Need to build an interface between an audiometer and a set of LED lights

[Gryd3]

Hi Adam,
Thanks for that breakdown, I think im following along for the most point, Im only really familiar with NPN transistors. Is the base not connected to the LED output on my LM3914? Therefore not getting 24V to begin with?
I may be being a complete ditz here so ill appologise in advance.

PNP and NPN are pretty darn close to the same.
The base still goes to the output pin on the LM3914, and will get whatever the output voltage is from this chip.
Adam's suggestion is to use a resistor between the LM3914 and transistor to limit current flow. Transistors are current driven and have such a high gain that you won't need to pump a ton of current into them (Or sink current from them) to be able to control the lights on the Light Tower you have. The additional pull-up resistor will help switch off the transistor at a greater rate as well.
What kind of experience have you had with the NPN transistors?

 

[xXAmyXx]

PNP and NPN are pretty darn close to the same.
The base still goes to the output pin on the LM3914, and will get whatever the output voltage is from this chip.
Adam's suggestion is to use a resistor between the LM3914 and transistor to limit current flow. Transistors are current driven and have such a high gain that you won't need to pump a ton of current into them (Or sink current from them) to be able to control the lights on the Light Tower you have. The additional pull-up resistor will help switch off the transistor at a greater rate as well.
What kind of experience have you had with the NPN transistors?

Oh OK, So am I right then in thinking hes used 24V as thats what the input to the LM3914 will be and therefore the output on each LED driver chip?

 

[Gryd3]

Oh OK, So am I right then in thinking hes used 24V as thats what the input to the LM3914 will be and therefore the output on each LED driver chip?

24V is approaching the maximum supply voltage for the LM3914, so it could be done that way. I'm not sure what the thought process was for expecting the voltage to be output from the LM3914, but regardless of the output being 5V, 12V, or 24V, it's good practice to put a resistor on the base of the transistor.

 

[xXAmyXx]

24V is approaching the maximum supply voltage for the LM3914, so it could be done that way. I'm not sure what the thought process was for expecting the voltage to be output from the LM3914, but regardless of the output being 5V, 12V, or 24V, it's good practice to put a resistor on the base of the transistor.

Ok that makes sense to me now! Because he put 24V that threw me for a bit, even though its close to the limit would it still be ok to use? I say that because then I can just connect the power supply directly up to it without having to worry about limiting the voltage.

 

[Gryd3]

Ok that makes sense to me now! Because he put 24V that threw me for a bit, even though its close to the limit would it still be ok to use? I say that because then I can just connect the power supply directly up to it without having to worry about limiting the voltage.

While it could support 24V, the absolute maximum is 25V at which the Datasheet claims damage may occur.
Although it can be done, the chip will run hotter and it's expected life-time will decrease. I would strongly suggest stepping the voltage down first for the V+ input. The current draw is pretty low so I'm sure you could get away with using a 3-pin voltage regulator to provide 5V or 12V (most common) to the LED driver.
( My bike red-lines at 13,000 , but I dare not drive there for any period long enough to count xD )

 

[xXAmyXx]

While it could support 24V, the absolute maximum is 25V at which the Datasheet claims damage may occur.
Although it can be done, the chip will run hotter and it's expected life-time will decrease. I would strongly suggest stepping the voltage down first for the V+ input. The current draw is pretty low so I'm sure you could get away with using a 3-pin voltage regulator to provide 5V or 12V (most common) to the LED driver.
( My bike red-lines at 13,000 , but I dare not drive there for any period long enough to count xD )

OK, dont want to be blowing anything up!
Would this be suitable http://uk.rs-online.com/web/p/linear-voltage-regulators/2988514/
Do I need any resistors before or after the Voltage regulator?
I can use resistors that you suggested earlier to get my 1V aswell then right?

 

[Arouse1973]

Because the output of the LM3914 is open collector (Comparator) then you could potentially run the IC from a lower voltage.
Adam

 

[Alec_t]

I'm not sure what the thought process was for expecting the voltage to be output from the LM3914, but regardless of the output being 5V, 12V, or 24V, it's good practice to put a resistor on the base of the transistor.

The LED-drive outputs of the 3914 are, as stated above, open collector, but they are configured as controlled constant-current sinks.

 

[Arouse1973]

OK, dont want to be blowing anything up!
Would this be suitable http://uk.rs-online.com/web/p/linear-voltage-regulators/2988514/
Do I need any resistors before or after the Voltage regulator?
I can use resistors that you suggested earlier to get my 1V aswell then right?

