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Need to build an interface between an audiometer and a set of LED lights
- Thread starter xXAmyXx
- Start date
I'm not sure I entirely understand... The output of the LED Driver IC we are looking at is controlled internally by a comparator that will essentially connect the 'LED output' pin to 'ground' (or negative) when active.
Because the pin goes 'low' when it's active we are connecting it to a PNP transistor that works by taking current 'from' the base to turn it on.
Our PNP transistor will then take care of providing 24V to one of the input wires for the LED Tower.
Use Adam's circuit for each stage on the LED Tower and you should be complete.
(If the LED Driver IC output 5V on it's 'LED Output' pin, we would slightly adjust the circuit and use an NPN transistor instead... It may seem odd that 'On' is connecting something to ground but you'll get used to it. It's called an 'Active Low' output and is quite common from simple ICs to automotive electrical.)
Ok I think I finally understand, so if I just copy that last section two more times over (for 3 total outputs) and use the resistors you recommended earlier (22k and 220K) i should be good to go? (adding in the stuff on the inputs such as the voltage reg, the 1volt IC chip and the two capacitors)
Yes. Here's a zip of the latest versions of my 3914 and 3915 models, together with some test jigs.
Hi Alec,
Thats super helpful thankyou, where abouts in the multisim folder do I copy these files too?
Arouse1973
Adam
Amy, the files look like they are for LT Spice. This is the simulator a lot of us use on here and we have a few experts that know it very well. You can download a copy from Linear Technology for free. You can then copy the files into the relevant folder and away you go...
Adam
Adam
Ok I think I finally understand, so if I just copy that last section two more times over (for 3 total outputs) and use the resistors you recommended earlier (22k and 220K) i should be good to go? (adding in the stuff on the inputs such as the voltage reg, the 1volt IC chip and the two capacitors)
For the 24V supply I could use 2 12 volt batteries but as they are big and bulky could I use 3 9 volt batteries and then attached a 24V voltage regulator?
Finally because Im using the IC chip to limit the Voltage to 1V for the reference voltage do I need any resistors on the input side of the LM3914? And how do I calculate the size of capacitor I need for the voltage regulator?
You will also need a resistor or two for the 'brightness' setting on the chip, this controls the current output of the LED pins, and was recommended that you pre-set it to a high value, then you can adjust afterwards.
For the 24V supply, you will be drawing 50mA per tier it looks like, so the peak draw will be 150mA.
The 9V batteries you plan to use range from a 400mAh to 1200mAh capacity, which means you can light the entire tower anywhere from 2.5 hours to maybe 7 hours if you buy the expensive Lithium based batteries.
The 12V Lead Acid batteries are bulky, but for good reason; they have a LOT more capacity. You could also look into purchasing the 12V version of the tower instead of the 24V version. This would give you more freedom when selecting a battery or battery pack. (8 AA batteries for example)
As far as the input side of the LM3914, there are 'two' inputs you need to worry about.
The signal input does have a maximum current input limit... how are you making your signal, we 'may' need a resistor on the input.
The Voltage reference input. This one does not need a resistor unless you want to adjust the input voltage... The reference voltage IN pin is quite literally just connected to resistors internally. Totalling 10kΩ . This is why you could use a little math and make a voltage divider to provide 1V instead of a dedicated chip. Of course, this is your call based on the accuracy you require.
Hi Guys,
Sorry for the delay, have been super busy!
I put together a prototype of this circuit at work and when I apply the power im getting all the LEDs on at the same time even if i disconnect the audiometer
Any ideas, heres a rough picture of the diagram ive built it off.
The 5v vcc should be the varying low voltage coming from the audiometer and the small box that says boost represents the 5v regulator.
Attachments
Arouse1973
Adam
To start with your PNP transistors look like they are connected differently to my drawing
Adam
Adam
Arouse1973
Adam
Do you have a multi-meter that you can use? We might need this later.
Adam
Adam
Yeah i realised thatTo start with your PNP transistors look like they are connected differently to my drawing
Adam
I do have a multimeter.
I did get it working but with the wrong decible readings, then i realised I had put the wrong resistors for my voltage divider to get 1v for the reference. I have since put the right resistors in but the lights are now permanantly on which makes no sense.
Arouse1973
Adam
Ok I have to go in a mo... as I am on lunch at work. Can you make Voltage measurements of all the nodes referenced to 0V common. So that's all the pins of the circuit including the power rail. Make a note of them and edit your drawing with these measurements. This really helps us see what's going on. Also some clear pictures of the assembly of the circuit always helps.
I will be back on tonight if you are free... Unless one of the other guys solve your issue first that is.
Cheers
Adam
I will be back on tonight if you are free... Unless one of the other guys solve your issue first that is.
Cheers
Adam
Transistors look wrong in the drawing, but if they all used to work (but at the wrong db level, then you must have the proper ones installed)
Other than that, my initial look at the circuit diagram looks correct.
Assuming you are providing 5V power to the LM3914 and voltage divider for the reference.
Keep your black lead on the negative terminal of the battery and poke literally everything with your positive lead.
I'm particularly interested in the voltage on the Vref pin of the LM3914, but if you can please measure all the pins and let us know it would be great .
Other than that, my initial look at the circuit diagram looks correct.
Assuming you are providing 5V power to the LM3914 and voltage divider for the reference.
Keep your black lead on the negative terminal of the battery and poke literally everything with your positive lead.
I'm particularly interested in the voltage on the Vref pin of the LM3914, but if you can please measure all the pins and let us know it would be great .
Arouse1973
Adam
Ok just looking at this now in more detail and I have some questions. If you are using your own RHI (1 Volt in this case) do you need all the other resistors around the reference pins. Looking at the example in the data sheet, could you connect Ref adj (pin 8) to common and then the LED current is just 12.5 / your 1.2K+10R resistors across pins 7 and 8 which is about 10 mA.
Adam
Adam
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