How to switch on a circuit with a 12v feed....

[benweston]

Hi all

This might be a simple question for someone with more knowledge than me, but I can't figure it out.

I have a sat nav mounted in my car, fed with a 12v feed. Essentially, what I'm trying to do is toggle the power switch when the power goes to the sat nav so that it turns itself on automatically when the car ignition is switched on.

Is there such a thing (I've been looking at relays) that I can connect in place of the power button that, upon accepting a constant 12v feed will bridge the two power button wires, essentially switching it on?

It can't stay on like a relay as the sat nav will turn itself off again.

I hope I'm making myself clear!

 

[Sceadwian]

It depends on your GPS device, there might be an external power up line for it, otherwise you'd have to disassemble the device, and use a timer and a transistor or a relay to trigger the power on after it perceives the main power is on. Not simple if the GPS doesn't have an external power up line.

 

[ljcox]

I have just realised that the circuit I drew for you won't do what you want.

I don't fully understand what you're trying to do.

Do you need a circuit that will give a short pulse that operates a relay to bridge the GPS power button contacts for say 1 second?

 

[benweston]

Thank you for the replies - really appreciate it

Do you need a circuit that will give a short pulse that operates a relay to bridge the GPS power button contacts for say 1 second?

Yes, that's exactly what I'm trying to do, sorry I wasn't clearer.

Basically, the only way I can get the GPS to power-on on power is to set it in firmware but unfortunately, with it being a generic Chinese GPS, I've no way of reliably doing this. I think it'll be quicker and easier to ultimately simulate a finger pressing the power button for a second when the GPS receives a power feed (which is actually 5v, not 12v, is that makes any difference). The reason I can't use a relay is that if you push and hold the power button, the GPS switches on and then promptly switches off again.

No need to worry about activating the power switch again to switch if off as it will simply go off with the car's ignition.

Hope that's a bit clearer?

 

[WTP Pepper]

Hi all

This might be a simple question for someone with more knowledge than me, but I can't figure it out.

I have a sat nav mounted in my car, fed with a 12v feed. Essentially, what I'm trying to do is toggle the power switch when the power goes to the sat nav so that it turns itself on automatically when the car ignition is switched on.

Is there such a thing (I've been looking at relays) that I can connect in place of the power button that, upon accepting a constant 12v feed will bridge the two power button wires, essentially switching it on?

It can't stay on like a relay as the sat nav will turn itself off again.

I hope I'm making myself clear!

Most cars only apply power to the cigarette lighter when the engine is running. Open your GPS and remove the battery. Plug into the lighter socket and it will only power when the car is running.

 

[ljcox]

A crude way to do it would be with a large capacitor (probably about 2200uF) in series with the relay coil & a diode across the coil.

But the capacitor would have to discharge through the 5 Volt line when the power is turned off, so I worry about what effect that may have - given that I don't know what circuitry there is in the car.

So a better & safer approach would be to use an IC that generates the pulse to operate the relay briefly.

The circuit would be fairly simple & I'll design it for you, but given your limited electronics knowledge, do you want to take this approach?

Do you know how much current the GPS draws through the button? I assume not, so do you have a multimeter so you can measure it?

 

[benweston]

Most cars only apply power to the cigarette lighter when the engine is running. Open your GPS and remove the battery. Plug into the lighter socket and it will only power when the car is running.

Unfortunately this is not the case - I've already done this! The GPS' firmware is set to remain off when power is applied, sadly. Just happens to be a 'feature' of this particular firmware. Certainly the firmware in other GPSes means the GPS automatically turns on when power is supplied if the battery is removed.

Len - thank you so much for that. My electronics knowledge is 'bitty' so I'd be able to build a simple circuit from a schematic - I just can't design it myself! I agree that discharging the capacitor back into the 5v line (which is the cigarette lighter feed) is probably not a great idea. It might be fine but as it's a BMW with lots of different computers and sensors, I wouldn't want to risk upsetting something!

I don't know the current through the power button but I can find out tomorrow - I'll measure it with a multimeter.

I really appreciate your help

 

[ljcox]

Len - thank you so much for that. You're welcome.

My electronics knowledge is 'bitty' so I'd be able to build a simple circuit from a schematic - I just can't design it myself! I agree that discharging the capacitor back into the 5v line (which is the cigarette lighter feed) is probably not a great idea. See below.

It might be fine but as it's a BMW with lots of different computers and sensors, I wouldn't want to risk upsetting something!Exactly.

I don't know the current through the power button but I can find out tomorrow - I'll measure it with a multimeter.

I really appreciate your help

I'm not an expert on cars, but my car supplies 12 Volt to the cigarette lighter.

I don't know what the GPS does when you press the button, so I don't know if the current will be present long enough for you to get a sensible reading.

Is the MM analogue or digital?

DMMs take some time to make a measurement, whereas analogue ones start the needle moving immediately, so even if the pulse is short, you will have some idea of its magnitude.

I suspect that the current will be relatively small, but set your MM or DMM to the maximum current range initially - just in case.

 

[benweston]

I'm not an expert on cars, but my car supplies 12 Volt to the cigarette lighter.

I don't know what the GPS does when you press the button, so I don't know if the current will be present long enough for you to get a sensible reading.

Is the MM analogue or digital?

