I want to install a car door lock annunciator in our van. The purpose is to sound a horn when a signal to lock the doors occurs.
The reason is that the "walk away auto lock" feature does not always lock the doors. So sometimes when I return to the van, I find that the doors were not auto locked. An annunciator that sounds a horn when the signal to auto lock the doors occurs, would provide audio feedback while I'm walking away from the van. If I don't hear the horn, I can then lock the doors using the key FOB. If I hear the horn, I can just continue walking away with assurance that the doors were auto locked.

Here's my design so far:

My first try was the lower circuit without the PNP transistor. When I close the driver's door and walk away from the van, after about a 7 second delay the auto lock sends +12 volt pulses to lock all of the doors. Tapping one door lock circuit, it activates a relay that closes a normally open switch. That applies +12 volts to the motorcycle horn. This all worked fine.

BUT, then I discovered that the van auto locks the doors the first time the van reaches 9 miles per hour. This sounds the horn while driving. It only happens the first time it reaches 9 mph and never happens again while driving but I don't want that 9 mph horn blast.

So I'm thinking about adding a PNP transistor as show in the upper circuit. I'll tap the fuse box and find a circuit that has zero volts when the engine is not running and +12 volts when the key is on (engine running). Zero volts on the transistor base when the key is off should turn the transistor on but no current will flow in the circuit until the door lock pulse occurs. IOW, while the car is parked with the engine off, the transistor will be on but no current will flow to the horn since there is no door lock pulse.

When the engine is running (key on), the +12 volts will remove the forward bias on the transistor turning it off so the door lock pulse cannot sound the horn while driving.

QUESTIONS:
1. It's easy to find a fuse that has +12 volts only when the key is on. BUT, how can I determine, when there is NO +12 volts, that the circuit is truly zero volts (or near zero), and not an open circuit. IOW, that the +12 volts was provided by relay contacts.

2. What value resistor should I use in the base circuit of the transistor to limit the base current to a very low level. I don't want to drain the car's battery while it is parked. Is there a better circuit that does not have a parasitic current draw when the car is parked?

3. Is there a simpler circuit that you can come up with? Like omit the relay and just use the one transistor?

Thanks for any help.

 

I skimmed, but you're over complicating. Pick up a couple of $5 12V relays that have normally open and normally closed contacts. On the first one use the normally open contacts to drive the horn, trigger the relay with the door lock signal. On the second one use the normally closed contacts to control the ground to the first relay, and trigger that relay from the 12V accessory line from your key switch. Now when your key is switched on, the second relay will open thereby disabling the first relay that controls the horn. There are multiple ways to accomplish your goal, but this is quick and easy using off the shelf parts.

 

1. It's easy to find a fuse that has +12 volts only when the key is on. BUT, how can I determine, when there is NO +12 volts, that the circuit is truly zero volts (or near zero), and not an open circuit. IOW, that the +12 volts was provided by relay contacts.

2. What value resistor should I use in the base circuit of the transistor to limit the base current to a very low level. I don't want to drain the car's battery while it is parked. Is there a better circuit that does not have a parasitic current draw when the car is parked?

3. Is there a simpler circuit that you can come up with? Like omit the relay and just use the one transistor?

1. This is the first thing I was going to point out. A solution is to add a resistor to assure solid drive to the transistor, and change the transistor so that resistor is a large value. Two options are a power darlington or a power MOSFET. Based on bad experiences back in the 70's, I'm still a bit nervous about putting FETs in cars. Sure, we've learned a lot since then about how to protect them, but my first choice still is a big fat bipolar device.

With this approach, the PNP transistor has a pull-down resistor to GND that turns it on when the+12 V signal goes open circuit. This could be as high as 10 K, so the battery drain current would be approx. 1 mA. The electronics in the car when it is completely off draw more than that, so drain is not a problem.

PNP darlington as shown in your drawing, a 1K - to - 10 K resistor from the base to GND, and a 1N4004 diode from the base to the +12 V source. The 12 V source pulls the base up to near the emitter to turn off the transistor. When that voltage drops to around 10 V, the diode is reverse-biased and disconnects the transistor from the source. At this point the pull-down resistor turns on the transistor, and the circuit waits for the relay contacts to close.

An upgrade to the circuit is to replace the relay with a small-signal NPN transistor between the base resistor and GND, that turns on the darlington. Now the static drain in the off condition is microamps.

