small-value resistor, placed in parallel with an inductive load is essential no matter what MOSFET you use.

To answer an earlier question you posted, the diode is identified by part number: 1N4007 is a general-purpose rectifier diode rated to hold off 1000 V in the reverse direction and conduct 1 A current in the forward direction. Any diode in the series 1N4001 through 1N4007 will generally work in automotive applications. They differ only in their rated voltage specification. ALL diodes from this series may actually come from the same manufacturing process. Either the vendor or the manufacturer would test and "bin" individual diodes from a particular lot and sell them as 1N4001, 1N4002, ... 1N4007 depending on what reverse breakdown voltage the diode exhibited. Since the price difference is either zero or minuscule, it is probably better to use the 1N4007 diode for everything.

 

[hevans1944]

In an earlier post I mentioned there was a simple way to obtain a PWM signal directly from the output of a 555 timer. Here is the circuit schematic:

The above image was copied from here.

The two diodes labeled "DUG" are unspecified germanium diodes in the original design, perhaps used there because germanium diodes have a lower forward voltage drop (at any given current) than a silicon diode. Or maybe the designer happened to have a boat-load on hand. Germanium diodes may be difficult to find. I would try this circuit with ordinary silicon diodes, such as 1N4148 small-signal diodes, or a power silicon diode such as 1N4007 (or any diode from the 1N400x series) to see if it works. If not, perhaps two 1N5911 Schottky diodes will work.

I would also experiment with using R1 to vary the frequency, perhaps placing a 10 KΩ trim-pot in series with R1 to provide an adjustment range of 1 kΩ to 11 kΩ. The value of R2 also affects the frequency, because one section (between the wiper contact and an end contact) is in the charge path and the other section is in the discharge path of C1. Just about any value from 1 kΩ up will probably work for R2, the lower values producing a higher frequency. Again, some experimentation on a breadboard will quickly let you know if you are in the right ball park. It is necessary to have an inexpensive oscilloscope or a frequency counter to set the operating frequency, unless you happen to have "perfect pitch" and can use a cheap ear-bud to listen for 262.5 Hz from the output of the 555, capacitive-coupled to the ear-bud through a 0.1 μF capacitor in series with a 1 kΩ potentiometer to ground. Connect ear-bud between potentiometer wiper and ground. This is to adjust volume level and prevent "blowing up" the ear-bud.

The value of C1 sets the frequency for whatever values you pick for R1 and R2. C1 can be an electrolytic capacitor, positive terminal connected to wiper arm of R2. C2 is a "noise bypass" capacitor and its value is not critical. It should be a small ceramic disk or plastic dielectric capacitor, mounted as close as practical to the pin to which it is connected and "ground.".

I don't know why I am reluctant to fully recommend this circuit. Perhaps it is too simple...

... When I see a diagram (see below) that has 1n4001 diode being used on the PWM solenoid wires High and low terminals (C high & D low). I understand that it is a recirculation diode or fly back diode to keep the circuit from being damaged by spiking the electronics.

· But is the 1n4001 a part # or a code for a certain type or kind of diode? ...

It is the type or part number of the diode. See my previous post #21 above.

... What is the 4.7 ohm, 5 watt resister for on the low side? ...

It is there to limit the current applied to the solenoid.

... Is that the specs for the solenoid itself? ...

No. The solenoid probably has a much lower resistance. You should measure the solenoid resistance with an ohmmeter on its lowest resistance range, preferable at the solenoid terminals after disconnecting the terminals from the wiring harness. Short the ohmmeter terminals together and note the resistance of the test leads if the meter does not read zero. Measure the solenoid resistance and subtract the test-lead resistance noted earlier.

... The reason I was looking at the Arduino boards is that I can make some simple coding for up and down shifts via 2 push buttons instead of two toggle switches. ...

So, one push button to shift up and the other push button to shift down? Perhaps four LEDs to indicate which gear you are in? Very doable with a simple Arduino sketch.

... Because you guys understand the electronics part of it better than I. What I was hoping for a basic guidance of individual components needed to accomplish this task. If I have not given adequate information please forgive me.

