See the attached schematic.

SW1 is actually your reed switch; it's normally open and closes for a brief moment when a magnet on a wheel passes by. I used a relay and signal source so that I could simulate it.

R2 is where you set the speed limit. Once you find the correct setting, you should really remove the pot, measure it, and replace it with a fixed resistor, as pots are not as reliable as fixed resistors.

U1 has a time-out of about 95mS when R2 is adjusted properly. If triggers from the reed switch occur too often, U1 does not time out, and it's output never goes low. This causes U2 to time out a short time later, and its' output goes low, energizing the relay.

As shown, the output to "points" would be connected to the high (coil) side of the points, but that might result in the coil getting overheated. I don't know what arrangement you have for your ignition, but a relay could be wired up in whatever arrangement that's appropriate for your ignition - perhaps to disconnect power from it.

[eta]
If your scooter is fuel injected, it would be preferable to interrupt the injector circuit. Otherwise, when the ignition system is disabled, you will be dumping a raw air/fuel mixture into the exhaust system that is extremely flammable, and will likely ignite with a loud "BANG!" when the ignition is again enabled.

I can't make this circuit work in simulator or real life. Can you check schematic? First IC trigger will not work and second IC trigger is not connected at all?



Mod:Created new thread, link to old Thread.
https://forum.allaboutcircuits.com/threads/speedlimiter-circuit-using-lm2917.45554/post-297791

 

I'm not surprised it won't work the first ic has a trigger and will keep resetting making pin 3 high all the time. The second IC has no trigger input.

 

Gee, I'm sure coming back into this cold, from 12 years ago. I don't have LTSpice installed on my current computer, nor any of the custom models I used to have installed; and I'd spent a lot of time acquiring. Perhaps I'll take a look at it in a few days; I'm currently installing a new service panel in my house and troubleshooting a leak in the pool, as well as fixing up my Hayward Navigator pool vacuum monster. No rest for the wicked...

Have a safe Labor Day Weekend, everyone!

 

I'm not surprised it won't work the first ic has a trigger and will keep resetting making pin 3 high all the time. The second IC has no trigger input.

That's because its using the 555s in an unconventional way. Doesn't mean it won't work....

As I see it:

The initial trigger contact SW1 pulls U1:Trig low triggering the monostable but also shorts C1 to ground via Q1, which holds U1:Out high as long as SW1 is closed. If the trigger rate is low, when the trigger contact opens eventually U1 times out and U1:Out goes low. Conversely, if the trigger off-times is below a certain limit U1 never times out and its output is always high.. If U1:Out is high Q2 & Q3 are off, allowing C5 to charge and once it reaches the threshold U2:Out goes low, pulling in RLY1 and shorting the points killing the spark.

Putting it simply: while the revs are low, U1:Out goes low regularly, shorting C5 thus holding U2:Out high and RLY1 de-energised. When they hit the limit, U1:Out remains high, U2:Out goes low after the 0.7R4C5 delay and kills the ignition thus limiting the revs.

U2 is simply used as a delay to stop the revs 'hunting' around the set point. Its unconventional, and U2:Trig should really be held high to prevent any noise issues, rather than left NC.

Certainly its using 555s in a creative and unusual way. U1 is validating only the length of the OFF portion of SW1s cycle, not the trigger rate as such.

 

That's because its using the 555s in an unconventional way. Doesn't mean it won't work....

As I see it:

The initial trigger contact SW1 pulls U1:Trig low triggering the monostable but also shorts C1 to ground via Q1, which holds U1:Out high as long as SW1 is closed. If the trigger rate is low, when the trigger contact opens eventually U1 times out and U1:Out goes low. Conversely, if the trigger off-times is below a certain limit U1 never times out and its output is always high.. If U1:Out is high Q2 & Q3 are off, allowing C5 to charge and once it reaches the threshold U2:Out goes low, pulling in RLY1 and shorting the points killing the spark.

Putting it simply: while the revs are low, U1:Out goes low regularly, shorting C5 thus holding U2:Out high and RLY1 de-energised. When they hit the limit, U1:Out remains high, U2:Out goes low after the 0.7R4C5 delay and kills the ignition thus limiting the revs.

U2 is simply used as a delay to stop the revs 'hunting' around the set point. Its unconventional, and U2:Trig should really be held high to prevent any noise issues, rather than left NC.

Certainly its using 555s in a creative and unusual way. U1 is validating only the length of the OFF portion of SW1s cycle, not the trigger rate as such.

Hello,

Thanks for reply! I got this working. I had short between Q3 B-C.

Now I have a problem with SW1. I have bench setup, where a small motor turns plastic wheel with a magnet attached and reed switch mounted near of that.

with high rpm, circuit works and U2 output is low (picture 1)
with midle rpm circuit also works and U2 output is high (picture 2)
but with low rpm circuit doesn't work right and U2 output is low (picture 3)

ch1 = C1
ch2 = C5

with lower rpm there is also strange double waveform at C1 but it doesn't show at C5 (picture 3).

If I connect U2 trigger to VCC delay will not work anymore.

 

Can you show C1 and U1:Trig together? I suspect I know the problem...

 

Can you show C1 and U1:Trig together? I suspect I know the problem...

I tried, but when I connect probe to U1 Trig C1 starts looks same signal as U1 Trig (see picture 4)

I also tested input signal off this circuit. I just connect 1k resistor series with reed switch @ +5vcc and it looks like this (see picture 5)

My scope probes are set to x10.