Schematic later.

OTOH, MrSoftware is correct - this can be done with two T9 type automotive relays and zero resistors, transistors, etc.

https://www.te.com/en/product-CAT-SCH691-T1.html

ak

 

OTOH, MrSoftware is correct - this can be done with two T9 type automotive relays and zero resistors, transistors, etc.

Yes, I agree, but—

I would prefer to see a high impedance connection to any of the circuits in the vehicle since I don’t know (and I don’t think anyone here knows) what effect drawing additional current from any particular appearance of a voltage in the fuse box might do—now or down the road.

Optoisolators, small MOSFETS, and the relays would be my approach to that solution—and the optos/MOSFETs (possibly een the relays) could probably be found already nicely modular on Amazon or AliExpress and they should be cheap.

 

I skimmed, but you're over complicating. Pick up a couple of $5 12V relays that have normally open and normally closed contacts. On the first one use the normally open contacts to drive the horn, trigger the relay with the door lock signal. On the second one use the normally closed contacts to control the ground to the first relay, and trigger that relay from the 12V accessory line from your key switch. Now when your key is switched on, the second relay will open thereby disabling the first relay that controls the horn. There are multiple ways to accomplish your goal, but this is quick and easy using off the shelf parts.

I decided to go with the two relay solution that you suggest.

Actually, this was the first design that I thought of since I already have two relays. What detered me is that each "relay plus the mounting socket" is 1-1/8" square x 2-1/8" long. The space under the driver's side dashboard where I mounted the relay for my first try is limited. I don't think two relays (plus their sockets) can fit in that space. That's why I thought just gluing a transistor to one relay would be a good solution (mounting space wise).

But using a two relay solution will solve both of my questions 1 and 2.

1. There will not be a concern whether the zero volts from the fuse tap is actually driven by a driver or is an open relay contact.​

2: There will not be a concern for parasitic current draw by a PNP transistor while the car is parked.​

For completeness, here is my final schematic: (NC=NormallyClosed, NO=NormallyOpen)


The top relay doesn't care if the key off is an open relay contact. It also has no current draw when the van is parked with key off.

I'll just have to find a space under the dashboard to mount the top relay.

 

1. This is the first thing I was going to point out. A solution is to add a resistor to assure solid drive to the transistor, and change the transistor so that resistor is a large value. Two options are a power darlington or a power MOSFET. Based on bad experiences back in the 70's, I'm still a bit nervous about putting FETs in cars. Sure, we've learned a lot since then about how to protect them, but my first choice still is a big fat bipolar device.

With this approach, the PNP transistor has a pull-down resistor to GND that turns it on when the+12 V signal goes open circuit. This could be as high as 10 K, so the battery drain current would be approx. 1 mA. The electronics in the car when it is completely off draw more than that, so drain is not a problem.

PNP darlington as shown in your drawing, a 1K - to - 10 K resistor from the base to GND, and a 1N4004 diode from the base to the +12 V source. The 12 V source pulls the base up to near the emitter to turn off the transistor. When that voltage drops to around 10 V, the diode is reverse-biased and disconnects the transistor from the source. At this point the pull-down resistor turns on the transistor, and the circuit waits for the relay contacts to close.

An upgrade to the circuit is to replace the relay with a small-signal NPN transistor between the base resistor and GND, that turns on the darlington. Now the static drain in the off condition is microamps.

Schematic later.

OTOH, MrSoftware is correct - this can be done with two T9 type automotive relays and zero resistors, transistors, etc.

https://www.te.com/en/product-CAT-SCH691-T1.html

ak

Thanks for taking the time to reply. I'm going with the 2 relay solution.

 

Yes, I agree, but—

I would prefer to see a high impedance connection to any of the circuits in the vehicle since I don’t know (and I don’t think anyone here knows) what effect drawing additional current from any particular appearance of a voltage in the fuse box might do—now or down the road.

Optoisolators, small MOSFETS, and the relays would be my approach to that solution—and the optos/MOSFETs (possibly een the relays) could probably be found already nicely modular on Amazon or AliExpress and they should be cheap.

Thanks for taking the time to reply. I'm going with the 2 relay solution.

 

Thanks for taking the time to reply. I'm going with the 2 relay solution.

