# 12v relay on 3v ??

Discussion in 'Electronic Basics' started by ahonda55, Sep 2, 2005.

1. ### ahonda55Guest

that drives a 12v relay with an input of 3v or less. i mean when th
circuit input reachs about 3v the output of it becoms 12v and unde
the 3v the output is 0v
may be using transistor or using LM741 ( i don't know how)
any ideas or drawings will be aprecciated

Plz help ..

thanks a lot :

2. ### Jonathan KirwanGuest

On Fri, 02 Sep 2005 03:35:13 -0500,
What supply voltages are available to you? Where does the 3V signal
come from?

Jon

3. ### ahonda55Guest

Thank you Jon for replying me

I made an RPM for my car using LM2907 , it's output is 1 to 3 volt
i need to make this output drive a 12v relay , but the main problem i
that i need to drive the relay on exactly > 3v , i mean 3v , 3.1
3.2 so on

4. ### Ralph MoweryGuest

What you need is a comparitor circuit. Most easially made by using an op
amp. Put a 3 volt refferance (can be set by a pot) on one input and feed
the 0-3 volts to the other input. I don't remember which way it is right
off, but you should be able to find the simple circuit. When the voltage
becomes greater than the refferance voltage , the op amp output will go from
about 0 to the supply voltage. You can use this to drive a low power relay
or more likely a transistor and then the relay.

5. ### Jonathan KirwanGuest

On Fri, 02 Sep 2005 19:35:26 -0500,
Have you examined the various output configurations on the data sheet?
It appears to me that the output transistor may very well be able to
drive your relay (it handles up to 50mA and up to 28V on the
collector.) I imagine that with the right voltage being provided on
the inverting input (3V), you are just about there. (You can also
adjust the generated Vs you are comparing with, if some other voltage
threshold would be more convenient to use, I think.)

What's the difficulty with the illustrated output configurations on
the data sheet?

Jon

6. ### Jonathan KirwanGuest

All that is pretty much already in the LM2907. Take a look at it.

Jon

7. ### JamieGuest

you don't specify the source voltage for the 12 Relay so i will
assume that its 12 volts and you need 3 volts control signal to
close it also, you didn't specify the amount of current the
relay coil consumes? so i will assume it to be aprox. 250 ma's for
now.
using a 2N2222 type transistor.
collector to one side of the relay coil, Emitter to ground.
330 ohm resister in series feeding the base.
the other side of the relay coil going to the 12 Volt source,
and a simple 1N4001 type diode across the relay coil connections with
the Cathode (line side) connected on the side that is connected to the
12 volt source! this is for protection.

apply the 3 volt signal to the other side of the resistor.

8. ### ahonda55Guest

thank you all dear friends :
i have made the transistor - relay circuit , it is working very goo
:
but there is a small problem , i wanted to adjust the relay ope
volt
i mean that i need to make the relay open on 3v or 2.5 etc
i got a 100k variable risitor but it BURNED because of the rel
current
what is the best way to make the open voltage for the relay variable
i mean to control the 3v or less , where should i put the variabl
risitor

thank you very very very much for helping me , and sorry may be i a
confused cuz i am still biginner ;

9. ### Jonathan KirwanGuest

On Sat, 03 Sep 2005 19:35:23 -0500,
Can you show us what you are doing now? Write up a description of how
you are connecting up the LM2907? I think you have everything you
already need in the LM2907 without burning up parts!

Jon

10. ### ehsjrGuest

The single transistor circuit is no good for what you want.
Go back and read Ralph Mowrey's post of 9/2. He correctly
identified a comparator circuit, and anticipated you wanting
to adjust the voltage. Here is a schematic and explanation.
I went into a lot of details - I apologize if I mention

0 + 12 V
|
+---------+----+-------+
| | | |
| | [Relay] [Diode]^
| | | |
| - |\ | +-------+
[POT]----| \| |
| | \ /c TIP 120
| |OP }---[R]---|
| | / \e
Input-- | -----| /| |
| + |/ | |
| | |
+---------+------------+
|
Gnd

R is a 2200 ohm resistor. OP is an op amp, like an LM324.
(Many different op amps will work - the 324 is very popular)
The plus and minus signs to the left of the op amp refer
to whether the output voltage will be inverted as compared
to the input voltage. - means inverted, + means non-inverted.
The banded end of the diode, a 1N400x, connects to +12 (as
indicated by the ^ symbol). The potientiometer can be 10K ohm
or higher.

