# Need Help, building a frequency circuit

Discussion in 'The Projects Forum' started by deathphoenix99, Sep 24, 2010.

1. ### deathphoenix99 Thread Starter New Member

Sep 24, 2010
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Hey guys, I'm building a circuit for automotive use so it'll be powered with 12-15 volts.

I need a circuit that will basically have an input, this will be a frequency from 0-8,000 pulses per second. Alright, when the frequency is above a certain rate, let's say 2,000 pulses per second, then it will open or close a circuit via relay and will hold that circuit in that state until the frequency is below 2,000 puleses again.

I'm trying to build a circuit for my electric fan, where when I'm driving over 30-35 mph it'll turn the fan off so that it can't turn back on unless I'm driving under 30-35mph. My truck's speed sensor sends out 4,000 pulses per mile, which is where I got the general frequency needed for the input.

The ouput would need to either be a ground or +voltage that will alternate between the two. I'm interest in your guys ideas, prefer inexpensive setups, but will look at all alternatives.

Also, I would like to be able to easily change at what frequency the circuit is looking for when to change.

Thanks for your help guys, I have experience with wiring and circuits, but I've never had to build this sort of a circuit before.

Phillip Chauvaux

2. ### retched AAC Fanatic!

Dec 5, 2009
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You could use a frequency to voltage IC.

Once you find the voltage that is produced @ 35mph, you can easily use a comparator or even a zener to operate the Normally Closed relay.

When the NC relay gets the signal, it will cut the fan. Once the voltage drops back below the zener voltage, the relay would re-engage the fan.

3. ### deathphoenix99 Thread Starter New Member

Sep 24, 2010
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Alright, any links or info on this? Sorry, I'm new to a lot of circuits. I've built simple circuits with 555 timers and similiar, but I've yet to use zeners or a comparator.

Thanks for your help, this means a lot to me. When I get the chance, I'll hook up my voltmeter to the VSS (vehicle speed sensor) and figure out the exact voltage it makes at around 30 mph.

Are either of the items you suggested easily "programmable" or "calibrated" with resitors or capacitors?

Sorry, I'm not very good with the specific termanology.

4. ### retched AAC Fanatic!

Dec 5, 2009
5,201
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A zener is a special diode.. Its just 1 part. just like a resistor.
You install it backwards. Most diodes start conducting at .7v when forward.

Zener diodes installed backward or reverse biased, have a breakdown voltage. You can get them in a variety of voltages. If you lucky, a zener will exist for the voltage you need.

Here is a link to our ebook page on zener diodes:

and here is where you can buy them:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=1377034&k=zener

There are 13,410 zener diodes to choose from on that site. The voltages available are from 1.8 to 200v in about .1 or .2v steps... so it shouldn't be a problem for you.

5. ### deathphoenix99 Thread Starter New Member

Sep 24, 2010
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Ok, I actually have a couple zener diodes laying around, I'll have to see what their numbers are.

If anyone has any other ideas, I'm always open for suggestions. I'll continue researching the zener diodes and look at comparators as well.

6. ### wayneh Expert

Sep 9, 2010
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I'm quite sure I once used the 555 to build a frequency-to-voltage device using a circuit from the little Forest Mimms book I got at Radio Shack. I remember it worked well in converting audio from my guitar tuner to voltages. Nicely linear.

My only point is, if you've already got the 555 and familiarity with it, you may not need a new device, except the comparator as noted. Those are great to know about on their own, BTW, and easy to get, even at the Shack.

Another way to set the voltage reference, that you will compare your measured frequency (now a voltage) to, is a voltage regulator (eg. 7805 or 7812) and a resistor divider. You might want to power your ICs with a steady 5v or 12v anyway, so this would kill two birds and give you both a stable power supply and a handy fixed voltage, which is all the zener does. These devices are in practically any old electrical junk you might have (old TVs, VCRs, answering machines, etc.) but also at the Shack. Not that I'm a big fan of the Shack, I'm just sayin'...

Last edited: Sep 25, 2010
7. ### deathphoenix99 Thread Starter New Member

Sep 24, 2010
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Well, I have a handful of 555 and 556 IC's laying around, I also have some voltage regulators and other misc chips. I should probably catalog what all i have laying around. If i could use a 555 timer, that'd probably be the easiest for me, now I just need to find a good schematic or diagrams and figure out what I'd need to make it work.

I"ll do more research as I get time, right now I'm in transition of temporarily moving from college (Michigan) back to Texas over the next couple of days.

8. ### Wendy Moderator

Mar 24, 2008
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What part of Texas?

9. ### SgtWookie Expert

Jul 17, 2007
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I don't think you can go by just the speed of the vehicle.

What happens if you have a 35MPH tailwind and you're doing 35MPH? No airflow; engine overheats.

I think what you really need is a way to measure the air pressure differential across your radiator.

One way to determine the pressure differential might be to cut a rectangular hole near one end of the sheet metal/plastic that separates the area behind the grille from the engine compartment, and mount the cut out rectangle using a hinge as a flap to allow the flap to apply pressure to a switch. The switch could energize a relay to turn off the fan when sufficient pressure was applied to the flap. The trip point could be adjusted by moving the mounting point of the switch closer to the hinge (trips at lower pressure) or away from the hinge (trips at higher pressure.)

I don't know offhand how much surface area you'd need for the flap vs the spring tension of the switch.