DMMs take some time to make a measurement, whereas analogue ones start the needle moving immediately, so even if the pulse is short, you will have some idea of its magnitude.

I suspect that the current will be relatively small, but set your MM or DMM to the maximum current range initially - just in case.

Absolutely - there is a 12v-5v transformer between the cigarette lighter and the GPS. This is why the feed to the GPS is 5v, but I could quite easily take a 12v feed too if required.

Couldn't you bypass the need for measuring the current by having the IC operate a relay for a second, which in turn would just bridge the two connections for the power button? This would at least provide some electrical isolation between the 'starter circuit' and the GPS itself.

Or am I barking up the wrong tree?

 

[ljcox]

You said "there is a 12v-5v transformer between the cigarette lighter and the GPS".

Transformers can't transform DC. It must be a DC - DC converter.

5 Volt is excellent for the electronics & 5 Volt relays are available.

The point about measuring the current is so that we know what relay to choose.

Relay contacts have both a minimum and a maximum current rating.

If, for example, you used a relay with contacts rated at one Amp, and the GPS current was say 100 micro Amp, then the operation may be unreliable.

In other words, it may work sometimes and not others, it may work for a few days or months & then fail or it may not work from the outset.

Alternatively, if you used a relay with contacts rated at 10 mA, and the GPS current was say 800 mA, then the contacts would eventually burn out.

So we need to know roughly what the current is.

eg. < 10 mA or > 10 mA.

 

[Sceadwian]

I would strongly recommend a transistor replacement rather than a relay.

 

[ljcox]

I would strongly recommend a transistor replacement rather than a relay.

Yes, a transistor would be a good solution, however we need to know how the button in the GPS is wired.
I presume it turns a transistor on inside the GPS that connects the 5 Volt supply line to the GPS circuitry.

A microprocessor inside the GPS would then hold the transistor on so the button can be released.

However, this is guesswork, and given the OP's limited electronics knowledge, I feel that a relay may be a safer option.

benweston,
Do you wish to persue the transistor option?

 

[audioguru]

There is another kid in his class called DAG (I think) with exactly the same school assignment.

 

[benweston]

Ha, I told you my electrical knowledge was 'bitty' - I never realised transformers only applied to AC!

Unfortunately, my MM is of the digital variety so the response is slow. From what I can gather from repeated tests, it seems to be between 4 and 5 mA which would seem about right as it's almost definitely just a logic switch to signal some IC.

I would have thought a transistor would do the trick but my limited knowledge dates back to old physics lessons at school!!

Re how the button is wired - the traces go straight to the main IC.

 

[benweston]

There is another kid in his class called DAG (I think) with exactly the same school assignment.

Unfortunately, it's been many years since I was at school! This is simply a project for a custom sat nav in my car!

 

[ljcox]

I suspected that the button tracks may go to an IC. So it would appear that the power switching transistor is inside the IC.

If you want to use a transistor, then I need to know the voltages on the button pins before the button is pressed and again after the button has been pressed and released.

Call the pins A and B.

4 ~ 5 mA sounds reasonable.

A transformer works because of a changing magnetic field, created by current in the primary. This creates an EMF (voltage) in the secondary winding.

So it needs AC to create the changing field. DC creates a constant field, hence there is no EMF at the secondary.

I suggest you do an internet search for transformers.

 

[benweston]

Pin A - 1.36v
Pin B - 2.76v

I measured the pins to ground - is that correct? Otherwise, it's a constant 4v (not 5v) across the button pins.

Voltages remain the same before and after button is pressed, but 0v whilst the button is being pressed.

Off to Google transformers now!!

 

[ljcox]

Pin A - 1.36v
Pin B - 2.76v

I measured the pins to ground - is that correct? Yes

Otherwise, it's a constant 4v (not 5v) across the button pins. This inconsistent with your measurements above. 2.76 - 1.36 = 1.40 Volt, not 4 Volt.

Voltages remain the same before and after button is pressed, but 0v whilst the button is being pressed. Do you mean the voltage between A & B while the button is pressed or did you measure from say A to Gnd & then see the voltage fall to 0V when the button is pressed?

Off to Google transformers now!!

As I wrote above, your figures don't add up.

To put it in other words, if you measured 4 Volt between A & B then Vb - Va should equal 4 Volt.

 

[ronv]

Maybe just a reed relay like this one: https://www.electro-tech-online.com/custompdfs/2012/02/105A_117SIP_107DIP_171DIP.pdf
Put about a 6800 Ufd capacitor with a 10k resistor across it in series with the coil (+ to + 5 volts) The relay will pick then release.
Hook the contacts across the GPS switch.

 

[ljcox]

Maybe just a reed relay like this one: https://www.electro-tech-online.com/custompdfs/2012/02/105A_117SIP_107DIP_171DIP.pdf
Put about a 6800 Ufd capacitor with a 10k resistor across it in series with the coil (+ to + 5 volts) The relay will pick then release.
Hook the contacts across the GPS switch.

Thanks for the suggestion but it has a few issues:-

1. the minimum switching current shown in the relay data sheet is 10 mA & the OP measured about 4 mA. See my comments in an earlier post (post #10) re the contact current issues.

2. I don't know if the pulse can be applied immediately the ignition is turned on. A short delay may be necesary.

3. It would be necessary to put a diode across the coil in order to limit the back EMF to about 0.7 Volt.