PLEASE help me

[ahonda55]

Hi all , i am new user to this great forum
and i am a hobby in electronic field , but i have a good skills and i made some nice circuits.
i need help please , what i need is a very simple switch circuit that triggered at only just 3v and that circuit can switch a 12v relay.
i mean when the voltage reches 3 to 4 volts the circuit run and closes the 12v relay.
i know that circuit is easy but i have no information to make it
i think it can be done with transistors or something like that.

:cry: :cry: HELP ME PLEASE

 

[audioguru]

Hi Ahonda,
A transistor might work, but it operates with current.
You don't say how much current the relay uses for its coil, and you don't say how much current that the 3V source can provide to a transistor.
Without these very important pieces of information we can't help you, or we will overdesign its circuit so it might cost a fortune.

 

[ahonda55]

the circuit is an LM2907 frequency to voltage converter
it works in my car , here is the circuit .
and the relay is regular 12 volt relay , i don't know the amperes
This is the circuit , i made it and it worked very good
**broken link removed**

Please help me

 

[audioguru]

Hi Ahonda,
Still not enough information.
What's a regular 12V relay? A high current one that switches your winch or headlights or a tiny one that switches on an instrument panel light? A huge difference.
If you don't know how much current the relay uses, do you know how much current for what it is switching? Maybe a relay isn't even needed.

The datasheet for the LM2907/2917 shows many circuits. One uses its output transistor to switch on a light at a certain speed or engine RPM. Its output transistor can turn on a very bright LED or a "regular" 50mA or less relay. Maybe that's what you need.
**broken link removed**

 

[ahonda55]

Dear , first THANK you very much for replying me.
the relay is small type , which used to switch small things not like the head light relay.
the circuit in the page i had posted gives 3v at max RPM , but the problem is that i want to turn on another small devices at that 3v , the relay is very good i had used it before , but now i have a POWER SUPPLY problem , how can i drive a 12v relay with a 3v source ????????

 

[audioguru]

Hi Ahonda,
Your circuit is exactly the same as National Semi's "tachometer" circuit which gives a variable output voltage depending on engine speed. To switch a relay when the engine reaches max RPM and your circuit gives a 3V output, you need to add a comparator that compares the variable output voltage of your circuit to a fixed 3V. Then you must add a driver transistor that is driven from the comparator to drive your relay.

I want you to understand that the LM2917 IC that you are using already has a comparator and a driver transistor that are not being used in your circuit! Very few changes to your circuit's wiring are required to connect them. National Semi's "speed switch" circuit shows the basics, but a resistor's value must be changed to make 3V, and the pins are showing the 8-pin IC instead of your 14 pin IC.

The key to whether the "speed switch" wiring will work for your application is determined by the current draw of your relay and whatever you are turning on with your relay.
Your relay probably has a manufacturer's name and part number that we can look up. Probably the transistor in the LM2917 can drive it.
If you do finally tell us what the relay switches and its current draw, we can determine if the transistor in the LM2917 can drive it directly without a relay.

 

[ahonda55]

The relay i am using is this
JZC-22F
www.escltd.ru/catalog/nhg/JZC-22F.pdf

 

[FRIED]

Try this

 

[audioguru]

Hi Ahonda,
The transistor circuit from Fried won't work with your tachometer circuit.

Your relay coil draws only 38mA so if you re-wire your tach circuit to be a "speed switch" then the 50mA-rated driver transistor in your LM2917 can easily drive your relay's coil directly, without anything else!
Do you want me to show you how?

What are you powering with such a heavy-duty relay?

 

[Boris]

Hi all , i am new user to this great forum
and i am a hobby in electronic field , but i have a good skills and i made some nice circuits.
i need help please , what i need is a very simple switch circuit that triggered at only just 3v and that circuit can switch a 12v relay.
i mean when the voltage reches 3 to 4 volts the circuit run and closes the 12v relay.
i know that circuit is easy but i have no information to make it
i think it can be done with transistors or something like that.

:cry: :cry: HELP ME PLEASE

For easy way you can use IC relay driver:uln2803.Works very good.

 

[audioguru]

Boris, why use an IC with seven 1/2A relay drivers in it to drive only one 38mA relay?

 

[ahonda55]

Thank you all , i am really want to thank you very very much.

audioguru , if there is no problem to you, show me how, i have seen the LM2907 data sheet , "speed switch circuit" page , it is easy and i may use it instead of my crazy things !!

if i did anything i will tell you , again THANK you all.

Sorry guys for my poor english , cuz i am from Egypt !!

Thank you , you all GREAT like this GREAT forum.