It is really hard to be specific about components, especially power components, without being there to make electrical measurements on an operating transmission at operating temperature. I wouldn't want to suggest a particular power MOSFET, for example, unless I was absolutely sure it wouldn't burn out during normal use... whatever "normal use" happens to be! As the end-user and builder of the test rig, you have to accept responsibility for its design. Sure, Adam or I or anyone else here can suggest circuits... but without real solenoids operating in a real transmission, under load and delivering power to a dynamometer, who knows what will happen? Well, you will know!

It may appear that some of us here <grin> complicate things unnecessarily. All that means is we are trying to cover all the bases from afar since none of can be there to lead you to victory. This is a hobby forum, among other things, so you should be prepared to get on the hobby horse and ride it. That may mean learning "stuff" you wouldn't need to know if you were to, say, purchase the thing you want to build as a kit of parts, complete with Heathkit-quality instruction manual. If @KrisBlueNZ were still alive and posting here, he may have been able to get you close to that goal. But so far no one has stepped up to take his place. So, go for the ride... and if you fall off, buy some fresh components and get back on again. Repeat until your project is complete and working to your satisfaction.

WARNING! If the electronics hobby bug bites you and you get infected, your project will NEVER be done. There will always be another "feature" to add at some later date.

 

[Randy Bligh]

Hop, thank you for the schematic I am going to research both yours and Adam's and see what I come up with.

So, one push button to shift up and the other push button to shift down? Perhaps four LEDs to indicate which gear you are in? Very doable with a simple Arduino sketch.

Yes on the 1 push button for up-shifts and 1 push button for downshifts and a digital read out for each gear selection. I saw a you tube video for a push button shifter that had what looked like a 10mm x 10mm read out for his gear selection. he had used an Arduino board for this. it had sparked my interest. so you are right about

WARNING! If the electronics hobby bug bites you and you get infected, your project will NEVER be done. There will always be another "feature" to add at some later date.

I thought if I have to have a circuit board to control the PC solenoid why not use it to change from the toggle switches to push buttons...
that's what I get for thinkin'
I do have one more thing to ask...
this is a quote from my service manual on the 4L60E could some one please make sure that i understand this correctly. I would like to control the PC solenoid like the factory does... if it really matters how its controlled at all.

"The PCM controls the PC solenoid valve on a positive duty cycle at a fixed frequency of 292.5 Hz (cycles per second). A
higher duty cycle provides a greater current flow through the solenoid. The high (positive) side of the PC solenoid valve
electrical circuit at the PCM controls the PC solenoid valve operation. The PCM provides a ground path for the circuit,
monitors average current and continuously varies the PC solenoid valve duty cycle to maintain the correct average current flowing
through the PC solenoid valve."

the PCM is controlling this solenoid as a high side drive with a low side sense. (maybe I have this backwards)
so if the high side is (this is where my ignorance shows) controlling the valve operation how is the ground side varying the duty cycle?

 

[Arouse1973]

In an earlier post I mentioned there was a simple way to obtain a PWM signal directly from the output of a 555 timer. Here is the circuit schematic:

The above image was copied from here.

The two diodes labeled "DUG" are unspecified germanium diodes in the original design, perhaps used there because germanium diodes have a lower forward voltage drop (at any given current) than a silicon diode. Or maybe the designer happened to have a boat-load on hand. Germanium diodes may be difficult to find. I would try this circuit with ordinary silicon diodes, such as 1N4148 small-signal diodes, or a power silicon diode such as 1N4007 (or any diode from the 1N400x series) to see if it works. If not, perhaps two 1N5911 Schottky diodes will work.

I would also experiment with using R1 to vary the frequency, perhaps placing a 10 KΩ trim-pot in series with R1 to provide an adjustment range of 1 kΩ to 11 kΩ. The value of R2 also affects the frequency, because one section (between the wiper contact and an end contact) is in the charge path and the other section is in the discharge path of C1. Just about any value from 1 kΩ up will probably work for R2, the lower values producing a higher frequency. Again, some experimentation on a breadboard will quickly let you know if you are in the right ball park. It is necessary to have an inexpensive oscilloscope or a frequency counter to set the operating frequency, unless you happen to have "perfect pitch" and can use a cheap ear-bud to listen for 262.5 Hz from the output of the 555, capacitive-coupled to the ear-bud through a 0.1 μF capacitor in series with a 1 kΩ potentiometer to ground. Connect ear-bud between potentiometer wiper and ground. This is to adjust volume level and prevent "blowing up" the ear-bud.