 

This is not a terribly difficult Circuit to design,
but shorting the Points in an Engine can be damaging to the Engine,
depending on exactly what type of Engine this is, and various other circumstances.

More information regarding the purpose, and details, of this project is needed.
.
.
.

 

This is not a terribly difficult Circuit to design,
but shorting the Points in an Engine can be damaging to the Engine,
depending on exactly what type of Engine this is, and various other circumstances.

More information regarding the purpose, and details, of this project is needed.
.
.
.

This is multipurpose speed limiter. I'm going to use with electric scooter where relay switching motor contoroller internal speed limiter to on / off, so it does not harm electric motor at all.

Later my boy is going to buy moped, so then we going to move this circuit for that. Then relay is going to cut ignition, so there is no spark and then speed going down. Moped has only carburator and it doesn't have cat, so no harm there also.

Can you show C1 and U1:Trig together? I suspect I know the problem...

Now I moved scope ground to this circuit +VCC so we get inverted signal and it looks like this (picture 6)

With no trigger signal it looks like this (picture 7)

 

If this is a scooter engine and the need is to limit RPM so as to limit the max speed, then shorting the points is how it is done. This is not an engine in a race car with a battery and coil ignition system. And even on real car engines, with a ballast resistor, coil shorting was done without damage, usually.
But getting a reliable trigger on a scooter engine can be a real challenge, and at even just 1000RPM a reed switch can wear out in a few hours. But to pass an inspection that can be adequate.

 

But getting a reliable trigger on a scooter engine can be a real challenge, and at even just 1000RPM a reed switch can wear out in a few hours. But to pass an inspection that can be adequate.

Reed switch gets signal from wheel and it's like 10Hz, so it doesn't wear out so fast. I also have a HAL sensor, but I don't know how to modifie this circuit to accept that signal.

 

I focused to U1 and monostable. There is no input signal for trigger but monostable is still active (look picture 8)

CH1 = C1
CH2 = U1 OUT

 

"" This is multipurpose speed limiter. I'm going to use with electric scooter where relay switching motor controller internal speed limiter to on / off, so it does not harm electric motor at all.

Later my boy is going to buy moped, so then we going to move this circuit for that. Then relay is going to cut ignition, so there is no spark and then speed going down. Moped has only carburetor and it doesn't have cat, so no harm there also. ""

Using a Relay is not a good method for controlling Speed on either one of these vehicles.

Also, how is your Boy going to learn how to crash ?
Crashing is a very valuable Teacher.

And, your Boy is going to buy a Moped,
I assume with his own Money,
and then You are going to cripple it.

If I understand this correctly,
You won't get any help from me.
.
.
.

 

A while back we had a thread from a person in another part of the world where those in a position to enforce their ideas had decreed that all scooter type vehicles had to be speed limited, because they decreed it so. These folks may possibly live in that same territory.
So it may simply be an effort to obey the edict of the rulers without limiting performance. Not all people are as free as some of us would choose to be, and as some actually are.

 

This is off topic, but I still reply.

I'm living in Finland, and our laws says, that electric scooter can be max 1kw and speed 25km/h. Restrictions to moped is 49cc and max speed 45km/h. Moped doesn't have a power restrictions.

Now this electric scooter has brushed 1kw motor and controller only has limitation settings for 33/66/100% duty cycle. So it limits also Power, not speed or rpm. I want it to work at 100% duty cycle for max power and when speed is 25km/h then this circuit kick in and switch duty cycle to 66%.

 

Today I'm study how 555 monostable multivibrator works. I figure out, that trigger needs to go below 1/3 of vcc to switch output LOW -> HIGH.

Why this circuit doesn't have pull up resistor for trigger? Where the voltage come to trigger?

In this picture trigger signal voltage is about 3v so with 12vcc it must be 4v or less to switch output to LOW -> HIGH.





Because it stays below 4v all the time, output is HIGH all the time?!

 

If you have access to the motor shaft you could arrange a magnetic pickup for the trigger pulse. If not, then a photo pickup inside the motor. One or two spots of white paint, the rest flat black paint. Then use a white LED and a photo transistor to sense the reflection from the white patches. That could be inside the motor away from any weather. OR, if there are any gears, a magnetic pickup sensing the gear teeth. So there are ways to get speed pulses, but not being at all aware of how your vehicle is built all I can give is these general suggestions.

 

Now I tried with lm2907n chip and this circuit:



Still not work =(

Here is magnetic pick-up signal for input. Relay will not energized.

 

For the circuit using the 8 pin IC, measure and compare the voltages on pin #7 and pin #3. Also, try removing that 500 K ohm resistor, R3, connected between pins 5 and 7. IIt is there to provide a latching function.
The relay should operate when the voltage on pin #3 iis higher than the voltage on pin #7

 

For the circuit using the 8 pin IC, measure and compare the voltages on pin #7 and pin #3. Also, try removing that 500 K ohm resistor, R3, connected between pins 5 and 7. IIt is there to provide a latching function.
The relay should operate when the voltage on pin #3 iis higher than the voltage on pin #7

Pin #7 has 1/2 of vcc about 6v
Pin #3 has no voltage at all
Removing 500k resistor will not help.

I also tested with signal generator (50Hz, 50% duty cycle, +/- 10v) nothing happens with that also =(