When your input voltage is goes positive with respect to
the voltage set by the pot, the output of the op amp
goes positive (close to 12 volts). The transistor is an
NPN, which means it needs positive on its base to turn
on, so the relay is energized. When your input voltage
goes negative with respect to the voltage set by the
pot, the output of the op amp goes negative (close to
0 volts) so your relay is de-energized.

To set it up, apply exactly +3 volts to the input. Then,
carefully adjust the pot until the relay just energizes.
Once you're sure you have it set properly, you can use it.

Later on, once you have this circuit working, it can be
improved to avoid relay "chatter" which will occur if
your +3 volt input single is not stable.

The reason a transistor is included is that the op amp
cannot provide enough current to energize most relays.
The transistor needs only a tiny current from the op amp
to control a large current for the relay.

To discover which pins to use on the op amp IC,
Google for the datasheet. For example:
http://www.national.com/pf/LM/LM324.html
I found that by putting "lm324 datasheet" in the
Google search box. On the search results, I clicked
on the first one and got to the link above. On that
It shows that the LM324 is actually 4 op amps inside
one chip. I strongly recommend you use the LM324
If you use the first op amp in the IC, the output
is pin 1, the - input is pin 2, and the + input
is pin 3. +12 volts connects to pin 4 and Gnd connects
to pin 11. Identifying the pins: Set the IC on a table,
standing on its pins, oriented like this:

14 13 12 11 10 9 8
| | | | | | |
-------------------
| |
) LM324 |
| |
-------------------
| | | | | | |
1 2 3 4 5 6 7

The end with the notch is always the pin 1 end.

You can use it over and over again, experimenting with
all kinds of circuits. Once you are sure you have a
circuit right, you can put it on a PC board like this:
or similar.

Ed

11. ### Jonathan KirwanGuest

I am curious why you think that the LM2907 doesn't already have the
necessary components for the OP's need. It already includes an opamp
that can be used as a comparator (with many examples provided on the
data sheet) and a 50mA floating transistor at its output.

By choosing the C and R connected to the node common to the
non-inverting input and the charge pump at pin 3, the OP can select
the desired F->V conversion desired and presented for comparison to
the opamp. By selecting the resistor divider, the inverting input can
adjusted to set the switch point. Another resistor can set the
desired hysteresis, if any. Finally, the load (relay) is connected
between the automotive supply rail and the internal floating
transistor collector (emitter tied to ground, for example.)

Under this circumstance, assuming that the transistor is capable of
handling the relay (and I don't know that it cannot, at this point,
with the data sheet specifically mentioning that it is designed to
directly drive relays), it should be done with what's already there in
the LM2907 except for a few external passives.

What do I miss, here?

Jon

12. ### ahonda55Guest

Hey guys , you are impressing me with your replys. you are talking a
if it is your circuit not my circuit , thank you very much thank yo
, really thank you

http://img326.imageshack.us/img326/7944/image28ie.jp
---
i know that LM2907 has a comparator in it , but there is two problem
with me , the same problems with LM324 .
1- i can't find the 8 pin version of the LM2907 here
2- i have the data sheet of many IC's but i don't know how to realiz
the circuits mentioned there
---
i have LM741 , will it work ? if yes would anyone show me a circui
for my usage
i think i need so much time to "design" a circuit , so plz do it fo
me
---
special thanks to ehsjr for his great work like all of you
but i don't understand your diagram , i am not proffeccional like yo
man
if you can plz draw a simple circuit for LM327 or LM741 the mos
simple
---

Thank you[/size

13. ### JamieGuest

for your project , you can use a 741.
Vcc = 12 volts from your source which i think is pin 7 but don't quote
me on that.
pin 4 goes to ground.
Op-AMP output goes to your 470 R
voltage divider on the - input of the OP-AMP set at your desired
trip point. 2 Resistors in series makes the divider and to get
3 Volts you need 1/4 of your 12 volts.
something like 7500 for the high end and 2500 for the low end to
ground, the tap connects to the - input.
the + input should have a small Resister in series for noise reasons

basically all you need to do is get the - input set at your trip point
level and use the + input from the source ref of your tach..
the 741 does not have a lot of gain how ever, the relay will help
prevent chatter.
that is the simplest way i can explain it.

