Increasing 0.15v to >1v?

[stevejnr]

Hi Folks

I'm quite a novice when it comes to circuits.. So hopefully someone could please help with the following problem I'm facing..

I've got a vehicle tracking system that logs when a vehicle door is unlocked. It is negative triggered and as such when the door is locked a voltage of 0.14-0.16V passes over the central locking unlock wire. When I unlock it drops to 0.

The issue is my tracking devices only pick up the door as locked if the voltage across the central locking wire is >1V and as such interperets the 0.14-0.16V as unlocked, even though it is not.

I have tapped in to the door wire and I'm running off a seperate feed, I also have a 12V DC supply if required. The input for my tracker can take up to 45V, 500mA max. Could someone kindly recommend a work around to get me from 0.14-0.16V input to >1V output while still allowing for the voltage to drop back to 0 when unlocked?

Thanks in advance
Steve

 

[luben111]

Use OpAmp rail to rail such as OPA340 to amplify the input - use non inverting schematic with amplification. You need ~20x amplification to get 0.14V to 2.8V

 

[alexan_e]

Check the TIP#15 level shifter in page 46 (4-16), it uses a comparator
https://ww1.microchip.com/downloads/en/DeviceDoc/01146B.pdf

Alex

 

[FvM]

It sounds really unlikely, that such a low voltage is used for signaling purposes in automotive electronics.

 

[Orson Cart]

Be careful about noise, mobile phones etc giving you false signals at these low voltages...

 

[stevejnr]

Hi folks

Thank you for your replies.

I've started looking at creating the non-inverting opamp circuit. with the following circuit:

Will using the following resistors and powering from a 12v DC supply work with the LM741 opamp? **broken link removed**

Vin = ~0.15V
Rf = 1000 ohm
Rg = 100 ohm
Vout = 0.15 * ((1000 / 100) + 1) = 1.65V

Many Thanks

---------- Post added at 16:17 ---------- Previous post was at 15:15 ----------

@FvM

I've double checked the voltage across the wire on 2 ford transits now and it has ~0.15V when the vehicle is locked and drops to 0 when unlocked.

Thanks

 

[alexan_e]

Why did you choose 741?
It is a vary basic old opamp and I think it doesn't work with single supply.

Alex

 

[stevejnr]

Hi Alex

Thanks for your reply. The main reason I used the 741 was the following blog.

How to Use an Operational Amplifier | eHow.com

It uses the 741 in an identical scenario to mine.

However lets say If I power the 741 V+ to a 9v battery and V- to its negative terminal the output on my pin 6 is 9V is this correct? :|

The issue I am having is that even although I have wired up following the blog I still get the input supply voltage as the out?

And I'm using the AA battery as a test input with a common ground

 

[stevejnr]

I've attached a photo of my breadboard. Using a hard disk 12VDC supply

I'm not sure as to why even though if only the V+ & V- of the opamp are hooked up to my supply the voltage on pin 6 is 12v?