The value of C1 sets the frequency for whatever values you pick for R1 and R2. C1 can be an electrolytic capacitor, positive terminal connected to wiper arm of R2. C2 is a "noise bypass" capacitor and its value is not critical. It should be a small ceramic disk or plastic dielectric capacitor, mounted as close as practical to the pin to which it is connected and "ground.".

I don't know why I am reluctant to fully recommend this circuit. Perhaps it is too simple...


It is the type or part number of the diode. See my previous post #21 above.

It is there to limit the current applied to the solenoid.


No. The solenoid probably has a much lower resistance. You should measure the solenoid resistance with an ohmmeter on its lowest resistance range, preferable at the solenoid terminals after disconnecting the terminals from the wiring harness. Short the ohmmeter terminals together and note the resistance of the test leads if the meter does not read zero. Measure the solenoid resistance and subtract the test-lead resistance noted earlier.


So, one push button to shift up and the other push button to shift down? Perhaps four LEDs to indicate which gear you are in? Very doable with a simple Arduino sketch.



It is really hard to be specific about components, especially power components, without being there to make electrical measurements on an operating transmission at operating temperature. I wouldn't want to suggest a particular power MOSFET, for example, unless I was absolutely sure it wouldn't burn out during normal use... whatever "normal use" happens to be! As the end-user and builder of the test rig, you have to accept responsibility for its design. Sure, Adam or I or anyone else here can suggest circuits... but without real solenoids operating in a real transmission, under load and delivering power to a dynamometer, who knows what will happen? Well, you will know!

It may appear that some of us here <grin> complicate things unnecessarily. All that means is we are trying to cover all the bases from afar since none of can be there to lead you to victory. This is a hobby forum, among other things, so you should be prepared to get on the hobby horse and ride it. That may mean learning "stuff" you wouldn't need to know if you were to, say, purchase the thing you want to build as a kit of parts, complete with Heathkit-quality instruction manual. If @KrisBlueNZ were still alive and posting here, he may have been able to get you close to that goal. But so far no one has stepped up to take his place. So, go for the ride... and if you fall off, buy some fresh components and get back on again. Repeat until your project is complete and working to your satisfaction.

WARNING! If the electronics hobby bug bites you and you get infected, your project will NEVER be done. There will always be another "feature" to add at some later date.

You said you wanted to explore both options, and now your saying it's too complex? And yes if Kris was still with us he would have put together something no doubt. But Kris had the time because he didn't work and for some that was a blessing because he could give them exactly what they needed. It's not about who is better than who, the fact that we are all having a go is a good thing. Their aren't many of us here that actually produce designs for others, I don't know why but I find that part the most fun. Knowing someone has used part of your design is most rewarding.

I on the other hand like to get people started and let them go off and do a bit of research themselves, I find that to be a nice way to learn. I don't use 555 timers so I can't comment much until I have researched the circuit but initially I can't see how the frequency can be fixed at 292.5 Hz and the duty cycle be adjusted also without changing the frequency. I need to look at this in further detail.

Thanks
Adam

 

[hevans1944]

You said you wanted to explore both options, and now your saying it's too complex? And yes if Kris was still with us he would have put together something no doubt. But Kris had the time because he didn't work and for some that was a blessing because he could give them exactly what they needed. It's not about who is better than who, the fact that we are all having a go is a good thing. Their aren't many of us here that actually produce designs for others, I don't know why but I find that part the most fun. Knowing someone has used part of your design is most rewarding.

I on the other hand like to get people started and let them go off and do a bit of research themselves, I find that to be a nice way to learn. I don't use 555 timers so I can't comment much until I have researched the circuit but initially I can't see how the frequency can be fixed at 292.5 Hz and the duty cycle be adjusted also without changing the frequency. I need to look at this in further detail.

