Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Looking for a pulse detection circuit

Status
Not open for further replies.

coderighter

New Member
I have a cooling fan on a motorcycle that I want to run in one direction when the motorcycle is stopped and the other direction when the motorcycle is moving, with a 5 second pause between changes to give the fan motor a chance to spool down before reversing directions. The speedo on my motorcycle is electric, a freq meter. The pulses are 5 Volt that I'm getting from the sprocket sensor. The fan is 12vdc 10 amps. I'm guessing the signal input should be a pretty high impedance to protect the sensor. Thanks for the help.
 
How long do you want the fan to run for when the bike has stopped ? I presume that it is only when you stop that you want the fan to pause and then run in reverse ?
 
First, by stopped I mean motorcycle running but not moving, like a stop sign. The fan only runs when temp switch closes, around 205 degrees. Once it cools below 200 degrees the switch opens and the fan goes off.
 
So when you stop moving AND the engine temperature is >= 205 degrees the fan runs in reverse.
When you are moving OR the engine temp is below 200 degrees the fan is OFF.

Is the fan an add on to the motorcycle or is it factory fitted and you want to modify it's behaviour ?
 
The fan is stock and as such it starts at 205 degrees and turns off at 200 degrees as long as its running, whether it's moving or not.

The motorcycle is a Honda VFR800. It a V4 engine with 2 side radiators. When the bike is moving air flows in the large front opening, then out the side through the radiators. Because the two front cylinders exhaust outlets are in that same opening Honda makes the fan pull air from the side into the front. The thought is that they don't want to pull warm air from the engine/exhaust through the radiator. This is fine if the bike is getting hot because you're waiting at a light. How ever if you're in bad traffic, moving 5-10 miles an hour, and the temp goes over 205, now the fan is working against the 5-10 mile an hour "wind" trying to come in from the front. If the fan turned in the reverse direction of stock (out the side) while the bike was moving, it would actually be helping the "wind" instead of fighting it.
 
Why not install a DPDT relay in line first and manualy operate it for now. If it does what you want then it can be automated later. You could use standard auto electrical connectors instead of cutting into the loom.
 
Thats how I got to this point. I'm now looking to automate. I really just need a circuit to give me a closure when the bikes moving. I think I have the rest under control.
 
I reckon a little charge pump would do, what is your construction level in terms of electronics ?
Missing Pulse Detector

Please explain relevance of 'charge pump' .... How does that relate to this problem?

The missing pulse detector circuit makes sense .... but not 'charge pump'.

It looks like what is required by the OP is a method to reverse the polarity of the DC fan motor, upon detection of the lower frequency sensor pulse condition from the sprocket gear ....

One possibility would be to use something like DC relays .... one pair for normal fan action, and a separate pair for reverse fan action ... devising some scheme to activate each directional relay pair according to the sprocket sensor output ... possibly using the Missing Pulse Detector Circuit ....and also including some means to delay the fan direction reversal for a few seconds while the blade rotation came to rest.
 
Last edited:
A 8-pin PIC driving an H-Bridge is a good solution.

It get messy using individual logic chips, monostable or timers and relays to get the same control function.
 
Hi coderighter,

reversing an axial fan is not the best idea at all. The fan blades are shaped aerodynamically (high air speed and low pressure on the upper side of the blade and low air speed and high pressure on the bottom side) and operated reversed the air flow reduces to 30% of the normal rotation.

Driving at low speed and "fighting" the wind with the proper fan rotation has more effect than having the airflow with the wind but reduced by 70%.

If the fan is a radial type you might as well forget about reversing air flow. It still produces air flow in the forward direction but also reduced by 70%.

Consider this before you apply any changes.

Boncuk
 
reversing an axial fan is not the best idea at all. The fan blades are shaped aerodynamically (high air speed and low pressure on the upper side of the blade and low air speed and high pressure on the bottom side) and operated reversed the air flow reduces to 30% of the normal rotation.

Driving at low speed and "fighting" the wind with the proper fan rotation has more effect than having the airflow with the wind but reduced by 70%.

Consider this before you apply any changes.

Boncuk

This point might require further examination ......

Are the blades of the fan contoured or shaped in an aerodynamic fashion?
.... or are they more or less a straight vane type?

Is Coderighter able to manually switch the rotational direction of the fan motor .... and to detect a noticeable difference in air flow .... as per Boncuk's comment?

