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RPM Pick up device

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Hi every1

I want to make a simple RPM pick up device. I want to use it to see if my car's fan is actually turning. Will this be possible to make?
 
Yes - how you do it depends on resources available to you - skills, materials, components being some of the "resources" necessary.

One possibility - visible light or IR on one side of fan might be interrupted as viewed from the other side when the fan is turning.
 
I have some skills in soldering and reading diagrams. My dad is also quite good with electronics. If I can find a simple diagram I think I'll be able to do it. But finding the diagram is a problem. I even thought of making one that works on a normal tachometer of a car. Basically just to let you know that the fan is actually turning.
 
I think the first thing you need to do is determine how you will sense that the fan is turning. That does not mean the circuit is not important.

A light source/sensor allows for non-contact indication. If there is any metal a magnetic sensor might be possible however proximity could be a problem. It has to be close enough to sense reliably yet not contact the moving parts under any condition.

If there are other conditions that strongly suggest that the fan is turning you might sense those. An example - if the wire to the fan is at 12 volts when the fan should be on - will an indication of that be sufficient for your needs? Note that if the fan motor has failed open there might not be an appropriate indication - would look like fan is "on" but won't indicate non-operation.

As I recall the current to those fans is considerable. When current flows thru any conductor (except superconductors) there is a corresponding voltage drop. If you could sense the voltage drop when the fan is operating normally you might use this with a comparator that is set up to sense that current is flowing and that the current is within a normal range. This might take some work as auto electrical systems are noisy and the noise could affect a circuit like this - or others.
 
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I think the first thing you need to do is determine how you will sense that the fan is turning. That does not mean the circuit is not important.

A light source/sensor allows for non-contact indication. If there is any metal a magnetic sensor might be possible however proximity could be a problem. It has to be close enough to sense reliably yet not contact the moving parts under any condition.

If there are other conditions that strongly suggest that the fan is turning you might sense those. An example - if the wire to the fan is at 12 volts when the fan should be on - will an indication of that be sufficient for your needs? Note that if the fan motor has failed open there might not be an appropriate indication - would look like fan is "on" but won't indicate non-operation.

As I recall the current to those fans is considerable. When current flows thru any conductor (except superconductors) there is a corresponding voltage drop. If you could sense the voltage drop when the fan is operating normally you might use this with a comparator that is set up to sense that current is flowing and that the current is within a normal range. This might take some work as auto electrical systems are noisy and the noise could affect a circuit like this - or others.

I did think of just putting a LED with a resistor on the fan's contacts to see when it comes on, but if the fan fails you will think it's on but it's not turning. Like you mentioned. The fan's blades are plastic. So I thought of something that can pick up reflector paper.
 
Seems like the light source and sensor (probably IR) would be a good way to sense the fan turning, either through the blades, or from it's reflection. You would look for an AC voltage from the sensor as an indicator that the fan is turning. AC couple the signal from the sensor using a capacitor to block any dc. Then amplifiy and rectify the signal to give a dc voltage that's an indication of fan rotation. Could use that to operate an indicator light.
 
Something you need to be aware of when using a light source and sensor application, is normal air movement through the radiator when travelling forward above slow urban speeds will likely drive the fan blades.

Motorway/highway speeds could probably drive the fan at a considerable speed, so a distinction between actual electrically driven speed, versus airflow driven speed needs to be made.

Measure the air driven speed signal, then electrically driven signal, and filter through a window comparator perhaps?
 
I did think of just putting a LED with a resistor on the fan's contacts to see when it comes on, but if the fan fails you will think it's on but it's not turning. Like you mentioned. The fan's blades are plastic. So I thought of something that can pick up reflector paper.

Also try not to add something to heavy to one fin, it'll cause a big imbalance at speed, that could have a negative effect on your fan motor.
I would rather look at a laser beam or something of sort that is cut by the blades. Just keep in mind, when driving, the wind will probably turn the fan also. Some cars even has a fan that always spins, non-electrical type, has a clutch mechanism in the block that actives with temperature to make it really spin.
Think yours would be electrical though.
Is you car overheating regularly? What is the condition of the thermostat valve?
 
Some cars even has a fan that always spins, non-electrical type, has a clutch mechanism in the block that actives with temperature to make it really spin.

That would be classed as a 'viscous-coupling', a device which is directly attached to one of the driven pulleys and contains an oily substance within, which expands to act upon closely spaced plates. During warmup, the contents are sufficiently cool to enable slip to occur between the driving plate and the driven plate directly connected to the fan assembly.

As the engine warms up to operating temperature, the thermostat opens and diverts hot coolant throught the radiator to be cooled. The resulting air flow through the radiator has an effect upon the viscous coupling...the hotter the air flow, the hotter the coupling becomes, until a point is reached where the driving and driven plates lock, due to thermal expansion. At this point, the fan speed is governed by engine speed and air flow is increased through the radiator. The increased air flow serves not only to reduce the coolant temperature, but also the viscous coupling temperature...to a point where cooling is no longer required, thus the plates separate and the fan free-wheels again.
 
There are many ways you can monitor the speed of a DC motor. Such as attaching a rotary sensor directly to the shaft or by choosing one of the many optical and inductive solutions that are well documented across the web.

http://www.discovercircuits.com/DJ-Circuits/motor-controller.htm
Here though is another less well known option for DC motors created by 'Dave Johnson' (loosely based on an EDN design idea but cannot find the original article) that could be useful for automotive application. The circuit uses the current pulses from the motor brushes to extract the motor speed.