Thanks
Adam

Adam, I was referring to my original suggestion as being too complex... I didn't actually provide a schematic like you did, just a verbose description of how to use a sawtooth waveform, a comparator, and a flip-flop to generate a PWM signal. I am sure your circuit will work too. There is always more than one solution to any electronics problem, so I threw out the idea from the guy in India for consideration. The only problem I have with it is it appears too simple. I tell myself, "There must be a catch somewhere!" As for how it can vary duty cycle without also changing the frequency... the 555 charges the timing capacitor through one path and discharges it through another path. The sum of the RC time constants for both paths is constant... one gets longer as the other gets shorter when the pot wiper arm is moved... so the frequency doesn't change over a wide range of duty cycles. The main limitation is the circuit doesn't go from 0% duty cycle to 100% duty cycle. There are other circuits that will do that, but it isn't necessary for this application.

You really should investigate the 555... the most popular and widely used analog integrated circuit ever to be invented. Now available in low-power CMOS versions.

There is a fourth option (besides the two I mentioned and your circuit): use an Arduino to generate PWM signals. But Randy did say to use the KISS principle, and he has zero experience programming an Arduino, so I didn't bring it up.

 

[Randy Bligh]

Gentleman

Right now all I am doing is research, no development. My thoughts on this are all random and changing as I learn. Somethings seem too difficult but when explained differently they can sound very easy. Easy is more doable for a novice like me. If either of you & I were to trade a few days’ work. I believe we would both come back to a disaster.

Electrical diagrams are similar to hydraulic diagrams easy to read and I understand 95% of it. Almost everything I look at on an electronic diagram of any kind is foreign to me. I have to have a dictionary of terms out to find what each individual component is. And then it is still like “Okay, whatever that means.” It would probably be the same for you if I gave you a hydraulic diagram or a pile of automatic transmission parts.

Adam, I agree that it is not about who is better than who. 2 diagrams from 2 different experts, to me is a humbling experience. And to be able to communicate with one that lives in the UK. Well I find that awesome in itself. I am glad that that you find the design part fun. Designing anything has always been a shortcoming of mine. Fixing things and improving the design flaws is what I have spent my life doing. But even the design flaws I come across someone else designed the fix, not me…

The Arduino approach could still be the easiest to do. After all I do have the computer friend that wants me to go that way. he would like to program it. He just wants to know or understand the requirements for the solenoid control so we don't let the smoke out of it from the get go.

Enough rambling thank you both for your expertise, I have a lot of learning and gathering to do. A parts list is next. You all did give me an idea of looking at a test vehicle and how it measures up. I bought an extension cable for testing one as well. now to find a donor vehicle to test. I'll keep in touch as I develop this out. I'm on vacation and it is only one more day then it's back to the grind stone.

Randy

 

[Arouse1973]

That's cool Randy.

"If either of you & I were to trade a few days’ work. I believe we would both come back to a disaster"

I am a qualified mechanic also, but to be honest I have not done much on auto boxes in a very long time

Just keep us posted with how it's going, if you need any help with the Arduino stuff just shout.

Adam

 

[hevans1944]

...I do have one more thing to ask...
this is a quote from my service manual on the 4L60E could some one please make sure that i understand this correctly. I would like to control the PC solenoid like the factory does... if it really matters how its controlled at all.

"The PCM controls the PC solenoid valve on a positive duty cycle at a fixed frequency of 292.5 Hz (cycles per second). A
higher duty cycle provides a greater current flow through the solenoid. The high (positive) side of the PC solenoid valve
electrical circuit at the PCM controls the PC solenoid valve operation. The PCM provides a ground path for the circuit,
monitors average current and continuously varies the PC solenoid valve duty cycle to maintain the correct average current flowing
through the PC solenoid valve."

the PCM is controlling this solenoid as a high side drive with a low side sense. (maybe I have this backwards)
so if the high side is (this is where my ignorance shows) controlling the valve operation how is the ground side varying the duty cycle?

You posted a schematic showing the wiring interface between the transmission and ...something. I'm not sure if the "something" is the ECU or another "black box" transmission controller like the SuperShifter box you also posted. In any case, the diagram shows one side of the solenoid valve (terminal C) connected to the +12 V DC auto power through a 2 A fuse. The other side of the solenoid valve (terminal D) is connected, through a 4.7 ohm, 5 watt, resistor to pin 33 of a 35-pin connector. There is also a 1N4001 "fly-back" diode connected across the two solenoid terminals. Clearly, from this wiring configuration, a low-side solenoid drive is implied since the only control line to the solenoid is in the low side. There are many ways to monitor average current in the low-side drive, perhaps the most common being a low-value resistor in series with the ground return. The voltage drop across this resistor is proportional to the current in the solenoid and MOSFET, all three being in series, along with the 4.7 ohm power resistor which is also in series with the solenoid.