The regulator will work, you don't need any resistors before or after. If accuracy is important for the 1V ref... you could look at a 1.024 V reference IC. You can get 1 Volt versions but they are not as common and quite expensive.
Adam

 

[Arouse1973]

The LED-drive outputs of the 3914 are, as stated above, open collector, but they are configured as controlled constant-current sinks.

Alec did you get your LM3914 spice model working? Might be good to upload it if you can..
Thanks
Adam

 

[Gryd3]

OK, dont want to be blowing anything up!
Would this be suitable http://uk.rs-online.com/web/p/linear-voltage-regulators/2988514/
Do I need any resistors before or after the Voltage regulator?
I can use resistors that you suggested earlier to get my 1V aswell then right?

It won't blow up, it just may not last as long and get warmer than expected.
You don't need any resistors before or after the Voltage regulator, but it would be a good idea to use a capacitor on the input and output of the voltage regulator. (1 in parallel with the input, and 1 in parallel with the output)
You can most certainly use resistors to get your 1V mentioned earlier, this part is up to you, but you should understand the limitations before you continue.
Using a simple voltage divider or resistor method relies on the input voltage to this part of the circuit. If the input voltage changes, so will the 1V reference... You should use a 'regulated' voltage for this. Additionally, resistors are not perfect, they may 'drift' a small amount with heat, and are not exactly the value they specify. It's a good idea to use a potentiometer to that you can fine-tune it after it is constructed, or during development.
Other solutions include purpose built ICs, as well as using a Zener Diode (in combination of a resistor)
Of course, there is already a reference voltage on that LED driver, so you could simply use that as a starting point, and use a potentiometer to tune.

 

[xXAmyXx]

The regulator will work, you don't need any resistors before or after. If accuracy is important for the 1V ref... you could look at a 1.024 V reference IC. You can get 1 Volt versions but they are not as common and quite expensive.
Adam

Ok thankyou! How does the fact the output is open collector effect the circuit, will the output still be the same as the input (5V) does this effect what resistors I need on transistors?

 

[Arouse1973]

Ok thankyou! How does the fact the output is open collector effect the circuit, will the output still be the same as the input (5V) does this effect what resistors I need on transistors?

The beauty of an open collector means you can control another device which maybe on a completely different voltage. Because it's open it doesn't have any reference to the supply voltage that's controlling the drive of that transistor. Does that make sense? Do you want me to do a drawing?
Adam

 

[xXAmyXx]

The beauty of an open collector means you can control another device which maybe on a completely different voltage. Because it's open it doesn't have any reference to the supply voltage that's controlling the drive of that transistor. Does that make sense? Do you want me to do a drawing?
Adam

A drawing would help immensely!
I'm feeling incredibly stupid on this thread.

 

[Arouse1973]

A drawing would help immensely!
I'm feeling incredibly stupid on this thread.

Don't feel stupid, we all have to start somewhere. I just wish this forum was around when I started 20 years ago.
Adam

 

[Arouse1973]

 

[xXAmyXx]

View attachment 25649

So is R2 deciding the voltage output at the open collector? Also I thought my LED output pins would connect to the transistor to allow the higher voltage to activate the LED, Am I still best of using a PNP over a NPN?

 

[Gryd3]

So is R2 deciding the voltage output at the open collector? Also I thought my LED output pins would connect to the transistor to allow the higher voltage to activate the LED, Am I still best of using a PNP over a NPN?

R2 would be inside your light tower, the picture is just an example to show how you can operate a higher voltage device from a lower voltage device.
Q1 is inside the LED driver we have been looking at, but it cannot handle the 50mA.. so we are going to daisy chain a second PNP to Q1 by inserting it on the line where the pin? label is. The second PNP will be more capable, and that is what you will connect your higher voltage device to.

 

[Arouse1973]

This is just an example to show you how it works, it's not your circuit. I'll draw you yours the best I can. The bottom end of R2 (connected to the IC) basically gets connected to your common point through the IC, it limits the current for the LED otherwise a standard LED would be destroyed. It basically takes away or drops the excess voltage required to allow the LED to operate correctly without damage.
Adam

 

[xXAmyXx]

R2 would be inside your light tower, the picture is just an example to show how you can operate a higher voltage device from a lower voltage device.
Q1 is inside the LED driver we have been looking at, but it cannot handle the 50mA.. so we are going to daisy chain a second PNP to Q1 by inserting it on the line where the pin? label is. The second PNP will be more capable, and that is what you will connect your higher voltage device to.

OK, so what determines the output from the LED output pins?