Looking for a 2 input 12V AND circuit which will take from each front indicator and drive a piezo 10ma(or more) 12V buzzer which aldready has the driver in it so only needs the 12V 10mA or greater output when both inputs triggered.

Thank you.

 

Sounds like a job for an AND gate.

What is a "front indicator"?
What is the resulting signal?
When is it high?
When is it low?
Which state do you want to drive the beeper?
Do you want to use diodes, discrete transistors, or an IC?

There are at least six different ways to do this; seven if you include my favorite part for automotive projects. Which one do you prefer?

ak

 

OK, a two input AND circuit driven by two sources that when not active have a low resistance pull down to zero volts, which are the two front indicator lights in a car. So a diode connected so that it pulls down the base of an NPN transistor will serve as an input to the AND gate, with one diode connected to each indicator light terminal. Then when both indicators are driven, the transistor will be biased ON, as the base will be biased on by a suitable pull-up resistor. That will allow collector current to flow, sounding the piezo buzzer connected between +12 volts and the collector.
A diode should be connected between the emitter and common to assure that the transistor is completely biased off when only one front indicator is switched on.
Sorry I can't post a drawing, but the circuit is very simple. Actually, it is a negative logic NOR GATE, to be really correct. But it will provide the desired function

 

A diode should be connected between the emitter and common to assure that the transistor is completely biased off when only one front indicator is switched on.

Better to put that third diode in series with the base so it doesn't reduce the drive voltage to the beeper.

For *real* noise immunity, make that diode a 5.1 V zener.

OR change the transistor to a FET.

We still need to know if the inputs are active high or active low.

ak

 

Car-Lock ???
or
"Indicator" ???
which is it ???
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And BTW, the original question is not at all clear. I think what he wants is an OR or NOR function, but the way it is worded any of the six standard logic gates could be used, with very different result sets.

To the TS:
There are two inputs: Left and Right.
There are two possible conditions for each input: 12 V and 0 V.
There are two possible outputs: Beep and Silent.

Please post a four-line truth table with each of the four possible input combinations and the four resulting outputs.

I have three different schematics worked up. The odds of any of them being correct is only 16.7%, so, as Paul said to George, "Now we sit."

ak

 

Indicators altenate between 12v and 0v.
Can anyone recommend a two input, or more, discreet AND IC which can handle more than 10mA out of the AND gate. I suppose maybe two AND gates in parallel as long as they switch at the same exact time so as not to have the load on one gate for any length of time. I'll work out the other components like any fuses/caps/Res/diodes while building it.
Thank you.

 

So, to be cleat:

Left Input 0 V, Right input 0 V > Silent
Left Input 0 V, Right input 12 V > Silent
Left Input 12 V, Right input 0 V > Silent
Left Input 12 V, Right input 12 V > Beep

Correct?

ak

 

Assuming post #8 is correct, here is a 1-transistor solution, a callback to the days of DTL. Diode Dn indicates that there is no theoretical limit to the number of inputs. Per the AND function, all inputs must be high (or open-circuit) for the output to activate. D3 can be anything from 4.7 V to 6.2 V.

Update: The right-side schematic is the same function with a small-signal MOSFET. The higher gate threshold voltage should be enough noise immunity to obviate the zener diode.

ak

 

Thank you but there seems to be voltage goung back to the indicatorsvua the DN diodes if the indicators are powered off.

 

Another alternative?

 

Thank you but there seems to be voltage goung back to the indicatorsvua the DN diodes if the indicators are powered off.

There is nothing in this thread to indicate that that is a problem. It would really help to know what the "indicators" are.

Here is a 2-input AND circuit that does not source current out its inputs.

ak

 

Just for fun, here is a 3-input version for rugged environments.

The ULN2004 is intended as a low-power peripheral driver. Internally it is seven Darlington-pair transistors (see the small schematic, lower left), each rated for 50 V and 0.5 A. Functionally, it is a high-voltage, high-current, open-collector sept-inverter with built in transient protection, reverse-voltage protection, and input pull-down resistors. It needs no power-supply decoupling, and unused inputs can be left floating.

ak

 

There is nothing in this thread to indicate that that is a problem. It would really help to know what the "indicators" are.

Here is a 2-input AND circuit that does not source current out its inputs.

ak

View attachment 320959

Indicators, the yanks might can them flashers. The directional indicators that flash when the car is locked/unlocked remotely.
Apologies for not being precise.
Thank you.

 

Indicators, the yanks might can them flashers. The directional indicators that flash when the car is locked/unlocked remotely.

That clears up some things. How do you distinguish between an unlock simultaneous flash and emergency simultaneous flashing?

One way would be to include the signal from the emergency flash button as an enable signal to suppress the output. Schematics 1 and 5 can do this with no additional parts.

ak

 

That clears up some things. How do you distinguish between an unlock simultaneous flash and emergency simultaneous flashing?

One way would be to include the signal from the emergency flash button as an enable signal to suppress the output. Schematics 1 and 5 can do this with no additional parts.

ak

I dont really mind if it sounds off when the hazards are ever on, since that will be a very rare occurrance and a switch could be put before the buzzer if I ever want it to not buzz.

I was thinking of trying this instead of tge uln2004 because it is already on a pcb and easy wired.
https://www.ebay.co.uk/itm/22593104...4g9lmQUQ8O&var=&widget_ver=artemis&media=COPY
Thank you.

 

I was thinking of trying this instead of tge uln2004 because it is already on a pcb and easy wired.

Should work as well as the 2004.
What do you think about my suggestion in post #11?

 

Another alternative?
View attachment 320954

I like this one. Nice and simple. Not sure though if cars canbus(ecu) will complain for indicator1 driving the buzzer even if the buzzer is only 10mA. I have no idea if the extra load compared to the led indicator woukd make the ecu complain, is why I want to make as small a footprint on the inputs as possible.
Thank you.

 

Buy a cheap Car-Alarm to get what You want, plus a ton of other built-in features.
Replace the Siren with what ever "12-Volt Noise-Maker" that You prefer.

With a modern Car, You may run into all sorts of crazy situations with the Factory-Wiring,
almost everything is going over to CAN-Buss-Control.

You'll need a Factory-Wiring-Schematic if your Car is less than around ~10-years old.
Get the Schematic first before You spend any Time or Money on
a project that may not be possible or practical in the first place.

Good-Luck .........
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I was thinking of trying this instead of the uln2004 because it is already on a pcb and easy wired.
https://www.ebay.co.uk/itm/22593104...4g9lmQUQ8O&var=&widget_ver=artemis&media=COPY

A bit expensive, but should work fine. The 2803 and 2804 are the same parts as the 2004/2004, but with 8 sections instead of 7. A 2003/2803 also will work in this application. The datasheet says it is a 5 V part, but that is not at all a firm spec. In a 12 V circuit it has a slightly larger input current than a 2004, not a bad thing in automotive environments because this translates into increased noise immunity.

I've put 2003's and 2004's in homes, cars, ambulances, firetrucks, industrial controls, telecom gear, medical sub-systems, and rugged military systems. Great little part.

ak