Will you be making your connections directly to the transmission connector (the 12-pin circular connector) or to the 35-pin rectangular connector?

If you decide to use the Arduino, here is a link to a power MOSFET that plays well with Arduino and will easily drive your solenoid. I have also attached its datasheet.

 

[Fish4Fun]

On an Arduino.....Connect something like this to one of the ADC inputs....



..... Obviously Set up//Enable the ADC in 10-Bit Free-Running Mode and a 16 bit timer with OCCRxA and OCCRxB IRQs Enabled.....Divider = 1


Assuming:
.....Clock = 16Mhz ==> 292.5Hz = 57400 MCU Clocks

.....5% = 171uS ==> 2736 MCU Clocks
.... 40% = 21.888mS ==> 21888 MCU Clocks

.....the Duty Cycle = 5% Low to 40% Low.....Otherwise Switch where "Set I/O Pin = High/Low" are located....

and in **Pseudo Code** for brevity//clarity

Set OCRxB = 54700 ;

IRQ_OCRxA:
Set I/O Pin = High
reti

IRQ_OCRxB:
Set I/O Pin = Low
ReadADC ; V = 0 to 1023
Set OCRxA = 2736 + (V * 16) + (V * 2) + (V/2) + (V/4) ; For V = 0 --> OCRxA = 2736 and for V = 1023 --> OCRxA = 21916
Set TCNTx = 0
reti

Same result can be achieved using a "PWM Mode" with no IRQs, but then there really isn't any code to show at all, lol, and one would have to set up some type of idle/delay loop to poll the ADC Anyway, I am not sure this would prove the easiest path for the OP, but it is certainly the easiest path from my perspective.....The "odd" formula for "scaling" the ADC value wrt the OCRxA value could easily be replaced with a math library and simply multiplying the ADC output by (21888 - 2736)/1023 ===> 18.7124 (and, of course, adding back in the 2736) but the example approximation is "easier/faster" in assembler, lol.

Good Luck!

Fish

 

[Randy Bligh]

You posted a schematic showing the wiring interface between the transmission and ...something. I'm not sure if the "something" is the ECU or another "black box" transmission controller like the SuperShifter box you also posted.

Hop, the 35 pin connector is an ECU for the complete set-up of a paddle shifter from Mega-Shift set up for street rods. I am using it for reference. I will be concerned with only the round pin connector. pin E is a common power for the shift solenoids and the TCC (Torque Converter Clutch) Solenoid. The ground side of the solenoids are Pins A,B & T. PWM PC solenoid pins C positive and D negative

 

[Randy Bligh]

Fish, thanks for the input as well. Now I have really have a ton of learning to do
Randy

 

[hevans1944]

Hop, the 35 pin connector is an ECU for the complete set-up of a paddle shifter from Mega-Shift set up for street rods. I am using it for reference. I will be concerned with only the round pin connector. pin E is a common power for the shift solenoids and the TCC (Torque Converter Clutch) Solenoid. The ground side of the solenoids are Pins A,B & T. PWM PC solenoid pins C positive and D negative

So the 1N4001 diode and the 4.7 Ω resistor would have to added to whatever circuitry you attached to the connector. I assume you have an extender cable to go between the transmission connector and your purpose-built box. Do you have a connectors on both ends of this cable? Or will you just hardwire one end into your box and connect the other end to the transmission connector? Will you derive the +12V DC and ground from a pair of battery clamps attached to wires connected to your box? There should be either fuses or circuit breakers (if you can afford them) in series with the main power line (at least) and the solenoid loads. I think that fuse blocks for automotive fuses (the little plastic color-coded ones with handles) are inexpensively available.

It's irrelevant to this project, but is the TCC solenoid used like a foot-clutch on a manual transmission? Is there a particular sequence for releasing the TCC, applying the clutch bands to get the planetary gears somewhere close to the right speed, and then engaging the TCC to provide power to the gear train after changing gears? It all seems pretty magical from my viewpoint and I am curious about what is done to implement a rapid "performance shift" in an "automatic" tranny. Are these used for racing? I guess I should visit our local Kil-Kare Raceway and talk to some mechanics... they just re-opened this year under new ownership who has promised a more "family-like" experience.

