Help: Circuit Design 4in NOR Gate

[Guest eminence]

I own an Alfa Romeo 75 car and I have fitted an alarm. Unfortunately Alfa decided to make the doors switches opposite to usual. When the doors are open they supply 0v GND (ground/negative) and when shut they supply 12v GND. So this makes it difficult to hook up to my alarm as it wants a 12v GND or 12v POS (positive) supply when the doors are open.

So not being that savy with electronics I figured I could get by with 4 relays (with NO/NC terminals), one for each trigger wire. I could not use one as there would still be 12v supplied if the other doors where shut and only one was opened. I rigged these up but as you would expect they chewed through the cars battery power pretty quickly as the coils were nearly always active because as you realise doors spend most of their time shut! :)

I went to a local electronics shop (JayCar) in Geelong, Australia and the person behind the counter recommended the following circuit layout.

[EDIT]Circuit diagram attached below[/EDIT]

I have put his suggestion together, but I am not getting the reaction I had hoped for. The basic idea of the above circuit is that if door switch 1 or 2 or 3 or 4 are not switched/pulled/toggled/on/down/together then the NOR gate will tell the transistor to switch the relay on or off. Where the lamp is hooked into the above circuit the alarms positive door trigger wire would be hooked up in the actual application.

I hope that I have given you enough information to help me come up with a circuit that works.

Thanks in advance,
Simon

[ante]

Hi Simon,
Welcome to this forum.

On most alarm installations with backwards door switches you just fit an extra switch to each door. This is the same as the one you use for the hood and the trunk and sometimes also for the fuel hatch.

[audioguru]

Hi Simon,
If you don't want to add doorswitches your circuit should work with a few changes (and if the relay coil's current is within the ratings of the transistor):

1) Add a diode across the relay coil to protect the transistor from the flyback voltage spike from the relay coil.
2) Replace the transistor that probably got damaged by the flyback voltage spike.
3) Remove R2 that keeps the transistor turned-on all the time.
4) Reduce the value of R1 so that the transistor gets enough base current.

Of course the IC should have its power pins connected to the car's power, and the inputs of the unused gate should be grounded.

See my corrected schematic.

[MP]

Simon,
Audioguru has given you some good advice. I only have one addition:
Add a resistor from the base of the transistor to the 12V supply. A good value is 10 times the value of the base resistor. You want to have a pull-up here at all times unless your chip pulls it low. This will insure against false triggering.

MP

[audioguru]

Thanks MP,
But I don't think an extra base resistor is necessary.
The gate's output is either "active low" which turns-on the transistor, or "active high" which turns-off the transistor and keeps it off. A Cmos gate's output is at the supply voltage (or ground) without any load current.

[MP]

A resistor from base to collector would be a good insurance policy. This problem has hit me in the past, so I highly recommend it from experience. You are correct that the gate should be either logic high or logic low, but when something causes it to become unstable, you sometimes see levels in the middle region. This is especially so on cars where grounding loops are such a problem.

Just my 2 cents.

MP

[audioguru]

Hi MP,
I think you mean, "base-emitter" resistor.
Its puny 7uA maximum current would be over-ridden by the gate driving current that is up to 161 times more. ;D

[Kevin Weddle]

There is a way to simulate the input of a gate so that it matches the output. You could load the output with another gate just to match it right. Otherwise, you should simulate the situation using the correct current draw which for TTL the source is less than the sink. If you use a common base transistor it should work. It's very easy and assures the correct current. This is just a guess. I really don't recall designs that utilize this, maybe because they are mostly CMOS.

[audioguru]

Hi Kevin,
The Cmos gate used in this project, like all Cmos gates, doesn't have any input current.

You can't "match" the input of a TTL gate to the output of another TTL gate since the output is designed to drive up to 10 inputs and still not be "matched".

[Guest eminence]

I have a solution to the problem from another forum: http://www.edaboard.com/ftopic99748-30.html Take a look if your interested

Thanks for all your suggestions,
Simon

[audioguru]

Hi Simon,
I didn't see any schematics posted on the EDA site so I don't know what they are talking about nor your solution. It is frustrating to jump from site to site.
On yet another site (Electro-tech-online) that you posted this problem, someone sketched a DIY DTL NOR gate. Maybe your solution is similar. Just think what those rectifiers will do if you drove under power lines or near a high-power radio transmitter. They might even set off the alarm!

[Guest eminence]

The schematics are attached to the posts on the other site. The solution is in the last post. I have added it here for you as well.

Simon

[audioguru]

Hi Simon,
Your description of the door switch's function sure caused a lot of confusion. No wonder the first few circuits didn't work.

If the door switch closes its contact to ground when the door is closed but becomes an open circuit (switch is turned off) when the door is open, then your last circuit should work. But only if the input current requirement for the alarm can be supplied by a 10K resistor and a diode voltage drop.

The current through all 10K resistors won't drain the battery too quickly. It should last for months.

[Guest eminence]

I have it working out in the car at the moment seems to be a nice simple solution.

Sorry for the confusion I caused.

Simon