Low parts count, essentially a couple of common op-amps and a logic flip-flop.
 
The fan's self generation (if it's a DC motor) could be all you need to know that the fan is OK.If the fan is blocked or failed, it won't produce a voltage at highway speeds; and if the electrical circuit has failed there won't be any 12V at rest.
 
Thanx alot for all the reply's. Ok let's make it simpler. Is there a circuit that I can make just to pick up movement above let's say 1500RPM? So if sitting in traffic (very often noisy and can't hear the fan) and the temp needle goes up a little, then there is lets say just an LED that shows you that the fan is actually on and rotating more than the 1500RPM turn on limit? So if the needle goes up and the LED is not on you know that the fan is not turning?

And once again, Thanx for every1's replies! Some good points there.
 
Hey Kadett, I think MProf has a good solution there.

Someone previously talked about an op-amp to check above certain speed, combine the two.
I used to have an Escort that also had the same thing with the needle going up and down a bit (always freaked me out), so once in peak traffic, the temp would climb slightly. It was fine for years like that, until the day the thermostat failed not opening to allowing water flow.
Needless to say, when my eye caught the temp gauge, it has gone right through red on the gauge.:eek:
I had to do block and all the arrangement of gaskets that came with.
Somehow I wonder if you had a similar experience.

The moral of the story, a R80 thermostat caused a R40 000 engine to come undone.
 
Hey Kadett, I think MProf has a good solution there.

Someone previously talked about an op-amp to check above certain speed, combine the two.
I used to have an Escort that also had the same thing with the needle going up and down a bit (always freaked me out), so once in peak traffic, the temp would climb slightly. It was fine for years like that, until the day the thermostat failed not opening to allowing water flow.
Needless to say, when my eye caught the temp gauge, it has gone right through red on the gauge.:eek:
I had to do block and all the arrangement of gaskets that came with.
Somehow I wonder if you had a similar experience.

The moral of the story, a R80 thermostat caused a R40 000 engine to come undone.

Luckilly my car didn't overheat. The overday it started to get hotter then normal. It was when this happend that I started thinking of this idea. I did buy a new thermostat and going to fit it this weekend. The waterpump is quite new so it must be thermostat. The problem I have is you never now if the blade is actuelly turning. The motor can run but slide on the shaft making the blade stand still or not rotating at the correct speed. So if one can make a RPM pick up thingy then you will know that the blade is actually rotating.
 
I hear what you are saying.
Then you have to count the fin cuts through a beam.
Your problem there will be mounting your sender and receiver, as it will be kinda diagonal in relation to the fan.
Then you also have to compensate for how the fan is turned by means of vehicle movement air. I think you are gonna have to experiment a lot there also, plus that might bring a mcu into the fray, but maybe you'll come right with the op-amp setup.
My Ikon actually has to speeds when the fan comes on. I'm still puzzled about that, but it's clearly audible. Keep that in mind also.
 
I think those fans use a lot of current - adding load to the alternator - added load to the alternator requires a change in throttle position as the load imposed by the alternator in terms or torque or horsepower is not insignificant at low RPM.

Lots of good thoughts here. I wonder if some effort should be applied to a simple sensor that would sound an alarm when temps exceed certain limits. Arrie points out the all too important item that we dont' stare at the temp gage so going off scale is almost useless. Simple klixon switches could be employed.

There is most certainly a range of temperatures for various points in the engine for various operating conditions. An example - when it's hot outside and we're in traffic we expect to see the temp gage a little higher - and where it stops is more or less in predictable way or pattern. I would say to that the temperature difference between the fluid entering the radiator and leaving is also something that has a relatively predictable pattern. One might call these variations quite normal.

With an MCU, PIC or other device and some sensors you might compare the operating conditions and establish a "normal" relationship between variables - then sound some kind of alarm when the relationships get too far out of alignment. This could be very simple or very complex - the simple has already been stated. Simple could be looking at engine temp - there has to be a point where the fan blades should be spinning regardless of vehicle speed - if not then sound an alarm. Another possibility - past a certain coolant temp one would expect a minimum temperature differential between the radiator inlet and outlet - if not then something is wrong.

Just some thoughts to suggest that there might be other ways to get what you need.
 
Steve, you are absolutely right, there are other possibilities.
After my incident with the 96 Escort I actually thought, but hey, why is there not an alarm going off somewhere when things start to go awry.
Trucks have them, if your engine gets to hot, a real loud and irritating alarm goes off, same thing when the vehicle is moving and not in gear (critical for trucks with air systems), etc.

Any case, I thought one could measure the water temp in the radiator, and also the engine, if the temp diff gets too big, you know water is not flowing.
Similarly, set a barrier, if the temp gets to say 89 - 90 sound an alarm, your engine temp should not be there, etc. But you only have a needle going of the face of its planet. Why!
There are other ways, but still complicated.
One can probably monitor engine oil temp in the same way, once engine oil overheats excessively, or used to couple years back, it has lost some of it's qualities, and this can also cause engine damage.
Don't know why vehicle manufacturers take this for granted.
Oh, I forgot, they make more money from selling parts, than from selling the car.
 
There should be a point in the temperature gauge wiring where a simple comparator would be able to trigger an audible alarm.

But if you put an alarm only on the fan, then you have done nothing to indicate coolant loss, thermostat failure, clogged lines, or water pump failure.
 
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