Need Help for the working principle of this speed monitor

Thread Starter

agroelectroman

Joined Feb 11, 2020
9
Hello to all members,Nice work for the forum,
My name is Stratos and I'm from Greece
I have a speed monitor for my harvester machine which consists of reed switch sensor to each axle and a speed monitor with ten warning lights
Can you explain the principle of work
The harvester machine is 1990 model  8a1c8b6f-b3d6-4e3c-a889-c1ba2275afbb.jpeg
 

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MaxHeadRoom

Joined Jul 18, 2013
20,057
Job to say from the Pic, but as it has only two conductors, some kind of contact sensor for the two apparent contacts, more is need on the sensor or a better view!
I doesnt look like a reed switch, but if it is then there must be a small magnet on the part being sensed.
Max.
 

Thread Starter

agroelectroman

Joined Feb 11, 2020
9
Job to say from the Pic, but as it has only two conductors, some kind of contact sensor for the two apparent contacts, more is need on the sensor or a better view!
I doesnt look like a reed switch, but if it is then there must be a small magnet on the part being sensed
Max.
Yes, it has ferrite-type magnet,sensor has a 820Ω resistor and a reed switch connected in series
When a drive belt fails, then rpm on axle drops and the corresponding
warning light iluminate
Now, i Have a problem with two lights
I put New sensors and problem resists
 

Thread Starter

agroelectroman

Joined Feb 11, 2020
9
Yes, it has ferrite-type magnet,sensor has a 820Ω resistor and a reed switch connected in series
When a drive belt fails, then rpm on axle drops and the corresponding
warning light iluminate
Now, i Have a problem with two lights
I put New sensors and problem resists
Average voltage is about 5.47 volts
Bulbs on monitor are 12V 1.5W
When i disconect the plug from sensor and measure the voltage on wiring lines is abou 7 Volts
I think it works with pwm but how a 12V Bulb glow up?
Is it work with the number of frequency?
Thank you
 

Alec_t

Joined Sep 17, 2013
10,918
Here's a possible scenario, but it's only guesswork in the absence of a schematic of your monitor.
The sensor is possibly causing pulses to charge a capacitor; the faster the pulse rate the higher the cap voltage. This voltage is fed to a comparator which switches a lamp on or off depending on whether the voltage is respectively below or above a set threshold value.
 
Last edited:

LesJones

Joined Jan 8, 2017
2,522
Another guess is that for each channel the trigger pulse from the sensor triggers a retriggerable monostable. If the monostable is in a non triggered state then the speed has dropped below the threshold. The knobs will set the time constant of the monostables. If you do not have the schematic then pictures of both the component side and the etch side of the circuit boards may help giving you an answer that is better than a guess.

Les
 

djsfantasi

Joined Apr 11, 2010
6,196
This is the sensor


How about a view of the other side? Without that view, it’s difficult to draw any conclusion. And is there something under the top cylindrical object? BTW, that object looks like a reed switch. Do you know what a reed switch us? And the object at the bottom of the picture is definitely a resistor.
 

Thread Starter

agroelectroman

Joined Feb 11, 2020
9


How about a view of the other side? Without that view, it’s difficult to draw any conclusion. And is there something under the top cylindrical object? BTW, that object looks like a reed switch. Do you know what a reed switch us? And the object at the bottom of the picture is definitely a resistor.
There is nothing on the other side
Each bulb has its own channel and sensor
 
In the sensor picture, the reed switch is broken! The glass end is cracked. They are fragile.
The resistor looks like in parallel (not in series) with the the reed switch. This is to give fault detection for open wire/cable going to the sensor. No magnet = 820 ohms, magnet = 0 ohms.
Check the magnet, it must not hit the reed switch. If there is metal shavings or chips, that can also cause problems.
The Speed Monitor must just compare the speed readings between the axles. If one axle sensor is reading low or nothing for speed, I'm not sure if it would make two sensors look bad.
 

Thread Starter

agroelectroman

Joined Feb 11, 2020
9
In the sensor picture, the reed switch is broken! The glass end is cracked. They are fragile.
The resistor looks like in parallel (not in series) with the the reed switch. This is to give fault detection for open wire/cable going to the sensor. No magnet = 820 ohms, magnet = 0 ohms.
Check the magnet, it must not hit the reed switch. If there is metal shavings or chips, that can also cause problems.
The Speed Monitor must just compare the speed readings between the axles. If one axle sensor is reading low or nothing for speed, I'm not sure if it would make two sensors look bad.
It is in series because(unfortunately i don't have a picture which shows the back side of the sensor
One wire is connected to the resistor and the other to the reed switch
Sensor is cover by PVC for protection
So the monitor sends a dc voltage and the reed switch convert it to square wave signal i guess as the axle rotates?
Because in threshing machine you adjust the speed of axles,you must also adjust the potentiometer on the monitor to change the values
 

Thread Starter

agroelectroman

Joined Feb 11, 2020
9
The reed switch is just a pair of contacts that close when a magnet is presented within about 3/8", typically.
Max.
Yes Mr.Max but contacts are open and close lot of times so we have a square signal if I'm not wrong????
So the monitor measure the frequency of the signal?
Best Regards
Stratos
 
It could be analog circuitry, which is converting the pulses to a voltage, as a frequency-to-voltage converter then using comparators to active the lamps. More modern circuits would use a microcontroller.
It looks like the Speed Monitor has one axle pair malfunctioning. If a sensor shorted to ground, I wonder if the circuit is protected.
Pictures inside would help.
 
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