 

[Randy Bligh]

So the 1N4001 diode and the 4.7 Ω resistor would have to added to whatever circuitry you attached to the connector. I assume you have an extender cable to go between the transmission connector and your purpose-built box. Do you have a connectors on both ends of this cable? Or will you just hardwire one end into your box and connect the other end to the transmission connector? Will you derive the +12V DC and ground from a pair of battery clamps attached to wires connected to your box? There should be either fuses or circuit breakers (if you can afford them) in series with the main power line (at least) and the solenoid loads. I think that fuse blocks for automotive fuses (the little plastic color-coded ones with handles) are inexpensively available.

I have the 1N4001 diode and the 4.7 ohm 5 watt resister on my parts list. Yes I purchased a GM test connecter I am going to scavenge into a 12 FT lead so I can drive the test vehicle. I am going to connect it into a male10 pin connector that will finger screw to the female pass through connecter. All wiring inside the box will get attached to the pass through connector. The 12V supply for the box will come from a separate 5FT two wire accessory plug with a 5 amp fuse. My son has scavenged a lot of connectors from his RC car days. all will get a very clean professional setup.

The torque converter is a torque multiplier. The TCC solenoid locks the 2 halves of the TC together much like a manual clutch does. It cannot be engaged at idle or the engine will die just like if you let the clutch out to fast on a manual transmission. the TCC engagement usually happens above 40 MPH and is computer controlled, most times it is applied in any gear range depending on load. the brake switch will unlock the TCC. sometimes newer vehicles unlock the TCC slightly (allowing it to slip) to make the gear change softer all while staying engaged. the GM vehicles have a PWM solenoid the pulses the TCC system as a feed/bleed hydraulic circuit to give a softer apply of the TCC. in the cylinder deactivation systems it allows for a certain slip. that way the misfire is not detected. because a transmission is more hydraulics, shift feel is calibrated through orificed feed holes, accumulators and servos. the racing transmissions are modified in these areas to allow a harder or firmer shift feel..
Randy

 

[Fish4Fun]

Hey Randy!

No worries.....there is absolutely NOTHING wrong with an analog circuit using a 555, Inverters, comparators, flip-flops or even discrete components....literally hundreds of ways to go, but .....if you plan on adding an Arduino board later on for other functions, then I think you should consider starting there rather than 'inventing' two completely different wheels.....of course if you have (or are beginning to have) an interest in electronics as a hobby then I would encourage you to prototype several analog solutions before reducing the task to a few lines of code....... Everything discussed so far will work; but, fair warning: building/trouble-shooting/tweaking is a time-sucking vortex....regardless of which path you choose.....perfect for a hobby....rarely a highlight in a business plan.....SO, I hope you are having fun!

Good Luck!

Fish

 

[hevans1944]

I recently happened upon some free samples from Linear Technology that I forgot I had. These are Universal Timing Devices from LT's TimerBlox product line. They have a website that walks you through the application design procedure. This may be the easiest solution yet to generating a constant frequency, variable duty cycle, PWM signal. It's worth a look IMO: one resistor to set frequency and a potentiometer to apply the PWM control signal.

Although it may appear on the outside to be an analog solution, inside it is (mostly) not. You program the duty cycle with a variable DC voltage and set the frequency with a resistor. The appropriate part number is LTC6992-2 (5% to 95% duty cycle). The only down-side is they are available only in very tiny packages. The sample I have is a 6-lead plastic TSOT-23 which is almost invisible to the naked eye. The other package available is worse, a 2mm x 3mm DCB (whatever that means) apparently designed for re-flow soldering to a circuit board. I have attached a datasheet in case you are interested.

This will require a professional board layout and manufacture for best results, although others here have had good results with DIY projects that use the SOT-23 form factor, in particular @Arouse1973 and @TenderTendon (the latter using a CNC milling machine). You could also purchase DIP (dual in-line plastic) headers that convert a 6-pin SOT-23 package to an 8-pin DIP package, suitable for incorporation on the prototyping section of (for example) an Arduino Uno board. I would do this if you intend to use an Arduino for push button switch control and 7-segment LED display of the gear selection. Gee! This is beginning to sound like a real project instead of blue sky speculation.