Instead of chopping a hole, you might use a couple of open-ended vacuum lines; one ahead of the radiator, one in the engine compartment. They'd need some kind of protection to keep bugs from crawling in them, and mounted with their open ends down to keep fluids out. Have the other ends of the lines connected to a diaphragm. The diaphragm would need to be made from something like thin surgical rubber.

Even more simple might be to use just one tube carrying the pressure ahead of the radiator to a pressure assembly in the engine compartment that's made from a couple of those black microwaveable trays that food items (such as Steuffer's Lasagna come in) that are glued together face-to-face using RTV silicone or the like. That would be really cheap material to use, and it's designed for use at fairly high temps. The bottoms of the trays would flex outwards depending on the pressure differential.

If the pressure differential is low and the engine is running, then
if the engine is up to normal operating temperature or the A/C is on, the radiator fan needs to be on, otherwise off.

Last edited: Sep 26, 2010
10. ### wayneh Expert

Sep 9, 2010
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I agree, but since the issue here is temperature, why not use engine temp directly? Just mount a temp sensor and control the fan with a thermostat. I wonder why it doesn't already control this way?

There's a chance there are already onboard temperature signals that could be used. And the comparator could be used to "OR" the signals, so for example the fan would turn off if the speed was above 35 or the temp was below x degrees.

Last edited: Sep 26, 2010
11. ### SgtWookie Expert

Jul 17, 2007
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Well, the thermostat(s) (if good) keep the engine temp quite constant over a wide range of operation. You'd only be able to tell if the fan(s) were needed if the engine temp went above the normal operating range. That doesn't seem to me to be a very good approach, as the cooling system would be kept operating at maximum coolant flow with the thermostat(s) running wide open.

It would be better to monitor the temperature of the coolant in the radiator near the point where the water pump inlet hose connects to it. If it got within, say, 30°F of the engines' normal operating temp (number pulled from a random hat), then you'd need the fan running to cool the radiator down.

12. ### wayneh Expert

Sep 9, 2010
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Agreed. In my sloppy language, I wrote "engine" while thinking of the coolant flow. As in anything under the hood is the "engine".

The fan exists to cool the coolant when needed. So it makes sense to control the fan based on the temperature of the coolant.

13. ### deathphoenix99 Thread Starter New Member

Sep 24, 2010
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Houston TX. I'm in Houston now.

First off, it's an aftermarket fan, my stock fan was mechanical. So I do have a temp probe that turns it on and off. But I have two fans, one for the radiator and one for the a/c. The fan I'm wanting to control is my A/C fan, it only comes on with the a/c on, but it always runs, even if I'm driving 70 mph. I'm not too worried about the wind differential as you say, for my purposes, i just need an electrical means to turn the fans off above 30ish mph and turn them back on when I"m slowing down.

I haven't been online much the past couple of days, and I probably won't be the next couple as I get resituated here in Houston.

14. ### wayneh Expert

Sep 9, 2010
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That may work but I think the strategy of controlling directly off temperature instead of indirectly off speed is still valid. You could even use the cold air made by the AC as an input; if it isn't cool enough, run the fan.

15. ### deathphoenix99 Thread Starter New Member

Sep 24, 2010
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Alright, could anyone point me to some schematics to wire up the circuit I described in post #1. Preferably using 555 timers.

I know I still need to figure out the voltage for the input, which I will hopefully tomorrow, just been busy with work.

I'd really like to go this route as I know it'll work and some stock fans work this way when running off of the computer.

16. ### marshallf3 Well-Known Member

Jul 26, 2010
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The "one fan on with the AC" is common among most any car I've ever seen, same with always staying on as the engineers must have determined there's a definite reason for it.

1) They know about how much HP it takes to run the compressor and how much extra heat it usually throws in.

2) They want a constant flow of air across the condensor coil so the air conditioner doesn't freeze up.

I don't know why you couldn't try switching it off at highway speeds but I know darn sure you don't want a switch controlling it, it absolutely has to be on whenever the compressor is running.

17. ### deathphoenix99 Thread Starter New Member

Sep 24, 2010
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I can assure you, at about 30 mph or faster, the fan is useless. Which is why I don't need it on, I know plenty of people who have aftermarket tuners that have the a/c fan off at above 35 mph. I don't want a toggle switch because I don't want to risk leaving the fans off when they need to be on. And I have driven my truck with the fans turned off, the a/c worked just fine. And if you didn't have the airflow, the a/c won't freeze up, it'll build pressure causing the high pressure switch to disengage the compressor. It will also cause the system to get hot, not cold.

18. ### marshallf3 Well-Known Member

Jul 26, 2010
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Well then, the previous advice of an F to V converter driving a comparator circuit sees to make sense.

19. ### wayneh Expert

Sep 9, 2010
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I built a F-to-V meter using the 555 and the circuit that appeared in the little "notebook" on the 555 that you used to be able to get at Radio Shack, authored by Forrest Mimms. It worked great, and very linear voltage vs frequency. Perhaps if you Google around on that you'll find the schematic online.

20. ### deathphoenix99 Thread Starter New Member

Sep 24, 2010
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Alright, so I hooked up my voltmeter to my trucks wiring today. The VSS wire for my cruise control gets anywhere from 4.40-5 volts when I'm driving. So the voltage is about 5 volts. I can only calculate the frequency though, since my only multimeter that measures frequency is broken. I'm still researching and trying to figure out the best way to go through with the setup.