14. ### ehsjrGuest

I said nothing whatsoever about the LM2907.

I don't know what you're missing, if anything. However, I
doubt the OP can follow the datasheet and work out the
circuit without help. The OP said "i am confused cuz i am
still biginner"

I think an explicit schematic with values and description
was necessary, based on that. I think guessing about
whether the chip can drive some unnamed relay is a bad
thing, which could lead to destruction of the existing
circuit. In fact, any changes to it could do that, due
to errors that a beginner should be expected to make.
The OP already experienced a burned out pot, although
(I assume) not in the original circuit. Somebody proposed
a one transistor circuit, apparently overlooking the 3
volt requirement as the OP intended it. That is what led
eventually to the burned out pot.

Ed

15. ### Jonathan KirwanGuest

No, you didn't. That's what confuses me. Here is a post from the op:
Do you see the mention of the LM2907??
Well, go back and read that post from the OP and the others and help
me understand. I think an LM2907 is being used. But?? I'm willing

Jon

16. ### Terry PinnellGuest

You have not provided enough detail of your RPM circuit for any of us
to be clear about what this extra circuit must do. But, as Jonathan
has already suggested, it seems likely that you can achieve all you
want from the LM2907 itself. Presumably you have looked at a datasheet
and built your RPM circuit using that? For example, there's a 'Minimum
Component Tachometer' described on page 7 and 8 of the datasheet here
http://www.national.com/ds.cgi/LM/LM2907.pdf

For me, one confusing part of your description is "it's output is 1 to
3 volts." Do you mean that represents the rpm range you want to cover?
Are you perhaps trying to activate an alert when the rpm ('speed')
reaches a certain maximum level?

It may well be that the simple relay driver you've shown (which looks
fine on its own) will indeed do the job for you. But I can only echo
Jonathan's last recommendation: draw your circuit and show us!

--------------

Jonathan:

I've never used an LM2907, but I have a couple of problems with the
above datasheet.

1) The +Vout figure of 67 Hz/V for the typical circuit shown at the
top of page 8 seems wrong. Using the formula given on page 7, I make
it 670 Hz/V. (That would also be a lot more practical for anything
other than steam engines!)

2) No component designations are given, which IMO is unacceptable
carelessness in a datasheet by a major supplier. However, it's fairly
obvious what R1, R2, C1, C2 are (which is how I worked out the 670
Hz). But what is 'I2' in the formula for max input f? Is it a
reference to 'Output Current'? If so, how is the formula applied in

17. ### Jonathan KirwanGuest

On Sun, 04 Sep 2005 19:35:14 -0500,
Looked at it, but it isn't your complete circuit. I'd like to see the
rest of what you are doing with the LM2907.
So you are using the 14 pin version. I don't think this matters
except for pin designations.
They provide the equations needed. What is confusing you? Do you
need a complete walkthrough?
I think most folks don't mind teaching someone, if it may lead to more
independence in the future. (At least, I hope so, since I'd like help
myself!) But just being asked to 'do it for' you isn't the better way
to influence.
I'm no professional at this, by any possible measure of the term. But
I was thinking something along these lines:

