Help: Circuit Design 4in NOR Gate

Oops, D1-D4 will defeat the purpouse of the switches... :wink:
The resistors will limit the current through the switches to 1,2 mA, i.e. a
non-destructive value.

I had another thought - maybe the input to the car alarm is "high impedance".
I.e. it might react on voltage (+12V) alone, not requiring current?
This is impossible to know without trying, but you could try by connecting a resistor
(10 kOhm or bigger) and diode in series to the alarm, as shown by test.jpg.

If the alarm reacts properly to this you can simplify the "OR gate" and use only
resistors and diodes, as shown by simple.jpg.

If however the alarm doesn't react to the "test" circuit, you will still need the
transistor and relay, as shown previoulsy. If you have the type number of the
relay you are using I might find it on the net and suggest a proper transistor type
for you.

/Rambo

My relay has the following serial number printed on its exterior: **broken link removed**

Thats DC 12v at 3A output.

The transistor I currently have is a **broken link removed**.

I think a **broken link removed** should do the job.

**broken link removed** contains the product range I can get here.

Thanks,
Simon

They relay coil is rated for only 30 mA @ 12V so basically any ol' NPN transistor
will do. Your BC557C is unfortunately a PNP device, but from Jaycar's website
I can see that they have plenty of NPN candidates: BC337, BC547 etc.

I have plenty of suitable transistors lying around, but I live in Sweden so... :wink:

/Rambo

It's been a good discussion. The second diagram by 'XNOX' should work. Wish you luck and pls update your final design result.

I will let you know how I have gone once I have bought the components and trialed it.

Thanks for all your help,
Simon

After building the second suggestion by XNOX and it failing; transistor would not switch, I moved on to his third suggestion. Which works well.

Thanks very much for all your suggestions and help. I have learnt a few things about electronics.