How about a picture ... closeup ... of the fan and fan blade assembly ....
 
Please explain relevance of 'charge pump' .... How does that relate to this problem?

The missing pulse detector circuit makes sense .... but not 'charge pump'.

The OP wanted a trigger when the pulse train from the tacho ceased. The circuit link supplied is for a missing pulse detector which will do the same sort of thing. If the pulse train was fed through a diode into an RC combination we get a charge pump, and the resulting charge fed into the base of a transistor to keep it biased on. When the pulse train ceases the transistor turns off. Charge pump / missing pulse TRAIN...

We could of course use CMOS gates, or a PIC micro or a myriad of other methods, all of which work. The OP already has a relay and manual switch which he is using, read the post.
 
The OP wanted a trigger when the pulse train from the tacho ceased. The circuit link supplied is for a missing pulse detector which will do the same sort of thing. If the pulse train was fed through a diode into an RC combination we get a charge pump, and the resulting charge fed into the base of a transistor to keep it biased on. When the pulse train ceases the transistor turns off. Charge pump / missing pulse TRAIN...

We could of course use CMOS gates, or a PIC micro or a myriad of other methods, all of which work. The OP already has a relay and manual switch which he is using, read the post.



It looks like you could use the 1M pot on the 555 Timer to achieve a continuous output high signal on pin 3 .... adjusting until a low rpm input from the sprocket produced the desired result ... with pin 3 going low upon a complete stop...This continuous high signal at pin 3 could then be used to turn on a transistor to activate the appropriate relay coil ... Then you might not require the charge pump ..

The argument for using individual relays ...set up as directional pairs, on the fan motor ....as opposed to the DPDT relay, presently used by the OP, is that you could utilize heavy duty horn relays, which would be able to withstand the frequent 10A DC current switching .... It would probably not be a good idea to use relays that were merely rated for AC duty in this application.
 
Then you might not require the charge pump ..

You miss the point here, either will do.

With the 555 you don't need a charge pump, as I pointed out a charge pump will function as a missing pulse train detector and the link I gave is for a missing pulse detector Either would be suitable in this case. The charge pump will require some experimentation and if the OP is not set up for this then it is pointless going that route.

As for the choice of relays, the OP asked for something to add to his present system, not replace it. You are right in saying that an automotive relay would be both more suitable and cost effective though.
 
I will post picture later today. There's alittle more to the air flow then the fan design. The fairing design plays into this also. I guessing I should be able to do this with 2 DPDT relays. One to reverse the pos and neg wired in the run in the normal rotation de-energized, reverse energized. The second relay, also DPDT, would be just before the fan to break both pos and neg when energized to "pause" the fan during the direction change. The most happen just before the direction change. How would I make this 5 sec "pause" on both forward and reverse trigger slightly before the actually change? Perhaps a cap across the coil of the relay just to make it a little lazy?
 
Perhaps a cap across the coil of the relay just to make it a little lazy?

.... not quite that simple ..... a cap in parallel with the coil would charge instantly.. that is, there would be no time delay at all.

You can get a 5 sec. delay by placing a 50k resistor in series with a 100 F
cap ....However, this won't work because the 50k resistor would block the coil activation current.....which is probably several hundred ma.

... I don't see how you can get any significant coil activation delay without setting up a transistor to activate the coil current .... You would have the 5 sec. RC circuit operating on the gate of the transistor .... Difficult to say exactly what would happen, since you would also have the transistor base current draining the cap .... probably have to try this out to see how it goes.....
 
Last edited by a moderator:
I wasn't trying to get 5 seconds with the cap. The cap was only to make the sure the power cut off relay happens before the polarity relay changes states. I need the 5 sec "timer" to hold the power relay on, there by cutting the power to the fan. I need a way to trigger that "timer" just a split sec before the polarity relay changes states, either energized or de-energized.
 
I wasn't trying to get 5 seconds with the cap. The cap was only to make the sure the power cut off relay happens before the polarity relay changes states. I need the 5 sec "timer" to hold the power relay on, there by cutting the power to the fan. I need a way to trigger that "timer" just a split sec before the polarity relay changes states, either energized or de-energized.


Try putting a 10 ohm resistor in series with the relay coil .... it may be that this would cause enough of a time delay to prevent the polarity conflict problem ..... but still allow the relay coil to have enough current to activate.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top