help building a circuit

[autosri]

hi im a newbie and came across your forum searching for some help

i like setting myself small projects to stop me getting bored but need some help

basicaly what i need to do is use a 1-3vdc varying trigger to make a relay coil powered but i need the relay coil to be isolated from the trigger so was thinking of using a transistor the 1-3vdc would connect to the base and then use the collector or emitor not sure which to feed the relay coil

could someone please help me with a circuit diagram please with resistor values

the relay coil is 12vdc and the resistance is 1k ohm and the only main supply i have access to is 12vdc

thanks in advance if you need any more details let me know please

 

[Reloadron]

basicaly what i need to do is use a 1-3vdc varying trigger to make a relay coil powered but i need the relay coil to be isolated from the trigger so was thinking of using a transistor the 1-3vdc would connect to the base and then use the collector or emitor not sure which to feed the relay coil

A transistor will not give you isolation. The device you would want is an opto-coupler for isolation. Next, isolation of the relay coil means the relay coil and triggering are totally independent of each other meaning they do not in any way (including common) share the same power source. So since you have a single 12 volt supply we have a problem. What exactly are you trying to do? Why is isolation required?

Ron

 

[autosri]

What I'm trying to do is make a relay fire from a cars throttle pedal the cars ecu feeds the pedle with 5vdc and the pot inside the pedal varies between 0.7-3.8vdc
Originally just connected a 5v relay to the varying output but it massively affected the pedals linear feel when the contacts closed
So next thought was a transistor but I though they needed an actual load to change position From open to closed but I don't have a load as such just a varying pulse signal from an ignition coil and the signal needs to stay unaltered

What I'm actually doing is fitting a nitrous oxide controller to my car and I only want the controller to see the rpm signal when the throttle is pressed

Thanks

 

[Reloadron]

OK, now I know what you are after. The way we do this is the ECU provides a 5 volt level to the TPS (Throttle Position Sensor) which outputs a linear voltage proportional to throttle position, so what we do is use a small simple comparator circuit. I'll try to get a circuit done later today. You want nitrous when the TPS reaches about 3.7 volts correct? Anyway, you want a simple small comparator circuit. The comparator will drive the transistor which will turn on the relay when nitrous is called for.

Ron

 

[autosri]

I have setup the controller to activate the nitrous at wide open throttle that bit is fine

I would like this relay to activate at about 1.5v and stay closed up to full throttle then when the trottle is released I would like it to deenergise at 1.5v

If you could draw a circuit up that would be great

 

[Reloadron]

No problem, if I am too busy at work I'll get you one this afternoon when I get home. It's a pretty straight forward circuit. We will use a LM311 (real common), a 7805 voltage regulator (also real common), a 2N2222 transistor (real common) and literally a few resistors including a variable resistor. The variable resistor will allow you to adjust for the best energize/de energize position you want be it 1.5 volts or whatever.

Ron

 

[autosri]

Thats brilliant thank you very much For your help I've spent the last few days trying to sort this lol

Just been reading up on the comparator circuits Trying to understand them

 

[Reloadron]

OK, the attached should do what you want. The parts are really very common and just about everything can likely be had at the local Radio Shack (depending on your location). Everything can very easily be mounted on one of these boards from Radio Shack also.

Resistor R1 is a 10K ohm adjustable pot. It would be nice to have a 10 turn trimmer pot for this mounted on the board. The output of R1 runs to pin 3 of the comparator. The voltage on that line will be your trip point. When the voltage in from the TPS exceeds the set voltage the transistor will turn on and energize your relay. The transistor will easily handle 1/2 amp. That should be adequate for your relay.

If you have any questions just ask.

Ron

 

[autosri]

Thanks for that it's much appreciated I will try and collect all the bits I need tonight and make a start on it and see how it goes lol

 

[Reloadron]

A small error in my drawing was brought to my attention. Pin 1 of the LM 311 needs to be connected to ground and I left that out. My bad on that so again Pin 1 of the LM 311 needs to go to ground for the comparator to work.

Ron

 

[alec_t]

Might be worth adding a small amount of positive feedback around the comparator, to provide some hysteresis and prevent possible relay chattering.

 

[autosri]

Thanks for that only have one problem I can't get th lm311 can only get the lm339 quad type I assume I can copy your diagram still just use different pin numbers and leave the other 9 op amp connections spare Also they don't stock the exact tramsistor number but assume any small npn one would do

Thanks

 

[alec_t]

If you do use just one op-amp out of a quad, make sure the unused pins are properly terminated as per the datasheet.

 

[Reloadron]

Thanks for that only have one problem I can't get th lm311 can only get the lm339 quad type I assume I can copy your diagram still just use different pin numbers and leave the other 9 op amp connections spare Also they don't stock the exact tramsistor number but assume any small npn one would do

Thanks

Yeah, you can use the LM339 just tie the unused inputs to common as was mentioned. You will just be using 1 of the 4 comparators in there. I'll give you an idea when I get home. Also you may want to give this a read.

**broken link removed**

I couldn't get a link to work.

Now as mentioned I would have suggested hysteresis but this time avoided it because I figure when you hammer the pedal to the floor the TPS will blow past the limit and hit the nitrous. Same when releasing the pedal. While I don't see a chatter problem in this app if there is we can add some hysteresis later. Hysteresis is covered in the link above. Additionally there are links to the LM339 in the above link.

Later
Ron