Hi Guys,

Newbie to group, trying to make a circuit board that will operate a relay to ground a low level LED in my speedo using the bikes factory NTC thermistor.

Factory wiring for the thermistor had an incadesant lamp in series...which glowed when the Thermistor is out of the fuel and heats up... resistance drops and lamp glowed...the lamp has 6.6ohm resistance

I have searched this topic and used a couple of designs and put together this circuit

I've tested it but the relay closes soon as I connect my 12v supply, and I'm a bit lost with it now.

Could anyone check out the circuit and advise where I have maybe gone wrong?

Attached is the drawing of what's been wired the second is what I'm trying to trying to control...

Thanks in advance...

 

You need to check that the input voltage range of your op amp will allow it to work within 1.2 volts of the 12 volt rail. You also need to check that the output voltage swing goes low enough to turn off the transistors.

Les.

 

You need to check that the input voltage range of your op amp will allow it to work within 1.2 volts of the 12 volt rail. You also need to check that the output voltage swing goes low enough to turn off the transistors.

Les.

In any case you need a resistor between pin 6 of the op amp and the base of the transistor to limit the current, say 4.7k.
Then you can add a resistor from the base of the transistor to 0V, say 1k. This will fix the issue of the op amp output swing.

 

A couple of thoughts which may help...

1) What is the chip? an Opamp or Comparator? I suggest using a comparator here.

2) The resistance of an incandescent lamp is nonlinear, it's lowest when cold, but increases when it lights up.
Measuring the resistance of a cold lamp yields misleading results.

3) The LED in your diagram is backwards an lacks a current limiting resistor.

4) If all you need it to do is illuminate an LED, you can do away with all those transistors and the relay.
A comparator with an open collector output (like the LM339) can drive an LED directly, just include a resistor to limit the current.

 

In any case you need a resistor between pin 6 of the op amp and the base of the transistor to limit the current, say 4.7k. will add this in thanks Albert..
Then you can add a resistor from the base of the transistor to 0V, say 1k. This will fix the issue of the op amp output swing.

A couple of thoughts which may help... maybe a silly Q but I dont know, what terminal is the base of the transistor?

1) What is the chip? an Opamp or Comparator? I suggest using a comparator here, Its a 741 opamp

2) The resistance of an incandescent lamp is nonlinear, it's lowest when cold, but increases when it lights up.
Measuring the resistance of a cold lamp yields misleading results.,



3) The LED in your diagram is backwards an lacks a current limiting resistor. ok Ill add one in thanks..

4) If all you need it to do is illuminate an LED, you can do away with all those transistors and the relay.
A comparator with an open collector output (like the LM339) can drive an LED directly, just include a resistor to limit the current.

Thanks for the option, however the 12v supply for the LED is inside the speedo housing, to make the speedo LED work all I have is a single wire which needs grounded to turn on the light. Hence the relay is required to ground it..

To operate the relay I have 12v supply and the NTC thermistor....

 

What op amp are you using?

 

What op amp are you using?

Its a 741 opamp

 

Below is a LTspice simulation of your circuit modified to correct some errors and make sure the transistor turns off when the op amp output is low.
The LED turns on when the thermistor resistance (R_Thm) goes below ≈1000Ω for the pot U2 set at the middle of its range.

You should be able to light the LED directly from the transistor.

R_Opt is an optional resistor if the LED does not already have one.

 

You don't need a relay!

Why spend the time, space and money on such overkill.

You say the output of your circuit never shuts off?
That's probably because the 741 cannot swing it's output lower than a volt or so from ground, so the transistor never turns off.

The solution is to use a voltage divider on the base of the transistor, raising the "on" threshold to about 3 volts.
This way the transistor only turns on when the opamp output goes above 3 volts.

This circuit solves the problem, and requires no relay.



the 741 is an antique, use a modern rail-to-rail opamp and you wont suffer these indignities.

 

Below is a LTspice simulation of your circuit modified to correct some errors and make sure the transistor turns off when the op amp output is low.
The LED turns on when the thermistor resistance (R_Thm) goes below ≈1000Ω for the pot U2 set at the middle of its range.