What country are you in?
I believe this is still illegal in the UK.
I've no idea of rules in any other country.

 

I decided to go with the two relay solution that you suggest.
For completeness, here is my final schematic: (NC=NormallyClosed, NO=NormallyOpen)
View attachment 328628

The top relay doesn't care if the key off is an open relay contact. It also has no current draw when the van is parked with key off.

How does this solve the problem if the van still auto locks after getting up to 9mph?

 

How does this solve the problem if the van still auto locks after getting up to 9mph?

The van auto locking all of the doors at 9mph is a good thing. It's an added safety thing. It's only the motorcycle horn sound at 9mph that's objectionable. Don't want to scare pedestrians with a horn blast when the van reaches 9mph.

 

How does this solve the problem if the van still auto locks after getting up to 9mph?

His drawing of the normally closed relay isn't drawn how you normally draw it, but imagine that it opens when you give it 12V. So when the ignition is on and relay 1 gets 12V, the locks will not cause the horn to honk.

 

I decided to go with the two relay solution that you suggest.

Actually, this was the first design that I thought of since I already have two relays. What detered me is that each "relay plus the mounting socket" is 1-1/8" square x 2-1/8" long. The space under the driver's side dashboard where I mounted the relay for my first try is limited. I don't think two relays (plus their sockets) can fit in that space. That's why I thought just gluing a transistor to one relay would be a good solution (mounting space wise).

But using a two relay solution will solve both of my questions 1 and 2.

1. There will not be a concern whether the zero volts from the fuse tap is actually driven by a driver or is an open relay contact.​

2: There will not be a concern for parasitic current draw by a PNP transistor while the car is parked.​

For completeness, here is my final schematic: (NC=NormallyClosed, NO=NormallyOpen)
View attachment 328628

The top relay doesn't care if the key off is an open relay contact. It also has no current draw when the van is parked with key off.

I'll just have to find a space under the dashboard to mount the top relay.

Finished the modification. Works perfectly now .
When parked, the horn sounds when the walk away auto lock engages. When driving, the horn does not sound at 9mph.

It's amazing that now I hear the door lock solenoids clicking at 9mph. Door lock solenoids must have always been clicking at 9mph all these years since 2019 but I never noticed it until now.
The way I got around the mounting problem for the "key on" relay is:

1. I removed the socket for the "key on" relay and taped the "key on" relay on top of the "door lock" relay.
2. I soldered the wires directly to the terminals of the "key on" relay.
3. Now it acts like one unit with 2 enclosed relays, with just one socket and one mounting bracket.

End result is that with the relays stacked, it's like fitting a skyscraper in a small lot.

Thanks all.

 

It's amazing that now I hear the door lock solenoids clicking

Note that many automotive door lock mechanisms are a motor driving a linear actuator.

https://www.ebay.com/itm/3755175430...MfpEfXUt/fGboM/PTFZlO3xA==|tkp:Bk9SR7Lz392kZA

ak

 

Note that many automotive door lock mechanisms are a motor driving a linear actuator.

https://www.ebay.com/itm/375517543073?itmmeta=01J4JXQZ6CX86FNP3D8X808A01&hash=item576e96faa1:g:n1kAAOSw9Mlk8ZVZ&itmprp=enc:AQAJAAAA8POXtyuZm1cDS9S+lwOcdBZa7fPYtAVt/FK2Fb8WZoWb28F8vPQqc6GxbdTNPBfDeikAILUv90vGBCrpkUqzCyfwpo4uQ8JfVSoURQv1UaO1hyagoqTC4PqY6SC+vQscFljXYt/yIDka7T4NxCrDsqYOL+1Mc0n4To7YRIWpAqyrLnw/2Ew4zF0sc+qNu4DRmGj2O5CgyoNQg8jM/tFurYq47N0ecEuTgmn8VXKCtdHCKr9e78ilZmg4Oo6waphxyZVpNnAMRnFastiZKs7t9MvZbK7XF+Pyruo0HuVuoNMfpEfXUt/fGboM/PTFZlO3xA==|tkp:Bk9SR7Lz392kZA

ak

Yes, you are right. Replace the word "solenoid" with "motor driven actuator" in my text above. That explains why the "door lock pulse" is rather long so the horn sounds for about 1.2 seconds instead of a momentary beep.