: ,-----------------------------,
: | |
: | |
: | |
: | |
: | +12V +12V |
: | | | |
: | | | |
: | | | |
: | )| --- D1 |
: | )| RELAY / \ |
: | +12V )| --- |
: ,--, ,--, | | )| | |
: | | | | | | | | |
: gnd | gnd | | | +-------+-' |
: | | | | | | |
: ,--+------+-------------------------------------, | |
: | X X | | | | | | | |+12V
: | | | | | | | | | |
: | COM | | | | | \ | |
: | | | | | | / R4 | |
: | ,--------------------' | | | | \ 220k | |
: | | | | | | / | \
: | | ,------------+---------------+-' | | | | / R2
: | | |+ | | | | | | | \ 10k
: | | |\ ,---+----, | | | | | | /
: | '--|-\ | charge | | |+ | | | | |
: | | >------+ pump | | |\ | | | | |
: | ,--|+/ '-+----+-' '---|-\ |/c | '-------+--+
: | | |/ o,-------' | | >----| | |
: | | |- | ,-----',----------|+/ |>e | |
: | | COM | | | |/ opamp2 | | |
: | | | | | |- | | \
: | | | | | COM | | / R3
: | | | | | | | \ 10k
: | | | | | ,-------------' | /
: | | | | | | | |
: | | | | | | X X | |
: '-------------------------------------+------+--' |
: | | | | | |
: | | | | | LM2907N gnd
: | | +------+, gnd
: | --- C1 | |
: |( --- 1000| |
: magnetic |( | | |
: pick-up |( | --- C2 \
: |( | --- .1u / R1
: | | | \ 100k
: | gnd | /
: | | |
: gnd | |
: | |
: gnd gnd

I do not recall what RPM you are looking for, to set your cross-over.
So the values of components above are for illustration only and almost
certainly will not really get what you want. But the basic ideas are
there. What is missing is how to create Vcc, I suppose. But if a
kind of ratiometric threshold is okay and if (and I don't know) the
LM2907 can withstand the various voltages in automotive applications
then you may just hook it directly to the car supply. Otherwise, you
may need something to provide protection for the LM2907.

C1 and R1 are designed based on knowing your RPM threshold and the
voltage threshold you want to set (perhaps 1/2 of your Vcc) and the
minimum current that the LM2907 can manage (140uA) to supply. C2 just
filters this and sets the ripple voltage. Bigger can be better, but
only up to a point. I included R4 so that some hysteresis would be
added to overwhelm whatever ripple may be present after C2 filters it,
and it needs to be designed to be small enough to do the job. But
that will depend on the resistors, R2 and R3, which set the threshold
voltage, across which you want the relay switched.

Anyway, you will definitely need to fully disclose what it is that you
are trying to achieve, what RPM or range of RPMs you want to cover,
where you are getting your relay supply voltage, where you intend to
get your LM2907 voltage, etc. It's just guess-work until then (and
maybe even after that.)

Jon

18. ### ahonda55Guest

dear Jonathan , i want to thank you very very much
i have some troubles and i will tell you all of them..
i made the LM2907 tachometer circuit , and the output goes to th
transistor-relay circuit , but there is the problem of the ripple
the relay start bounceing before going ON or Off , i had used a 100u
capacitor , it is ok but the reaction of the relay becomes little bi
slow , what should i do to remove that ripple
that's all about the LM2907 circuit which i will post in the bottom o
this

the second aim of my dreams is (( speed switch )) , using LM2907 t
make it open the 12v relay on a desired RPM and closes it when th
RPM becomes low
the problems i have are..

i have the 14 pin version and i don't have the circuit for it , i jus
have the 8 pin

here is all supplies ..
Input voltage 8v or whatever below 12v will work
the relay is a small 12v rela
i want to make the relay On when the frequency reachs 150 hert

i want to know how to use the 14 pin instead of the 9 pin and will i
work

and if you can help me plz send me any circuit drawings not the tex
because i can't understand it and sorry for that

here is the circuit...

http://www.niksula.cs.hut.fi/~mdobruck/siililand/mini/diy/alien/tacho/tacho-big.gi

and the relay photo by me camera ;
[img:81ff149aa6]http://img386.imageshack.us/img386/5472/image12nh.jpg[/img:81ff149aa6

Thank you very much and really sorry to ask all that help , but thi
is the biginner me , need help 24/7 in something i ike to do
Thank you agai

19. ### Terry PinnellGuest

Anyone? I'm curious to know whether it's the datasheet or me that's
wrong

20. ### mikeGuest

One way out of your problem is to redefine it. If you use a counter and
compare the counts, you can have a very accurate threshold determination
anywhere you want. If you need analog output, a PWM can provide that
from the counter. And all this is built into any number of cheap
microcontrollers. One chip and you're done.
mike

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