You should be able to light the LED directly from the transistor.

R_Opt is an optional resistor if the LED does not already have one.

View attachment 125833

This is fantastic, many thanks for all your help guys.... Ill be sourcing these parts asap and I will let you know how I go.
Much Appreciated !

 

Below is a LTspice simulation of your circuit modified to correct some errors and make sure the transistor turns off when the op amp output is low.
The LED turns on when the thermistor resistance (R_Thm) goes below ≈1000Ω for the pot U2 set at the middle of its range.

You should be able to light the LED directly from the transistor.

R_Opt is an optional resistor if the LED does not already have one.

View attachment 125833

Hi Crutschow

I've made up the circuit board and tried this morning, it would turn in the LED

I tried adjusting the 2k pot, should the bottom of the pot be common with the - on the OP amp and the wiper?

Is there anything you can suggest to check?
I've gone over the wiring a few times and used my meter to confirm all connections

 

Hi Crutschow

I've made up the circuit board and tried this morning, it would turn in the LED

I tried adjusting the 2k pot, should the bottom of the pot be common with the - on the OP amp and the wiper?

Is there anything you can suggest to check?
I've gone over the wiring a few times and used my meter to confirm all connections

(It wouldn't turn on the LED)

 

Likely a wiring error.
Post a schematic of the circuit exactly as you built it with the voltages measured at all the nodes.

 

Likely a wiring error.
Post a schematic of the circuit exactly as you built it with the voltages measured at all the nodes.

Hi Mate

Thanks for the reply
I checked the wiring and have attached a photo of the schematic and voltages

The OP amp is switching when the fuel thermistor heats up...

What's else could I check?

 

You didn't post the transistor node voltages, which is the part of the circuit that's apparently not working.

 

In post #11 you say "it would turn in the LED" (I assume that you mean it would turn ON the LED)
In post #12 you say It wouldn't turn on the LED
Which statement is correct. or has the circuits behaviour changed between the two posts ?
In post #13 crutschow asked for the voltage on ALL nodes.
Can you post the voltage on the OP Amp output when it is in the low state. (In post #14 you say "The OP amp is switching when the fuel thermistor heats up" I assume the 11.23 volts is when it is in the high state.)
Can you post the voltages an the nodes you have missed out (All with respect to the negative rail.) both when the Op Amp output is high AND low. Can you take these readings directly on the three pins of each transistor (I know you will be measuring the same node twice if it is wired exactly as the schematic.) and directly on the pins of the LED.

Les

 

H

In post #11 you say "it would turn in the LED" (I assume that you mean it would turn ON the LED)
In post #12 you say It wouldn't turn on the LED
Which statement is correct. or has the circuits behaviour changed between the two posts ?
In post #13 crutschow asked for the voltage on ALL nodes.
Can you post the voltage on the OP Amp output when it is in the low state. (In post #14 you say "The OP amp is switching when the fuel thermistor heats up" I assume the 11.23 volts is when it is in the high state.)
Can you post the voltages an the nodes you have missed out (All with respect to the negative rail.) both when the Op Amp output is high AND low. Can you take these readings directly on the three pins of each transistor (I know you will be measuring the same node twice if it is wired exactly as the schematic.) and directly on the pins of the LED.

Les

Hi Les

Thanks for the reply, Yes that was a typo and I could not edit it after 10mins..
The LED wouldn't turn on..

I will measure all the nodes and repost voltages..

After I posted the diagram with incomplete voltages, I checked my transistors and relised I had them reversed
E&C wrong polarity

My transistors are PN2222A I original wired for N2222A
I have changed them around now, could this reversal mistake damage them?

 

My transistors are PN2222A I original wired for N2222A
I have changed them around now, could this reversal mistake damage them?

As long as the rest of the circuit is wired as drawn then, no, that would not damage the transistors or anything else.

 

Please find updated drawing.
Have I missed anything?

 

You have missed the voltage on the negative end of the LED (The end that is connected to +V) There are three possible causes for the LED not being lit.
1 There is no voltage on the +V end of the LED
2 The LED is faulty
3 The led is connected the wrong way round.

Les.